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ILENR/RE-EA-94/05(1)

The Changing Illinois Environment:
Critical Trends

Technical Report of the Critical Trends Assessment Project
Volume 1: Air Resources

Illinois Department of Energy and Natural Resources
Illinois State Water Survey Division

2204 Griffith Drive

Champaign, Illinois 61820-7495

June 1994

Jim Edgar, Governor
State of Illinois

John S. Moore, Director

Illinois Department of Energy and Natural Resources
325 West Adams Street, Room 300

Springfield, Illinois 62704-1892

Printed by Authority of the State of Dlinois
Printed on Recycled and Recyclable Paper

Illinois Department of Energy and Natural Resources
325 West Adams Street, Room 300
Springfield, Illinois 62704-1892

Citation: Illinois Department of Energy and Natural Resources, 1994. The Changing Illinois Environ-
ment: Critical Trends. Summary Report and Volumes 1 - 7 Technical Report. Ilinois Department of
Energy and Natural Resources, Springfield, IL, ILENR/RE-EA-94/0S.

Volume 1: Air Resources

Volume 2: Water Resources

Volume 3: Ecological Resources

Volume 4: Earth Resources

Volume 5: Waste Generation and Management

Volume 6: Sources of Environmental Stress

Volume 7: Bibliography

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Volume 1
Air Resources

CONTRIBUTORS

Illinois State Water Survey

Van C. Bowersox
Donald A. Dolske
Donald F. Gatz
Kenneth E. Kunkel
Stephen J. Vermette
Wayne M. Wendland

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ABOUT THE CRITICAL
TRENDS ASSESSMENT
PROJECT

The Critical Trends Assessment Project (CTAP) is an
on-going process established to describe changes in
ecological conditions in Illinois. The initial two-year
effort involved staff of the Illinois Department of
Energy and Natural Resources (ENR), including the
Office of Research and Planning, the Geological,
Natural History and Water surveys and the Hazardous
Waste Research and Information Center. They
worked with the assistance of the Illinois Environ-
mental Protection Agency and the Illinois depart-
ments of Agriculture, Conservation, Mines and
Minerals, Nuclear Safety, Public Health, and Trans-
portation (Division of Water Resources), among
other agencies.

CTAP investigators adopted a “source-receptor”
model as the basis for analysis. Sources were defined
as human activities that affect environmental and
ecological conditions and were split into categories
as follows: manufacturing, transportation, urban
dynamics, resource extraction, electricity generation
and transmission, and waste systems. Receptors in-
cluded forests, agro-ecosystems, streams and rivers,
lakes, prairies and savannas, wetlands, and human
populations.

The results are contained in a seven-volume technical
report, The Changing Illinois Environment: Critical
Trends, consisting of Volume 1: Air Resources,
Volume 2: Water Resources, Volume 3: Ecological
Resources, Volume 4: Earth Resources, Volume 5:
Waste Generation and Management, Volume 6:
Sources of Environmental Stress, and Volume 7:
Bibliography. Volumes 1-6 are synopsized in a
summary report.

ABOUT CTAP

The next step in the CTAP process is to develop,
test, and implement tools to systematically monitor
changes in ecological and environmental conditions
in Illinois. Given real-world constraints on budgets
and human resources, this has to be done in a practi-
cal and cost-effective way, using new technologies
for monitoring, data collection and assessments.

As part of this effort, CTAP participants have begun
to use advanced geographic information systems
(GIS) and satellite imagery to map changes in Illi-
nois’ ecosystems and to develop ecological indicators
(similar in concept to economic indicators) that can
be evaluated for their use in long-term monitoring.
The intent is to recruit, train, and organize networks
of people — high school science classes, citizen vol-
unteer groups — to supplement scientific data collec-
tion to help gauge trends in ecological conditions.

Many of the databases developed during the project
are available to the public as either spreadsheet files
or ARC-INFO files. Individuals who wish to obtain
additional information or participate in CTAP
programs may call 217/785-0138, TDD customers
may call 217/785-0211, or persons may write:

Critical Trends Assessment Project

Office of Research and Planning

Illinois Department of Energy and Natural Resources
325 West Adams Street, Room 300

Springfield, IL 62704-1892

Copies of the summary report and volumes 1-7 of
the technical report are available from the ENR
Clearinghouse at 1/800/252-8955. TDD customers
call 1/800/526-0844, the Illinois Relay Center.
CTAP information and forum discussions can also
be accessed electronically at 1/800/528-5486.

FOREWORD

FOREWORD

"If we could first know where we are
and whither we are tending,
we could better judge what we do
and how to do it..."

Abraham Lincoln

Imagine that we knew nothing about the size,
direction, and composition of our economy. We
would each know a little, i.e., what was happening to
us directly, but none of us would know much about
the broader trends in the economy — the level or rate
of housing starts, interest rates, retail sales, trade
deficits, or unemployment rates. We might react to
things that happened to us directly, or react to events
that we had heard about — events that may or may
not have actually occurred.

Fortunately, the information base on economic trends
is extensive, is updated regularly, and is easily
accessible. Designed to describe the condition of the
economy and how it is changing, the information
base provides the foundation for both economic
policy and personal finance decisions. Typical
economic decisions are all framed by empirical
knowledge about what is happening in the general
economy. Without it, we would have no rational way
of timing these decisions and no way of judging
whether they were correct relative to trends in the
general economy.

Unfortunately, this is not the case with regard to
changes in environmental conditions. Environmental
data has generally been collected for regulatory and
management purposes, using information systems
designed to answer very site-, pollutant-, or species-
specific questions. This effort has been essential in
achieving the many pollution control successes of the
last generation. However, it does not provide a
systematic, empirical database similar to the eco-
nomic database which describes trends in the general
environment and provides a foundation for both
environmental policy and, perhaps more importantly,
personal decisions. The Critical Trends Assessment
Project (CTAP) is designed to begin developing such
a database.

As a first step, CTAP investigators inventoried
existing data to determine what is known and not
known about historical ecological conditions and to
identify meaningful trends. Three general conclu-
sions can be drawn from CTAP’s initial investiga-
tions:

Conclusion No. 1: The emission and discharge of
regulated pollutants over the past 20 years has
declined, in some cases dramatically. Among the
findings:

¢ Between 1973 and 1989, air emissions of
particulate matter from manufacturing have
dropped 87%, those of sulfur oxides 67%,
nitrogen oxides 69%, hydrocarbons 45%, and
carbon monoxide 59%.

¢ Emissions from cars and light trucks of both
carbon monoxide and volatile organic com-
pounds were down 47% in 1991 from 1973
levels.

e Lead concentrations were down substantially in
all areas of the state over the 1978-1990 period,
reflecting the phase-out of leaded gasoline.

¢ From 1987 to 1992, major municipal sewage
treatment facilities showed reductions in loading
of biological/carbonaceous oxygen demand,
ammonia, total suspended solids and chlorine
residuals that ranged from 25 to 72%.

e Emissions into streams of chromium, copper,
cyanide, and phenols from major non-municipal
manufacturing and utility facilities (most of them
industrial) also showed declines over the years
1987-1992 ranging from 37% to 53%.

Conclusion No. 2: Existing data suggest that the
condition of natural ecosystems in Illinois is rapidly
declining as a result of fragmentation and continual
stress. Among the findings:

¢ Forest fragmentation has reduced the ability of
Illinois forests to maintain biological integrity.
In one Illinois forest, neotropical migrant birds
that once accounted for more than 75% of
breeding birds now make up less than half those
numbers.

FOREWORD

¢ In the past century, one in seven native fish
species in Lake Michigan was either extirpated
or suffered severe population crashes and exotics
have assumed the roles of major predators and
major forage species.

¢ Four of five of the state’s prairie remnants are
smaller than ten acres and one in three is smaller
than one acre — too small to function as self-
sustaining ecosystems.

¢ Long-term records of mussel populations for four
rivers in east central Illinois reveal large reduc-
tions in numbers of all species over the last 40
years, apparently as suitable habitat was lost to
siltation and other changes.

¢ Exotic species invasions of Illinois forests are
increasing in severity and scope.

Conclusion No. 3: Data designed to monitor compli-
ance with environmental regulations or the status of
individual species are not sufficient to assess ecosys-
tem health statewide. Among the findings:

¢ Researchers must describe the spatial contours of
air pollutant concentrations statewide using a
limited number of sampling sites concentrated in
Chicago and the East St. Louis metro area.

¢ Much more research is needed on the ecology of
large rivers, in particular the effects of human
manipulation.

e The length of Illinois’ longest stream gaging
records is generally not sufficient to identify
fluctuations that recur less frequently than every
few decades.

¢ The Sediment Benchmark Network was set up in
1981 with some 120 instream sediment data
stations; by 1990 the network had shrunk to 40
stations, the majority of which have data for only
one to three years.

CTAP is designed to begin to help address the
complex problems Illinois faces in making environ-
mental policy on a sound ecosystem basis. The next
edition of the Critical Trends Assessment Project, two
years hence, should have more answers about trends
in Illinois’ environmental and ecological conditions
to help determine an effective and economical
environmental policy for Illinois.

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CONTENTS

CONTENTS
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AIR RESOURCES: VOLUME SUMMARY

AIR RESOURCES:
VOLUME SUMMARY

REPORT OVERVIEW

Although considerable progress has been made in im-
proving the environment, national and state goals for
environmental quality still have not been completely
met, and additional effort and expenditures will be
necessary. To use public funds as effectively as possi-
ble, Illinois environmental officials must set priorities
so that the most serious problems receive the most
attention. The priorities should be based on an assess-
ment of the current status of Illinois’ environment. This
technical report on air resources is one of a series
aimed at documenting and assessing the status of
Illinois’ environment. It includes comprehensive infor-
mation about Illinois’ climate and air quality and the
deposition of atmospheric constituents on the earth’s
surface. In each subject area, currently available data
have been assembled to provide a picture of how air
resources have changed over time and how they vary
spatially from place to place around the state.

SUMMARY AND SIGNIFICANT RESULTS
Climate Trends

This report presents an analysis of climate trends since
the late nineteenth century in Illinois. This work is
based on a variety of climate data and information
from Illinois sites. Long-term daily temperature and
precipitation data available from 41 locations formed
the basis for much of the analysis of temperature and
precipitation in this report. More detailed observations
from National Weather Service office sites (including
Chicago, Springfield, Peoria, Moline, St. Louis, and
Evansville) were used to assess trends in cloud cover,
freezing precipitation, and visibility. Citizen reports
were used to assess trends in tornado frequency.

Although long-term trends can be identified in various
climate parameters, the dominant characteristic of
Illinois’ climate is the presence of variability on all
time scales from decades to a few years or less. That is,
the magnitude of long-term trends is in general consid-
erably less than the changes that can occur from one
year or a few years to the next. With that being said, how-
ever, some identifiable long-term features can be noted.

The most persistent and extreme period of summertime
drought and heat occurred during the 1930s, the Dust
Bowl era. This mirrors the long-term average tempera-
tures in Illinois, which increased by 4 to 5°F from the
mid- to late 1800s to the 1930s, and then cooled by
about half that amount to the present. This trend in
temperature is consistent with trends found in global
records. Consistent with the cooling trend after about
1930, generally benign and moderate summers
occurred during the 1960s and 1970s. However,
since that time, the 1980s were characterized by
more frequent hot and dry summers, similar to the
first half of the century. In fact, the severity of the
1988 growing-season drought in Illinois has only
been equaled in two other years since the turn of

the century.

In general, the frequency of extreme cold events has
increased from about 1930 to the present. This upward
trend reached its peak in the late 1970s and early
1980s, with an unprecedented string of extremely cold
winters. However since the early 1980s, winters have
generally been on the benign side.

Tornado frequency records exhibit large variability.
Reliable records of tornado frequencies are limited to
the past three decades. In that time, no upward or
downward trend in frequencies has been identified.

It is clear that climate has not been stable in Illinois
during the last 150 years, nor should it have been so
anticipated. Many of the changes, although abrupt,
were of relatively small magnitude. Yet those relatively
small-scale changes levied a substantial toll on the
inhabitants of the state through discomfort, lost

income, increased costs, and impediments to commerce
and agricultural production, in spite of major strides.
The past gives little hint as to the direction and
magnitude of any possible future changes, but it does
suggest the degree to which climate may change in the
future, and demonstrate the magnitude of that impact
on human activities.

Air Quality

Air quality can have direct effects on human and
ecosystem health. Thus it is necessary to examine
current air pollutant concentrations and their spatial
variations over the state. We also need to know
whether concentrations are increasing, decreasing, or
remaining constant, especially in the major population
centers. Finally, we seek to identify gaps in air quality
data and research that need to be filled to permit wise
planning for the future.

AIR RESOURCES: VOLUME SUMMARY

This assessment of air quality is based primarily on
Illinois Environmental Protection Agency (IEPA)
measurements of seven pollutants for which state or
national standards have been set, plus eight additional
pollutants for which standards have not been set. These
data cover the time period from 1978-1990. Summary
data published in IEPA annual reports were tabulated
in computer spreadsheet and Geographic Information
System (GIS) files and are available to others.

Temporal changes in pollutant concentrations are
depicted graphically using box plots, which concisely
show several features of observed distributions of
concentrations at sampling sites within various geo-
graphic areas. Spatial variations of average pollutant
concentrations in the Chicago area are shown by plots
of concentration contours. Trends in concentrations
over time were tested for statistical significance using
the nonparametric Spearman Rank Correlation Coeffi-
cient. Statistical tests were also carried out to identify
significant differences in concentrations between geo-
graphic regions.

For the criteria pollutants tested for time trend in any
geographic region of the state, results indicated only
decreasing trends or no significant (5 percent) trends.
No increasing trends in criteria pollutants were de-
tected. For the state as a whole, seven of twelve
pollutant/averaging time datasets tested for time trend
showed significant trends (5 percent or better) toward
decreasing concentrations over the 1978-1990 period.
After accounting for temperature effects, O, showed a
significant decreasing trend (2 percent level), rather
than no trend.

In the Chicago area, eight of the twelve datasets
showed decreasing trends at the 5 percent level (all but
one of these were at the 1 or 2 percent level). After
accounting for temperature, O, (ozone) in the Chicago
area showed a decreasing trend (2 percent level, table 3
of Air Quality Trends in Illinois chapter), rather than no
trend. In the MetroEast area, only four pollutant/aver-
aging time data sets were measured at enough sites to
warrant testing for time trend. Only Pb (lead) showed a
significant (5 percent) trend toward decreasing concen-
trations. After accounting for temperature, O, showed a
decreasing trend at the 6 percent level, but not the 5
percent level. Three of nine datasets tested for time
trend in the “remainder” region showed decreasing
trends at the 5 percent level or better.

The eight noncriteria pollutants were all tested for
trend in all four areas mentioned above, except for the
MetroEast region, where the data were not adequate to

test Cr (chromium) and Ni (nickel). Of the 30 trend
tests, 20 showed no significant (5 percent) trend. Over
the state as a whole, and in the Chicago area, two
species showed significant decreases—SO,” (sulfaate
ion) and As (arsenic). In the MetroEast area, Fe (iron)
and Mn (manganese) showed significant increases (the
only increasing trends found in this study). In the
“remainder” region, SO,*, As, Cd, and Mn showed
significant (5 percent or better) decreases.

Comparison of median and maximum pollutant con-
centrations within geographic regions, from the yearly
box plots in figures 1-30 (Air Quality Trends in Illinois
chapter), indicates which geographic areas of the state
experienced the highest concentrations of air pollutants.
Chicago generally had higher median regional values
of annual mean NO, (nitrate ion), and annual mean
and 24-hour Cr. It also experienced higher median
annual mean Ni and the highest individual 24-hour Ni
concentrations. On the other hand, the Chicago area
generally experienced lower concentrations of 3-hour
and 24-hour SO, than the rest of the state.

The MetroEast area generally experienced higher con-
centrations than the rest of the state for annual mean
Pb, annual mean TSP (total suspended particulate mat-
ter), and both 24-hour and annual mean As, Cd, Fe, and
Mn. The Chicago and MetroEast areas experienced
higher concentrations than the rest of the state for 1-hour
maximum O, and annual mean SO.,,.

The analyses of spatial distribution of pollutant con-
centrations in the Chicago area showed that only one
location stands out for its high concentrations of multi-
ple pollutants. This is the industrial area of southeast
Chicago around Lake Calumet. This area has persistent,
relatively high annual mean or 24-hour concentrations,
or both, of SO), Fe, Mn, and Pb, and possibly Cd, Cr,
and Mn. The evidence for the latter three is somewhat
weaker than for the others, however. Other locations in
the Chicago area appeared to have persistent high
concentrations of only one or two pollutants.

Atmospheric Deposition

Atmospheric deposition is an ensemble of environmen-
tal processes by which airborne pollutants from various
sources are delivered to receptor systems at the earth’s
surface. Among the six natural environmental receptors
treated by the Critical Trends project, atmospheric
deposition is considered for two, forest ecosystems and
lakes and impoundments, because research has shown
possible damage to these receptors from certain kinds
and amounts of atmospheric deposition. The character-

istics of atmospheric deposition in Illinois, and how it
varies across the state and throughout the year are
described in this chapter. Where there are data suffi-
cient for an analysis, changes over several years are
calculated and trends are inferred, if these changes are
significant. Also shown are maps of deposition
loadings, which together with the concentration data
provide information necessary for assessments of the
exposure of Illinois’ natural environment to atmo-
spheric deposition. While an explicit description of the
source-receptor relationships for major pollutants, such
as sulfur dioxide (SO,) and nitrogen oxides (NO,), is
not considered, the sources of these pollutants in
Illinois and surrounding states are compared to their
occurrence in atmospheric deposition. Finally,
additional work is discussed that is needed to improve
our assessment of atmospheric deposition in Illinois
and over Lake Michigan.

Atmospheric deposition includes gases and aerosols
that are solid, liquid, or mixed phase. It includes both
primary pollutants, which retain their chemical identity
between source and receptor, and secondary pollutants,
which undergo transformation during transport in the
atmosphere. Deposition of pollutants from the atmo-
sphere is a continuous process, though there are large
temporal variations in the deposition rate or flux. These
variations relate to the kind of deposition that is occur-
ring and to surface and atmospheric conditions. There
are two kinds of atmospheric deposition, wet and dry.
Wet deposition is defined as the delivery of pollutants
to the surface by precipitation. Dry deposition is

the delivery of gases and aerosols to the surface by
mass transfer processes other than precipitation.

In principle, dry deposition occurs continuously,
while wet deposition occurs episodically, e.g., when
it rains.

Wet deposition is measured by chemically analyzing
precipitation. For this project, wet deposition data
from the national network, the National Atmospheric
Deposition Program/National Trends Network (NADP/
NTN) were used. The NADP/NTN reports the concen-
trations of dissolved calcium (Ca*), magnesium
(Mg”*), sodium (Na*), potassium (K*), ammonium
(NH,*), sulfate (SO,), nitrate (NO,>), chloride (CI),
orthophosphate (PO,*), and free hydrogen ion (H’*),
measured in pH units. No trace metals or organics are
reported by the NADP/NTN. Samples are filtered to
remove insoluble materials, so NADP/NTN provides
data for the soluble major inorganic ions found in
precipitation, those chemicals that result in acidic
deposition or “acid rain,” which occurs over all

of Illinois.

AIR RESOURCES: VOLUME SUMMARY

Dry deposition includes the mass transfer of pollutants
to the surface by a variety of physicochemical pro-
cesses: turbulent diffusion, diffusion followed by
surface sorption of gases, gravitational settling of large
particles, impaction, and interception of solid and
liquid particles. Dry deposition fluxes are strongly
affected by atmospheric factors, which influence the
rate at which pollutants are delivered to a receptor
surface; and by surface factors, which influence the
efficiency with which pollutants “stick” to a receptor
surface. Among the atmospheric factors is wind speed
and turbulence, air temperature, solar radiation, and
relative humidity. Among the surface factors are
roughness, wetness, surface-to-air temperature differ-
ence, and type of surface, which includes whether the
surface is animate (plant) or inanimate.

The relative importance of these factors in determining
the dry deposition rate depends also on the physical
and chemical nature of the pollutant. For example,
factors that affect the mass transfer of carbonaceous
soot, an unreactive, insoluble particle, are much differ-
ent than the ones affecting nitric acid, a highly reactive,
soluble gas. The dry deposition of gases and submi-
cron aerosols involves highly complex processes, and
direct measurements are intractable on a spatial domain
the size and complexity of Illinois. For this reason, an
indirect method is applied to infer, rather than measure,
dry deposition fluxes.

This inferential method employs a conceptual model
that estimates an atmosphere-to-surface coupling pa-
rameter known as the “deposition velocity” V,. The
dry deposition flux is the product of V,, and the
measured air concentration of a particular pollutant.
Model inputs, including the atmospheric and surface
factors discussed earlier, are measured at a network of
sites sponsored by the National Oceanic and Atmo-
spheric Administration’s Atmospheric Turbulence and
Diffusion Laboratory (NOAA/ATDD). Land-use and
vegetation type and status are also reported at these
sites, along with the airborne concentrations of CI,
SO,;, particulate NO,, nitric acid vapor (HNO,), and
SO,. The NOAA/ATDD sampling system is especially
designed to exclude large particles, since the inferential
method of calculating dry deposition applies specifi-
cally to gases and submicron aerosols.

The dry deposition of large particles, which have an
aerodynamic diameter greater than | micrometer

(um), is typically estimated from an analysis of the
mass of a pollutant accumulated on a surrogate surface.
To estimate the dry deposition of large particles (i.e.,
sedimentation or dryfall) for CTAP, data from the

AIR RESOURCES: VOLUME SUMMARY

NADP/NTN were used. NADP/NTN measures dryfall
in samples taken from the same collector used for
precipitation. This device, a wet/dry collector, has two
identical containers; it discriminates between wet and
dry conditions, exposing the wet deposition container
during precipitation and the dryfall container at all
other times. Dryfall samples are sent for analysis of
the same analytes measured in precipitation.

Wet deposition in Illinois has been monitored for ten or
more years at eight NADP/NTN sites. NADP/NTN
reports the concentrations of ten separate chemical
pollutants in precipitation. Just five of these are
needed to account for about 90 percent of the chemical
composition that causes Illinois precipitation to be
acidic. They are, in order of importance: sulfate (SO,”)
> hydrogen ion (H*) > nitrate (NO,) > ammonium
(NH,"*) > calcium (Ca**). Illinois precipitation is most
simply described as a dilute solution of mineral sulfuric
and nitric acids, partly neutralized by ammonium and
calcium. Based on statistical tests of time series data
alone, there is no unambiguous trend that applies to all
of the important pollutants causing acid rain in Illinois.
Based on a “weight of the evidence” analysis, however,
several points can be made about Illinois precipitation
chemistry changes during the 1980s:

1. Sulfate has decreased 2 to 4 percent per year in the
southern third of the state, with smaller decreases
elsewhere.

2. Calcium decreased by 3 to 7 percent per year, except at
Argonne (suburban Chicago), where it remained steady.

3. Nitrogen species, ammonium and nitrate, remained
unchanged.

4. pH increased, but the increase is too small and too
variable to be quantified.

5. Sulfur dioxide and NO, emissions have decreased
slightly.

Dry deposition in Illinois tends to be somewhat higher
in the Chicago area, both due to higher airborne
concentrations of most pollutants and higher deposition
velocities. For sulfate, nitrate, sulfur dioxide, ozone,
arsenic, and manganese, the differences are on the
order of 10 to 30 percent. For cadmium, chromium,
iron, nickel, lead, and total suspended particles, dry
deposition in the Chicago area exceeds the rest of the
state by 200 to 400 percent, which is primarily caused
by the differences in air quality (see Air Quality Trends
in Illinois chapter). Temporal trends in dry deposition
generally follow air quality trends, although additional
variability is introduced into the time series data by

interannual variation in deposition velocities. More
important for ecological impacts is the seasonal nature
of dry deposition loadings, with higher deposition velo-
cities for many pollutants occuring during the warm
season, when biological impacts may also be the greatest.

The total deposition of sulfur in the Chicago area is
about 15 percent higher than in the rest of the state. For
sulfur (sulfate plus sulfur dioxide), the ratio of wet to
dry deposition is about 1 part wet to 1.5 parts dry. The
deposition of nitrogen in the Chicago area is about 30
percent higher than in the rest of the state. For nitrogen
(nitrate plus ammonium plus nitric acid vapor), the ratio of
wet to dry deposition is about 1 part wet to 3 parts dry.

The spatial and temporal variation information is most
useful in describing the coupling of the atmosphere to
receptors that are also distributed in space and have
temporally-varying sensitivity to the depositing
pollutants. Acid deposition to forests for example, is
most likely to have an effect during the growing
season, and much less likely to be harmful in the
dormant season. Toxic deposition to forests, however,
may act through a cumulative effect, where the
temporal variation is less important to understanding
the impact on the receptor system.

Acid deposition to Lake Michigan presents a special
difficulty in this analysis, since neither wet nor dry
deposition is measured over the lake, although the
refinement of estimates based on shoreline measure-
ments is an ongoing research topic.

Agricultural systems have been shown to be relatively
insensitive to current levels of acid deposition, but the
impact of toxic deposition and dry deposition of many
pollutants is unknown. Ozone has been demonstrated
to have negative impacts on yield and quality of cash
crops in several areas of the United States. The role of
wet and dry toxics deposition as a contributor to
nonpoint source pollution in surface and ground-water
supplies for human consumption is also unknown at
this time. The impact of atmospheric deposition (acid
rain, toxic pollutants, and biological nutrients) to lakes
and streams in Illinois (i.e., other than Lake Michigan
waters) has not been documented, although consider-
ation of the magnitude of deposition for many chemi-
cals would indicate that significant impacts are
possible. Finally, the impacts of SO,*, SO,, acids, and
NO, deposition, both in precipitation and dry deposi-
tion, has been demonstrated in recent federally
sponsored research in many areas of the United States.
Materials impacts in Illinois are as yet unquantified,
but they are potentially large.

INTRODUCTION

INTRODUCTION

CRITICAL TRENDS ASSESSMENT
PROGRAM (CTAP) OVERVIEW

Large amounts of money have been and are being spent
on pollution control efforts to protect human health and
the environment. Although these efforts have had con-
siderable benefits, many environmental goals still have
not been met nationally and in Illinois. Thus, we may
expect that the expenditures for environmental im-
provement will continue and perhaps increase.

It is incumbent upon state officials and environmental
policy makers to use public funds for environmental
improvement wisely. To use funds wisely in this con-
text means to effect the greatest possible environmental
improvement from the available resources. This implies
that priorities must be set for the use of public funds for
environmental improvement. What criteria should
govern the setting of priorities? The U.S. EPA’s Sci-
ence Advisory Board (1990) stated that “policy affecting
the environment must become more integrated and more
focused on opportunities for environmental improvement
than it has in the past. Integration in this case means that
government agencies should assess the range of environ-
mental problems of concern and then target protective
efforts at the problems that seem to be the most serious.”

The first step in addressing these issues is to assess the
state of the environment. By executive order, Illinois’
Governor Edgar directed the Department of Energy and
Natural Resources (ENR) and the Governor’s Science
Advisory Committee (GSAC) to “conduct an environmen-
tal trends analysis and to report to the public on the state of
the Illinois environment.” The goals of this effort are:

1) to document techniques for searching, organizing,
analyzing, interpreting, and evaluating data for geogra-
phic and temporal presentation,

2) to document assessment methodologies for analyz-
ing environmental and human health information,

3) to identify critical data gaps, for use in prioritizing
future research,

4) to establish an integrated data structure that will pro-
vide the means to judge trade-offs associated with anti-
cipated changes in the state’s natural environment or
infrastructure, and

5) to produce a final report that documents past conditions
and assesses the present status of the Illinois environment.

RELATIONSHIP OF AIR RESOURCES
TO CTAP PROJECT GOALS

The atmospheric environment is an important compo-
nent of the overall environment of the state. Weather
and climate affect a broad range of ecosystem and
human health issues. For example, temperature and
moisture conditions and their seasonal changes deter-
mine to a great extent the geographical distribution of
plant and animal species. Droughts and floods have
great impacts on agriculture and transportation. Simi-
larly, extremes of temperature and precipitation and
severe weather are major hazards to human health.

Exposure to airborne contaminants also has impacts on
the environment and human health, as evidenced by the
use of human and environmental health as criteria in
setting air pollution standards. Cases of injury to plants
and human health caused by extreme concentrations of
airborne pollutants are well documented in the literature.

Atmospheric deposition of airborne nutrients and
contaminants is more important to ecosystem health
than human health at current levels. Benefits may be
realized from the atmospheric deposition of nutrients to
agricultural lands, although excess nutrients in water
bodies can lead to problems. Deposition of acidifying
substances can lead to acidification of water bodies and
ecosystem damage in certain situations.

REPORT COMPONENTS AND STRUCTURE

An assessment of the status of the atmospheric environ-
ment must include statements related to both its
physical features (which over the short term we call
weather, and over the long term, climate), its chemical
composition, primarily as related to the concentrations
of pollutants in air and precipitation, and the deposition
of airborne materials back to the earth’s surface. An
assessment of the state of Illinois’ atmospheric environ-
ment should include representations of both the spatial
distributions and the time history of important features
of weather and climate, as well as pollutant concentra-
tions and deposition.

This report is composed of three major sections: Cli-
mate Trends in Illinois, Air Quality Trends in Illinois,
and Atmospheric Deposition Trends in Illinois. Each
presents information on the current status and temporal
and spatial trends of important features of the Illinois
environment, based on currently available data.

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CLIMATE TRENDS IN ILLINOIS

CLIMATE TRENDS IN ILLINOIS

Kenneth E. Kunkel, Wayne M. Wendland,
and Stephen J. Vermette
Illinois State Water Survey

INTRODUCTION

This report presents an analysis of climate trends since
the late nineteenth century in Illinois. This work was
pursued under the Critical Trends Assessment Project
(CTAP) of the Illinois Department of Energy and
Natural Resources. Weather and climate events have
significant impacts on many socioeconomic and
environmental aspects of Illinois. Therefore, an anal-
ysis of climate trends provides the necessary back-
ground for understanding trends in other aspects of the
Illinois environment.

Significant climate variability can occur at virtually all
time scales. Therefore, a variety of climate data and
information from Illinois sites are assembled here in
order to investigate the temporal variability of various
climate parameters. In addition to studying statewide
changes in annual temperature, precipitation, and
other climate parameters since the turn of the century,
changes by month and by season and in the frequency
of extreme events will be investigated over the
period of record.

CAUSES OF CLIMATE

Climate is a description of the overall character of
daily weather conditions that is manifested from season
to season and from year to year. It is the sum of all
Statistical weather information that describes the
average and variability of weather at a given place.
Climate does not include information on the weather of
a particular day.

We investigated changes in the frequency and magni-
tude of several climatic parameters. Some of these
yield a clear indication of a trend. For example, all
Stations indicate a common temperature trend over the
past 100 or more years, with associated changes in
related climatic parameters such as degree-day totals,
cloud cover, frequency of frozen precipitation, etc. It is
not yet possible to definitively identify the specific

causes of these trends. However, it is useful to discuss
in a general way some of the possible causes of climate
variability to provide a perspective on the analyses
presented in this report.

Ultimately, the climate of the earth is driven by energy
received from the sun. Although some of that energy is
reflected back to space, most of it remains in the atmo-
sphere to provide the fuel for global circulation
patterns. The single most important variable affecting
climate at a particular point is latitude because this
affects the total amount and seasonal distribution of
solar radiation reaching the surface at that point. The
mid-latitude location of Illinois results in a substantial
variation in the amount of solar radiation received
between winter and summer. Thus, the seasonal vari-
ation in climate is relatively large.

It is known that variations in the orbit of the earth
around the sun result in cyclic variations in solar
radiation on time scales of greater than 10,000 years.
Many scientists believe this to be a major factor in the
periodic occurrences and retreat of ice ages. However,
on shorter time scales (100 years or less), it is not
known with certainty whether significant variations in
solar radiation occur. Measurements of solar radiation
are not accurate enough nor of long enough duration to
identify significant fluctuations.

Another major factor affecting climate is the distribu-
tion of continents and oceans. It is known that tempera-
ture differences between the ocean surface and the land
surface are a major factor in establishing certain circu-
lation patterns in the atmosphere. For instance, during
the summertime, the Atlantic Ocean is cooler than the
North American continent. This results in a pressure
difference (high pressure over the ocean, low pressure
over the continent) that forces a southerly flow of air
over eastern North America, transporting significant
moisture from the Gulf of Mexico into Illinois. This
gives us our humid summertime climate with generally
abundant rainfall.

It is known that fluctuations in ocean surface tempera-
tures can occur and that these can cause fluctuations in
climate that are of significance. The El Nifio phenom-
enon is one example, where a warm pool of water
develops aperiodically over the eastern equatorial
Pacific. This warm pool of water and the associated
disruptions in equatorial wind patterns are known to
have effects on climate in many other parts of the
globe. It is possible, even probable, that fluctuations in
ocean surface temperatures in the Pacific and the
Atlantic have effects on climates thousands of miles

CLIMATE TRENDS IN ILLINOIS

away and may be responsible for some significant
fluctuations that have occurred in the past. However, it
is very difficult to identify such ocean temperature
changes as the clear cause of any particular climate
fluctuation experienced in Illinois.

The radiation balance and energy transfer within the
atmosphere are influenced by the composition of the
atmosphere. Some constituents of the atmosphere,
notably carbon dioxide, methane, chlorofluorocarbons,
and water vapor, limit the loss of infrared radiation by
the earth to space. These have a net warming effect on
surface temperatures. It is well known that the concen-
tration of many of these gases are increasing because of
human activities, such as the burning of fossil fuels. If
all other causes of climate were to remain constant, we
would expect that the temperature at the surface of the
earth would increase as the concentration of these gases
increased. However, it is still too early to ascribe ob-
served changes in Illinois climate solely to this cause.

Other factors may also play a role in modulating
Illinois’ climate. For instance, volcanic eruptions can
inject large amounts of dust into the earth’s strato-
sphere. This can decrease the amount of solar radiation
reaching the earth’s surface for a year or more in some
cases. It is likely that past volcanic eruptions have
caused significant changes in the earth’s climate. For
instance, the recent interruption of Mt. Pinatubo is
believed to have resulted in a temporary decrease in
global surface temperature of 1°F. In large urban areas
such as Chicago, the change from a natural or agricul-
tural land surface to large percentages of paved areas
and buildings can cause a local change in climate.
Urban areas tend to be warmer than nearby rural areas.

Since there are multiple factors that can affect Illinois’
climate, it is difficult, if not impossible, to cite a
specific cause for any particular feature or trend in
Illinois’ climate.

DATA

Data from two major categories of climate stations are
analyzed in this report. The first category includes the
National Weather Service (NWS) First Order Sites
(FOS). They provide data from at least hourly measure-
ments taken at major airports. The data are derived
from continuous observation of cloud cover, visibility,
pressure, temperature, dew-point temperature, winds,
hydrometeors, and other obstructions to visibility. This
very detailed set of observations is archived in digital
form for only five sites in Illinois.

The second major category of observing sites are those
of the NWS Cooperative Observer Network (CON).
This network is composed primarily of individual and
institutional volunteers who obtain measurements on a
daily basis for the NWS. The NWS provides the mea-
surement equipment, and for the most part, the
observers are not paid. The observations include daily
maximum and minimum temperatures, daily precipita-
tion, daily snowfall and snow depth, and in a few cases
soil temperature and pan evaporation. Although these
observations are not as detailed as those for the FOS
network (they provide daily rather than hourly observa-
tions), the network is much denser and therefore offers
a much-used, quality dataset. For this report, 41 CON
stations have been selected, each with long-term data
extending back to the turn of the century.

For purposes of climate applications, the NWS has
divided the state into nine regions, known as “climate
divisions.” They are shown in figure 1. The National
Climatic Data Center has developed a special dataset
that consists of temperature and precipitation values
averaged for all stations, as available in each of these
climate divisions. These values are available monthly
from 1895 through the present. Because this is a rela-
tively compact dataset and incorporates information for
all available stations, these data were used to assess
overall trends in precipitation and temperature.

Data from individual stations were used to assess trends
in more detailed aspects of temperature and precip-
itation that are not available by climate divisions.
Although climate division values are available for
monthly average temperature and monthly total precip-
itation, no such data are available on a daily time scale.
Therefore, trends in the following climatic elements
were assessed from the data from individual stations:

Temperature

¢ = Monthly maximum and minimum temperatures.

© Number of days with daily mean temperature
above 50°F. This threshold is examined because
the rate of growth and development of corn and
soybeans becomes significant when the daily mean
temperature is above 50°F.

© Extreme daily maximum temperature. This is the
highest single daily value occurring in a month.

¢ Extreme minimum daily temperature. This is the
lowest single daily value occurring in a month.

¢ Number of days with daily minimum temperature
above 70°F. This is a somewhat arbitrary threshold
that is meant to represent summer nights of abnor-
mal warmth.

CLIMATE TRENDS IN ILLINOIS

Northwest

Scale of Miles
0 10 20 30 40 50
——— — —— os —— |

Figure 1. Illinois climate divisions, designated by the National Weather Service.

CLIMATE TRENDS IN ILLINOIS

e Number of days with daily minimum temperature
below (°F. This threshold is examined as a repre-
sentation of particularly “cold” nights.

¢ Number of days with daily minimum temperature
below 32°F. This threshold is examined because
the freezing point affects many processes.

¢ Number of days with daily maximum temperature
above 86°F. This threshold is examined because
the rate of corn and soybean growth and develop-
ment typically reach a plateau near this tempera-
ture threshold. That is, above this threshold, the
rate of crop growth and development no longer
increases with rising temperature.

¢ Number of days with daily maximum temperature
above 90°F. This is a somewhat arbitrary thresh-
old, representing particularly warm days of dis-
comfort to many people.

¢ Number of days with daily maximum temperature
above 100°F. This threshold is examined as a
representation of the frequency of days of great
discomfort to many people.

Precipitation

¢ Number of days with precipitation.

¢ Number of days with precipitation of I inch or
more. This threshold is examined as a somewhat
arbitrary representation of rather heavy rain days.

¢ =Total monthly snowfall.

¢ Number of days with snowfall.

METHODOLOGY -
STATISTICAL TECHNIQUES

Trends in Means

In order to detect potential trends in climate, several
variables were subjected to a mathematical process to
remove the seasonal cycle in the data. This process was
performed on climate division values of monthly
temperature and monthly precipitation and on station
values of monthly minimum and maximum tempera-
tures. Specifically, the data were “standardized” with
the following formula:

x, = (x, - x,/s,

where x, is the value for year i and month j; x, is the
average over all years of the values in month J; and s, is
the corresponding standard deviation for those values.
These converted values are referred to as “Standard-

10

ized” temperature and precipitation. They express the
values in terms of the number of standard deviations
above or below the mean.

The climate division time series consisted of 1,164 data
values for each element (97 years x 12 months). Over-
all trends were assessed by subjecting the time series to
a linear regression; the statistical significance of the
resulting trends was tested using a t-test for signifi-
cance of slope of the regression (Draper and Smith,
1981). Trends in individual seasons were also assessed.
The meteorological definition of seasons was used:

¢ Winter (December 1 through February 28/29)
¢ Spring (March 1 through May 31)

¢ Summer (June | through August 31)

¢ Fall (September 1 through November 30)

Seasonal values were obtained by averaging tempera-
ture over the three months and totaling precipitation
over the three months. Thus, the resulting seasonal
time series each consisted of 97 data points. For the
time series of individual stations, only 91 years of data
were available.

For variables involving the number of exceedances in
temperature and precipitation and for monthly snowfall
totals, the monthly values were summed for each year.
The resulting data (91 data points) were subjected to
the regression analysis for trend indicated above. The
extreme monthly maximum temperatures were ana-
lyzed by taking the highest value in each year and
subjecting each of the 91 values to the regression anal-
ysis for trend. The lowest extreme monthly minimum
temperature value for each year was also assessed for
trend in this manner.

In all of the above regressions, the trends were tested
for significance at the 10 percent level using a two-
sided t-test.

Variance Assessments

In addition to testing the time series for trends in the
mean, the data were also analyzed for changes in the
variability of temperature and precipitation over the
period of record. A subjective analysis of the time
series suggested that certain periods of time, a decade
or more in length, were characterized by significantly
less or more variability than the rest of the time series.
To assess this, the climate division values of tempera-
ture and precipitation were broken into six 15-year
segments, beginning in 1901. For each 15-year subset

CLIMATE TRENDS IN ILLINOIS

of data, the standard deviation of the time series was
calculated. The resulting data were plotted and sub-
jectively analyzed.

Temperature and Precipitation Index

An additional objective of this study was to identify
certain periods of time characterized by specific cli-
mate conditions—in particular, multiyear periods
characterized as:

¢ Warm and wet
e Warm and dry
© Cool and wet
© Cool and dry

To assess these conditions, two combined temperature/
precipitation indices were defined and calculated as
follows:

¢ Index 1 = standardized temperature + standardized
precipitation

° Index 2 = standardized temperature - standardized
precipitation

The association between these indices and the above
scenarios is:

¢ Warm and wet (large positive values of Index 1)
¢ Warm and dry (large positive values of Index 2)
¢ Cold and wet (large negative values of Index 2)
* Cold and dry (large negative values of Index 1)

Modifications to this rather straightforward categoriza-
tion scheme were required, as will be explained as part
of the section Anomalous Episodes from the Illinois
Climate Record. Time series of the index were plotted
and examined to identify multiyear periods with the
above climate conditions.

TRENDS IN PRECIPITATION

Trends in Average Precipitation

Mean annual statewide precipitation, averaged for all
available stations since 1840 is shown in figure 2. The
data are considered to well represent statewide condi-
tions since about 1890, by which time the annual
values were derived from at least 113 stations in the
State. The “‘statewide” totals during the 1880s were

based on 18 stations; on six or seven stations from
1852 to 1880; and on only one station (Athens, Sang-
amon County) from 1840 to 1851. Because of the
paucity of data prior to about 1890, some care must be
exercised in its interpretation. If representative, they
indicate a slight decline in annual precipitation from
the mid-1800s to about the turn of the century, with an
increase thereafter. However, as shown, the year-to-
year variation is the dominant characteristic of statewide
precipitation, as with an individual station’s record.

The statewide mean annual precipitation record based
on 150 to 200 sites for each year (figure 3) has varied
widely from 25 to 50 inches. (Figure 3 duplicates the
more recent portion of figure 2. This time frame facili-
tates comparison with other figures in this report, most
of which cover the period from 1901 to 1991.) Occa-
sionally, consecutive years exhibit similar anomalies,
but in the vast majority of cases, one year’s precipita-
tion is unrelated to that of the next. The statewide
average is about 37 inches, with about 5 inches less in
northern counties and about 5 inches more in the south.

No obvious discontinuity in statewide annual precipita-
tion has occurred since 1901. Changnon (1984), in an
analysis of data through 1980, also noted that year-to-
year variability was the dominant characteristic of the
time series. He noted a slight tendency for lower values
during the period 1910 to 1940 compared to values
both before and after that 30-year period. A close
examination of figure 3 does suggest this. In addition,
an analysis of trends of individual stations (reported
later) also suggests a general upward trend in precipita-
tion during this century. However, this trend is weak
compared to the short-term variability.

The number of days per year with precipitation
(averaged for all long-term stations) exhibits a some-
what stronger upward trend since 1901 (figure 4) than
does annual precipitation. The number of days per year
with precipitation appears to have increased by about
10 during the 90 years of record, although variability
appears to increase as well.

The annual number of days per year with precipitation
trended upward with time at nearly all the 41 long-term
stations; only Carbondale (Jackson County) and Wal-
nut (Bureau County) digressed. Thus, the trend has
been rather ubiquitous throughout the state. However,
several of these upward trends were rather weak and
not statistically significant.

The number of days per year with at least 1 inch of
precipitation (with a present recurrence frequency of

11

CLIMATE TRENDS IN ILLINOIS

0861

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Precipitation (in)

CLIMATE TRENDS IN ILLINOIS

0)
1Set 1910 1921 19881. 4 941. ».1951,.41961,.. 1971.- 1981 1991
Year

Figure 3. Statewide average annual Illinois precipitation composed of available sites, 1901-1991 (inches).

CLIMATE TRENDS IN ILLINOIS

# of Days

50
1901 1911 1921 1931 1941 1951 1961 1971 1981 1991
Year

Figure 4. Average annual number of days per year in Illinois with precipitation.

14

CLIMATE TRENDS IN ILLINOIS

about once every other month in Illinois) is shown in
figure 5. As with the above two precipitation measures,
identification of trends is difficult. Nonetheless, a slight
decline appears during the first few decades with an
increase during recent decades. The days per year with
a least 1 inch of precipitation at individual stations in
the state all exhibit upward trends, except for White
Hall (in Greene County). Changnon (1984) found a
general increase in the number of days with rains of 2
inches or more during the warm season. In addition,
Huff and Angel (1989) found that the frequency of very
heavy rainstorms had increased from the early part of
this century to the latter part.

To summarize temporal trends in precipitation over
the state, magnitude (figure 2) appears to decline
from 1840 to about the 1930s, increasing thereafter.
The frequency of precipitation events appears to
have increased slightly in the 90 years since 1901,
paralleling the trend in annual precipitation. Much
more significant is the substantial year-to-year
variation.

Trends significant at the 10 percent level, were
identified for 9 to 21 of the 41 sites for the five
precipitation parameters. Trends for individual
stations are listed in appendix table A.1, and a
summary is given in table 1. Interestingly, nearly all
these trends were positive, except for the number of
days with measurable snowfall. Sites in northern and
east-central Illinois were somewhat more likely to
have experienced significant trends than sites in
other parts of the state.

Trends in Snowfall

Annual snowfall for the state since 1901 (figure 6)
shows some rather clear characteristics:

Decline from 1901 to the 1930s.

Increase from the 1930s to the 1980s.

Decline in the last decade.

Continuity from year-to-year over episodes of
three to six years. For example, note similar
snowfall amounts from 1903-1906, 1919-1923,
1939-1945, 1952-1959, 1961-1965, and 1987-1991.

se Gay hae

Because the year-to-year variation is so large, five-year
running averages of statewide annual snowfall were
calculated (figure 7). This figure exhibits a slight
increase with time, but much more apparent is the
fluctuating nature, as with annual precipitation. Fluc-
tuations in annual snowfall exhibit secular episodes of
about 13 years, varying from 7 to 17 years since 1901.

In general, years with greater than average snowfall
also experienced a greater than average number of days
with snowfall (figure 8). A decline in the number of
days with snowfall is apparent from the turn of the cen-
tury to the early 1930s (coincident with the warming
shown in figure 11), with a possible increase thereafter.
More prominent than trends are the several years with
relatively high frequencies of days with snowfall,
notably, 1914, 1924, 1951, 1960, 1977, 1978, and
1980.

Of 16 Illinois locations with statistically significant
trends in annual snowfall (table A.1, p.72), five (four in
east-central Illinois) exhibited declines, whereas 11
indicated increases in annual snowfall from 1901 to
1991 (table 2). Such differences in trend are not likely
within such a relatively small area, emphasizing a
problem with snowfall observations, i.e., they are
extremely sensitive to the exposure of the site, as well
as the method used to determine snowfall (precipitation
in a gage, average of several depth measurements, or
density of snow cores).

Trends in Variability

Trends in year-to-year variability in precipitation were
examined by separating the precipitation data into 15-
year segments, beginning with the period 1901-1915.
For each 15-year segment, the interannual standard
deviation of precipitation was calculated for each sea-
son (figure 9).

For the summer, a notable feature is the low variability
during the period 1961-1975, as opposed to much
higher variability from 1901 to 1945 and 1976 to 1990.
Figure 10 shows summer precipitation for this century.
The low variability during 1961-1975 is the result of an
absence of severe summer droughts during the 1960s
and 1970s. More frequent droughts during the period
1976-1990 have made variability similar to that during
the earlier part of the century.

A period of relatively low variability also occurred
during the fall in the 1940s, 1950s, and 1960s. By
contrast, the period 1916-1945 was characterized by
the highest variability. For the winter season, the first
half of the century exhibited very low variability com-
pared to the second half. The greatest variability was
exhibited from 1946 to 1960, virtually opposite to the
trends for summer. Finally, during the spring, high
variability was experienced between 1916 and 1945,
with low values before and after, essentially the same
trends as those of fall.

15

CLIMATE TRENDS IN ILLINOIS

# of Days

16

¢)
1904 6 1919 91921.) 1931 1941 21951, 1961 J971 219611321

Year

Figure 5. Average number of days per year in Illinois with at least I inch of precipitation.

Table 1. Summary of Precipitation Parameter Trends

Number of stations
with statistically significant

Precipitation parameter upward trends
Days with precipitation 19
Days with precipitation >1 inch 8
Total precipitation 11
Total snowfall 1]
Days with snowfall 10

Number of stations
with statistically significant
downward trends

Snowfall (in)

Snowfall (in)

0
1901

1901

CLIMATE TRENDS IN ILLINOIS

1911 1921 1931 1941 1951 1961 1971 1981 1991
Year

Figure 6. Average annual snowfall in Illinois (inches).

evenrrenseneenedieeserreesesenns dysescunees assensfieoes.

pes feceacseecceseet

|
1961 1971 1981

1941 1951 1991

Year

1911 1921 1931

Figure 7. Running five-year averages of Illinois annual snowfall (inches).

17

CLIMATE TRENDS IN ILLINOIS

30

# of Days

=O MO Ff O

SOT 1911. 1921 | SST 1941) 1951 1S6T ASTI Sage vase
Year

Figure 8. Statewide average number of days per year in Illinois with snowfall.

Table 2. Stations where Annual Snowfall Significantly Increased or Decreased (Linear Regression Analysis), 1901-1991

Increased Decreased
Aledo Palestine Flora
Anna Pana Hoopeston
Dixon Paris Olney
Effingham Urbana Pontiac
Griggsville Walnut White Hall

Marengo

Note: Significant at the 10 percent level.

18

Figure 9. Standard deviation of 15-year segments of Illinois statewide precipitation by season,

Precipitation (in) - Dev. from Average

CLIMATE TRENDS IN ILLINOIS

pS

oR Winter &~ Spring

Precipitation Standard Deviation (in.)

0 1
1901-1915 1931-1945 1961-1975
1916-1930 1946-1960 1976-1990
Year

1901-1915 to 1976-1990.

107

f-freeeseeescencersesen

1911 1921 1931 1941 1951 1961 1971 1981 1991
Year

1901

Figure 10. Statewide summer precipitation (inches) for Illinois, 1901-1991,
expressed as the deviation from the period average.

19

CLIMATE TRENDS IN ILLINOIS

TRENDS IN TEMPERATURE
Trends in Average Temperature

Changnon (1984) presented an analysis of trends in
statewide average annual temperatures for Illinois for
the period 1940 to 1980. His analysis is extended here
to 1991 and presented in figure 11. The number of
observing sites prior to 1890 is limited, as they were for
statewide precipitation. Temperatures in Illinois follow
a similar trend to those of the nation, i.e., rather stable
temperatures during the first few decades (with a state-
wide mean of about 50°F). A warming trend follows
until about 1930 (to about 53°F), after which cooling
reduces the mean to about 52°F by 1980, with warming
suggested thereafter. Cool temperatures during the first
few decades reflect an expression of what has been
called the “Little Ice Age” (Lamb, 1966). It is impor-
tant to recognize that temperatures cooled substantially
from the 1930s to 1980 or after (a similar trend for the
Northern Hemisphere), five decades during which
carbon dioxide was increasing!

Mean annual Illinois maximum and minimum tempera-
tures since 1901 (figures 12 and 13, respectively) both
exhibit similar trends, i.e., warming of 2 to 3°F from
1901 to the 1930s, followed by an equivalent cooling
through about 1980, and the suggestion of warming
again during the last few years. These trends are not
surprising, since they are found in temperature records
from the entire United States, the Northern Hemi-
sphere, and the world, although the magnitudes of
change decrease with ever-increasing areas of interest
(e.g., Wigley and Barnett, 1990).

The average maxima and minima exhibit extremes dur-
ing the same years, i.e., those much warmer or colder
than the long-term average, e.g., 1917, 1921, 1924,
1931, 1978, 1979, etc., throughout the period of record.
There is no evidence of periodicities.

The interseasonal fluctuations of average statewide sum-
mer maxima (figure 14) and those of average statewide
winter maxima (figure 15), are quite different from
each other. This is not unexpected, since temperature
anomalies seldom continue longer than just a few months,
and, meteorologically, one would not expect similar
temperature characteristics to prevail from one season
to another. Similar comments apply to statewide winter
and summer minima (not shown), i.e., the interseasonal
fluctuations need not parallel each other, and they do not.

The mean number of days (averaged over all stations)
with temperatures above 100°F (figure 16) are indeed

20

few, particularly during the most recent decades,
which are exhibiting cooling. Several years, namely
1901, 1913, 1930, 1934, and particularly 1936, ex-
hibited high frequencies of such days, all during the
first four decades of the record. Years with a high
frequency of days with temperature greater than
100°F tended to occur in episodes composed of a
few sequential years, e.g., 1913-1914, 1930-1936,
1939-1941, and 1952-1954, all of which represented
major droughts!

The statewide average number of days per year with
maximum temperatures above 86°F (figure 17) exhibits
trends similar to those shown in figure 16, although the
absolute frequencies are greater. The 1930s experi-
enced more days with higher temperatures than any
other decade on record.

Figures 18 and 19, showing the number of days per
year with maximum temperatures below 32° and below
O°F, respectively, exhibit inverse trends to those found
in figures 16 and 17. More frequent cooler maxima
have occurred since about 1960 than before. This trend
suggests that the cooler 1960s, 1970s, and early 1980s
comprised more frequent cool days, as opposed to
cooler extremes only. Note the relatively high fre-
quency of cold maxima in 1912, 1924, and 1936 in
both figures 18 and 19. The relatively high frequency
of maxima below 32°F in the late 1960s, 1970s, and
early 1980s is also seen in the frequency of maxima
below 0°F (figure 19).

Figure 20 presents the statewide average number of
days per year when the minimum temperature ex-
ceeded 70°F. Not surprisingly, the trend follows that of
average temperature and maximum and minimum
temperatures, with increasing frequency from 1901
until the 1930s, a decline thereafter, with a possible
increase again during the last few years. It should be
noted that the high frequencies of 1913, 1931, 1934,
and 1936 have not been equaled in Illinois since.

Figure 21 shows the statewide average number of days
per year when the minimum temperature was below
32°F. A declining trend may be suggested from 1901 to
perhaps the mid-1940s, with either stable frequencies
or perhaps a slight increase thereafter.

Figure 22 presents the statewide average number of
days per year when the minimum temperature was
below O°F. Such frequencies were decidedly minimal
during the 1940s and 1950s, which were warm dec-
ades. The highest frequency of such days occurred in
1936, 1963, 1977-1979, and 1985.

CLIMATE TRENDS IN ILLINOIS

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21

CLIMATE TRENDS IN ILLINOIS

Temperature ( F)
ep)
ro)

52

|
58 |

50
1901 1914-—1921-— 192-1044 1951. «1961: oe in eens eee
Year

Figure 12. Statewide average annual maximum temperatures for Illinois (°F), 1901-1991.

22

Temperature ( F)

CLIMATE TRENDS IN ILLINOIS

30
1901 1911 1921 1931 1941 1951 1961 1971 1981 1991
Year

Figure 13. Statewide average annual minimum temperatures for Illinois (°F), 1901-1991.

23

CLIMATE TRENDS IN ILLINOIS

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Temperature ( F)
S

15
7074

65

60

1901 1911 1921 1981 1941 1951 1961 i971 1981
Year

Figure 14. Statewide average maximum summer temperatures for Illinois (°F), 1901-1991.

24

1991

Temperature ( F)

CLIMATE TRENDS IN ILLINOIS

11 1921 1931 1941 1951 1961 1971 1981 1991
Year

Figure 15. Statewide average maximum winter temperatures for Illinois (°F), 1901-1991.

25

CLIMATE TRENDS IN ILLINOIS

# of Days

26

1901 1911 1921 19381 1941 1951 19671 1971 1981. 1991

Year

Figure 16. Statewide average number of days per year in Illinois
with maximum temperatures greater than 100°F, 1901-1991.

# of Days

CLIMATE TRENDS IN ILLINOIS

0
1901 1911 1921 19381 1941 1951 1961 1971 1981 1991
Year

Figure 17. Statewide average number of days per year in Illinois
with maximum temperature greater than 86°F, 1901-1991.

27

CLIMATE TRENDS IN ILLINOIS

# of Days

0
1901 1911 1921 1931 1941 1951 1961
Year

Figure 18. Statewide average number of days per year in Illinois
with maximum temperature less than 32°F, 1901-1991.

28

1971. 1981 1991

# of Days

CLIMATE TRENDS IN ILLINOIS

1941 1951 1961 1971 1981 1991

Year

1911 1921 1931

Figure 19. Statewide average number of days per year in Illinois
with maximum temperature less than 0’F, 1901-1991].

29

CLIMATE TRENDS IN ILLINOIS

# of Days

1931 1941 1951 1961 1971 1981 1991
Year

)
1901 1911 1921

Figure 20. Statewide average number of days per year in Illinois
with minimum temperature greater than 70°F, 1901-1991].

30

# of Days

70

60

50
1901

1911

CLIMATE TRENDS IN ILLINOIS

1921 19381 1941 1951 1961 1971
Year

Figure 21. Statewide average number of days per year
with minimum temperature less than 32°F, 1901-1991.

1981

1991

31

CLIMATE TRENDS IN ILLINOIS

307,

25 pod ah Fee ee es ee ee ae BE ers! ce i

Hay Arend | SE ua A AES PEE

# of Days

sodult-—-t- |.

1931 1941 1951 1961 1971 1981 1991
Year

0 i
1901 1911 1921

Figure 22. Statewide average number of days per year with minimum temperature less than 0°F, 1901-1991].

32

CLIMATE TRENDS IN ILLINOIS

Change in Parameters Related to Temperature

Mean Statewide Annual Heating Degree-Days. Heat-
ing degree-days (HDDs) are derived from the accumu-

lated positive differences of 65°F minus the mean daily
temperature. This rather simple calculation results in a

value that closely parallels heating fuel needs.

Average annual HDDs, shown in figure 23, demon-
strate a crude inverse correlation to annual temperature.
As one would expect, the fewest HDDs are found in the
1930s and 1940s. HDD totals during the warmest
decades are about 25 percent fewer than during the
colder years. Five-year running averages of this
parameter (presented in deviations from the period
average in figure 24) rather clearly demonstrate fewer
HDDs in Illinois from the 1920s through the 1950s, the
warmest decades of the century-long Illinois tempera-
ture record.

Mean Statewide Annual Cooling Degree-Days. Cool-
ing degree-days (CDDs) represent a measure of elec-
tricity required for cooling buildings during the warm
season. They represent accumulated positive differ-
ences of daily mean temperature minus 65°F.

Annual CDDs (figure 25) also show the 1930s and
adjacent decades as warmer than those earlier or later,
as did the HDD record. The total CDD five-year
running averages (figure 26) show more CDDs during
the 1930s and early 1940s, but they also show peaks
during the first few years of record, in the early teens
and twenties, in the 1930s and early 1940s particularly,
and the mid-1950s.

The five-year running averages of heating and cooling
degree days (figures 24 and 26, respectively) demon-
strate a rather clear inverse relationship to each other,
except for the late 1970s and 1980s.

Mean Statewide Corn Growing Degree-Days. Corn
growing degree-days (GDDs) are derived from the
accumulated positive differences of daily mean
temperature minus 50°F (when maxima exceed 86°F,
the maximum is set to 86°F; when minima are less than
50°F, the minimum is set to 50°F). Trends of tempera-
ture, mentioned earlier, are also apparent in this
measure (figure 27) of growth. Annual statewide corn
GDDs since 1878 have been lower than those of 1992
in only six other years, 1882, 1883, 1915, 1917, 1924,
and 1967, and then only marginally! The relatively
few GDDs in 1992 virtually devastated the northern-
most 100 miles of the Corn Belt. As a result, corn in
that area was of lower quality and did not fully mature

everywhere within that region. Interestingly, 1992
witnessed a record total corn crop in the United States
by a substantial margin, largely because of reduced
heat and moisture stress in the heart of the Corn Belt.

Trends in the Growing Season. Trends in the length
of the growing season (figure 28), the date of the last
spring freeze (figure 29), and the first fall freeze (figure
30) correspond only weakly to the trends indicated in
average annual temperature. This is due to the fact that
averages reflect the overall anomaly of the period
included in the average.

Individual events, e.g., the date of first or last frost, are
often poorly correlated with monthly or seasonal
temperature averages, which are composed of many
days surrounding the frost event, because the frost
event is short-lived (only hours in duration). Moreover,
the event is also dependent upon other specific local
conditions occurring during the time of the frost, e.g.,
low cloud cover, wind speeds, humidity, etc.

The dominant characteristic of the growing season time
series is the year-to-year variability. Changnon (1984)
had noted a slight increase in the length of the growing
season caused primarily by a trend towards earlier
spring freezes. The results of that study, based on data
from nine stations, are also seen in this analysis of 41
stations.

Table 3 (p.40) summarizes a statistical analysis of the
trends from these 41 stations for two freeze thresholds
(32°F and 28°F). The most obvious feature of these
results is that all stations with significant trends in the
date of the last spring freeze exhibit a tendency for ear-
lier spring freezes. There is also a tendency towards
later first fall freezes. The combination of these two
trends leads to a tendency toward longer growing seasons.

Linear Trends in Temperature Records
at Illinois Sites

Linear trends in the temperature parameters are listed
for all 41 long-term stations in appendix A. Average
temperature parameters, minimum temperature
parameters, and maximum temperature parameters are
listed in tables A.2-A.4, respectively, beginning on
p.74). These trends are summarized in table 4. For
most temperature parameters, statistically significant
trends (at the 10 percent level) are found for less than
half of the stations. However, in many cases, the
direction of the trend is similar for many stations. For
instance, 27 stations exhibited significant downward
trends for the highest temperature of the year. This

33

CLIMATE TRENDS IN ILLINOIS

Heating Degree Days

1901 1911 1921 1931 1941 1951 1961 1971 1981
Year

Figure 23. Statewide average annual heating degree-days for Illinois, 1901-1991.

34

1991

Degree Days (Deviation from Average)

-1400+4 | :
1901 1911. 1921 19381 1941 1951 1961

CLIMATE TRENDS IN ILLINOIS

{ i
1971 1981 1991
Year

Figure 24. Five-year running averages of statewide heating degree-days,
presented as deviations from period mean, 1901-1991.

35

CLIMATE TRENDS IN ILLINOIS

sfeq soiseq Surjoo)

1978

1958

Year

Figure 25. Statewide average annual cooling degree-days for Illinois, 1878-1992.

36

Degree Days - Deviation from Average

-700-4
1901

CLIMATE TRENDS IN ILLINOIS

1941 1951 1961 1971 1981 1991

Year

1911 1921 1931

Figure 26. Five-year running averages of statewide mean cooling degree-days for Illinois,
presented as deviations from period mean, 1901-1991.

37

CLIMATE TRENDS IN ILLINOIS

38

Growing Degree Days

1878 1898 1918 1938 1958 1978
Year

Figure 27. Average Illinois annual corn growing degree-days, 1878-1992.

CLIMATE TRENDS IN ILLINOIS

is aa -A4— ae

Peach mation Meno

210 oe vit Naf in :

Length of Season (days)

200 ; Dy Dara GEL WT WAY, r ey Hah atte Eye on ‘| 7 ioe Yo
|

190 ‘bees tel esp al eel a AY eae ie aa =!
H | i i

I INV AA AN Mw

| |

160 aaa ais ee ee
| |

H H } H H i

150 H i Hy H
1901 1911 1921 1931 1941 1951 1961 1971 1981 1991
Year

Figure 28. Statewide average length (days) of the growing season for Illinois
(defined for both 28°F and 32°F), 1901-1991.

150+ 7 7 T

j
i
i
i
H

Day of Year
°
oO

1
'

a ae ae ai

70 t t
4901 1911 1921 19381 1941 1951 1961 1971 1981 1991
Year

Figure 29. Statewide average day (Julian day number) of last spring frost in Illinois
(defined for both 28°F and 32°F), 1901-1991.

Day of Year

250
1901 1911 1921 1931 1941 1951 1961 1971 1981 1991
Year

Figure 30. Statewide average day (Julian day number) of first autumn frost in Illinois
(28°F and 32°F), 1901-1991.

39

CLIMATE TRENDS IN ILLINOIS

Table 3. Summary of Growing Season Trends

Number of stations with trends Number of stations with trends
toward later (longer) freezes (growing toward earlier (shorter) freezes (growing
season) (Statistically significant trends season) (statistically significant trends

Freeze thresholds given in parentheses) given in parentheses)
Last spring freeze 5 (0) 36 (24)
(threshold = 32°F)
Last spring freeze 6 (0) 35 (11)
(threshold = 28°F)
First fall freeze 24 (10) 17 (5)
(threshold = 32°F)
First fall freeze 24 (13) 17 (4)
(threshold = 28°F)
Growing Season Length 30 (17) 11 (3)
(threshold = 32°F)
Growing Season Length 26 (16) 15 (4)

(threshold = 28°F)

Table 4. Summary of Trends in Temperature Parameters of 40 Illinois Sites

Number of stations with upward Number of stations with downward
trends (statistically significant number trends (statistically significant number
Temperature parameter in parentheses) in parentheses)

Average monthly temperature 16 (5) 25 (9)
Average monthly minimum 18 (9) 23 (10)
Average monthly maximum 13 (3) 26 (12)
Highest temperature of year 2 (0) 39 (27)
Lowest temperature of year 23 (3) 18 (0)
Days with mean T > 50°F 22 (12) 19 (1)
Days with T-max > 86°F 7 (0) 34 (18)
Days with T-max > 90°F 2 (0) 39 (29)
Days with T-max > 100°F 0 (0) 41 (21)
Days with T-max < 0°F 40 (18) 1 (0)

Days with T-max < 32°F 36 (19) 5 (1)

Days with T-min > 70°F 18 (7) 23 (4)
Days with T-min < 0°F 38 (7) 3 (1)

Days with T-min < 32°F 21 (6) 20 (9)

40

reflects the high summer temperatures experienced
during the 1930s and the relative absence of extreme
high temperatures during the last three to four decades.
This same feature is also evident in half or more of the
sites in the number of days with maximum tempera-
tures above 86°F, 90°F, and 100°F. This feature is in
general agreement with the general downward trend in
average temperature from the 1930s into the 1970s
(figure 11). Although the average temperature record
and the number of “hot” days in the last decade suggest
a reversal of this trend, this change has not been of long
enough duration or magnitude to overcome the large
downward trend in the earlier period.

Somewhat surprisingly, the number of days of extreme
cold, identified as the number of days with maximum
temperatures below 0°F (figure 19), has exhibited an
upward trend at many stations. This was caused by a
significant number of extremely cold arctic air outbreaks
during the last 15 years. These outbreaks have even
occurred with some frequency during the 1980s, a per-
iod of generally very warm winters. This also reflects the
strong influence of the very cold winters in Illinois dur-
ing an extended period in the late 1970s and early 1980s.

Trends in selected parameters (seasonal average,
minimum, and maximum temperature and seasonal
precipitation) were also examined by individual
seasons. The results of this analysis are given in tables
A.5-A.8 (beginning on p.82) for the autumn, winter,
spring, and summer seasons, respectively, and summa-
rized in table 5. Very few stations show statistically
significant seasonal trends in total precipitation. How-
ever, a number of stations show downward trends in
temperature for autumn. Also, seven to eight stations
show downward trends in winter temperature. Trends
in spring are weakly positive.

Trends in Variability

An analysis of the trends in temperature variability are
shown in figure 31 by season. For the summer season,
the period 1961-1975 was characterized by the lowest
variability. Figure 32 shows summer temperatures for
this century. The low variability during 1961-1975 was
the result of the absence of severe summer heat waves.
By contrast, variability was much higher from 1976
through 1990, caused by more frequent summer heat
waves. This latter 15-year period was quite similar to
the first half of the century. For the fall season, low
variability was experienced during 1901-1915 and
1945-1990, while higher variability characterized the
years 1931-1945. For winter, 1976 through 1990
exhibited the greatest variability. This was the result of

CLIMATE TRENDS IN ILLINOIS

the very cold winters during the late 1970s, contrasted
with the mild winters of the middle and late 1980s. The
spring season was also characterized by high variability
during the period 1976-1990, as was winter.

Interestingly, trends of seasonal temperature variability
in summer and fall parallel those of precipitation
(figure 9), whereas those of winter and spring do not
appear to be related.

TRENDS IN CLOUD COVER

Changnon (1984) presented the number of days per
five years with cloudy skies (any cloud type or combi-
nation covering 70 percent or more of the sky) at
Springfield, St. Louis, and Evansville, IN (figure 33)
and Peoria, Moline, and Chicago (figure 34), from
1901 through 1980. All six sites showed a rather steady
increase in cloudy days for the period of record, from
about 90 days per year for the first few decades of the
century, to about 160 days per year after about 1940
and continuing to 1980. Additional data from 1981
through 1990 maintain the higher frequencies (figure
33), except those of Evansville, IN (figure 34). Chang-
non (1984) questioned whether the increases might be
due to greater frequencies of jet contrails, perhaps
instead of or in addition to natural clouds. The question
is as yet unanswered.

Although Petersen (1990) reported that cloud observing
techniques were changed in June 1951 by the National
Weather Service, the cloud cover record does not exhi-
bit any discontinuity during the period shown. Prior to
June 1951, fractional amounts of only two possible
cloud layers were observed and therefore recorded,
plus the height of the lowest scattered layer. Following
1951, sky cover and cloud height were reported.
Whether this procedural change may have biased the
record has not been evaluated.

Changnon et al. (1980) investigated temporal changes
in cloud cover over Illinois from the turn of the century
to 1977 (not shown). They demonstrated that the num-
ber of cloudy days increased by about 50 percent from
the early years (about 110 days per year in southern IIli-
nois, and 140 days per year in the north) to about 160
days per year in the south, and 180 such days in the north.

Petersen (1990) further calculated seasonal solar radia-
tion since 1948 at several locations about the Midwest,
based upon surface pressure, dew-point temperature,
cloud height, and fractional sky cover. The results

41

CLIMATE TRENDS IN ILLINOIS

Table 5. Summary of Seasonal Trends in Selected Temperature and Precipitation Parameters (Number of Stations)
(Statistically Significant Counts in Parentheses)

Average Average minimum Average maximum Total
temperature temperature temperature precipitation
Season Trend (°F/decade) (°F/decade) (°F/decade) (inches/decade)

Autumn Upward 14 (1) 17 (6) 10 (0) 35 (3)
Downward 27 (14) 23 (11) 30 (14) 6 (0)

Winter Upward 8 (0) 10 (0) 9 (0) 27 (0)
Downward 33 (7) 31 (8) 32 (7) 13 (0)

Spring Upward 26 (5) 26 (11) 24 (1) 35 (5)
Downward 15 (2) 15 (2) 17 (3) 6 (0)

Summer Upward 18 (5) 20 (13) 13 (0) 31 (5)
Downward 22 (9) 21 (6) 28 (10) 10 (0)

£&

Temperature Standard Deviation (deg.F)
NN)

6)
1901-1915 1931-1945 1961-1975
1916-1930 1946-1960 1976-1990
Year

Figure 31. Standard deviations of 15-year segments of Illinois statewide average temperature,
1901-1915 to 1976-1990.

42

CLIMATE TRENDS IN ILLINOIS

ecocsensnaes sevasScoseovenscsecsssefoenseonenseceesedisssasasessassere:S sosecerevees

Jeveccceececs: sesseheseeseceseeneeeeedonwesenernenees bessnnenseteenessaspacseeneens

Ww wv oO N - [e) — N oO

eBeseny WO ‘Aq - (4 ) ounyesedwis |

1971. 1981 1991

1961

1921 1931 1941

1911

Year

Figure 32. Statewide summer temperature (°F) for Illinois, 1901-1991,

expressed as a deviation from the period average.

43

CLIMATE TRENDS IN ILLINOIS

900

bo |

ro)

ro)
f
i
j
i
i
i

ea]
fo)
fe)

—#— SPRINGFIELD —*— ST.LOUIS —&— EVANSVILLE

# of Cloudy Days (per 5—year period)
o

(00
1901 1911 1921 1931 1941 1951 1961 1971 1981
First of five-year periods

Figure 33. Average number of days per five years with cloudy skies
at Springfield, IL; St. Louis, MO; and Evansville, IN, 1901-1990.

_
te)
fo)
fe)

| —®— Chicago —*— Moline ~—— Peoria |

1901 1911 1921 1931 1941 1951 1961 1971 1981
First of five-year periods

# of Cloudy Days (per 5—year period)

400

Figure 34. Average number of days per five years with cloudy skies
at Chicago, Moline, and Peoria, IL, 1901-1990.

relevant to Illinois are presented in table 6. Positive
trends in solar radiation over much the same areas were
noted at most sites and during all seasons except spring
and fall. However, the increase in cloud cover occurred
primarily during the first half of the century, a period
not covered in the Petersen study.

As summarized by Folland et al. (1990), the upward
trend in cloudiness has been observed in many other
parts of the globe. They point out that some of the in-
crease observed in the 1940s may be due to a change in
observing practice; obscuring effects of smoke, haze,
dust, and fog were included from the 1940s onward.
However, the increases thereafter are believed to be real.

TRENDS IN SEVERE WEATHER

Trends in Freezing Precipitation
since the Turn of the Century

Changnon (1984) presented the number of days per
year with observations of glaze (freezing precipitation)
for six NWS stations (Peoria, Moline, Chicago, Spring-
field, St. Louis, and Evansville, IN) from 1901 through
1980. At six sites, annual frequencies virtually doubled
or more, beginning rather abruptly during the early
1940s. The higher frequencies continued until 1980,
the end of the record. The absolute frequencies varied
from site to site, with the greatest at Springfield,
Peoria, and St. Louis.

Data were collected from 1981 through 1990 to bring
Changnon’s figures to the present (figures 35 and 36).
Although the “extreme” frequencies reported in the
1970s at Springfield and Peoria have not continued,
they remain within the range of values reported after
the increase noted in the 1940s or 1950s.

Procedures for observing or reporting freezing precipi-
tation have not changed significantly during the 90-
year period. As a result, no explanation is forthcoming
for the doubling (or more) of frequencies of glaze
beginning in the early 1940s. A question remains
whether these abrupt changes are real or an artifact of a
procedural change in observations. Discussions with
the NWS did not uncover any procedural changes
coincident with the upward trend.

Temporal Change in Thunderstorm Frequency

Thunderstorms form in unstable air, the larger the area
of instability, the greater the area of thunderstorms.

CLIMATE TRENDS IN ILLINOIS

Further, greater instability leads to greater thunder-
storm severity, i.e., strong, gusty winds, and possible
hail and tornadoes. Changnon (1984) investigated the
frequency of thunderstorms within and near Illinois
from the turn of the century to 1980. We have updated
those data through 1990, and present five-year totals of
thunderstorm days for Chicago O’ Hare and Peoria
(figure 37), and Springfield, Moline, and St. Louis,
MO, from 1901 through 1990 (figure 38), the pentad
totals plotted at the first of the five years.

First, there are no clear temporal trends. Thunderstorm
frequencies were relatively low during the late 1940s
and early 1950s at both Chicago and Peoria, the very
years when thunderstorms at St. Louis were most
frequent! Although thunderstorms are somewhat
related to temperature, i.e., warmer temperatures lead
to more thunderstorms, they also depend on instability
and available moisture. Also, thunderstorms are
relatively small-scale phenomena, i.e., the area of any
one event is usually a linear feature (associated with a
cold front), confined to a one- to three-state area. Since
mean frontal locations shift northward and southward
with warming and cooling, respectively, the area
favoring thunderstorms tends to be located north of
usual during warm episodes, and south of usual during
cold episodes.

Trends in Tornado Frequency

Illinois lies at the northeastern limit of “Tornado Alley,”
the group of states that exhibits the greatest tornado
frequency on average, generally Texas through Illinois.
Tornado frequencies change rather dramatically from
year to year. Since 1955, the period during which tor-
nado observations are thought to be essentially com-
plete, Illinois has experienced 28 tornadoes per year on
average. But as many as 107 were recorded in 1974,
and as few as seven and eight were recorded in 1964
and 1968, respectively (Wendland and Guinan, 1988).

The annual frequency of tornadoes in Illinois is not
well known prior to 1955, a period when the NWS
“received” as opposed to “sought out” data on tornado
occurrences within the United States. Annual Illinois
tornadoes since 1881 are presented in figure 39. The
data from 1845 to 1954 are known to be incomplete,
having been compiled from newspapers and U.S. Weather
Bureau publications, as available and summarized by
Wendland and Hoffman (1993). However, it is interest-
ing that some annual tornado frequencies prior to 1955,
such as 1883, 1886, and 1890, were as great or greater
than some years since 1955, the time during which
tornado records in Illinois are thought to be complete.

45

CLIMATE TRENDS IN ILLINOIS

Table 6. Period Trend (Positive or Negative) in Solar Radiation from 1948 to 1987, by Season

Location January April July October
Rockford, IL positive positive positive negative
Chicago, IL positive positive positive negative
Moline, IL positive positive positive negative
Peoria, IL positive positive positive negative
St. Louis, MO positive positive positive negative
Springfield, IL positive positive positive negative
Evansville, IN positive positive positive negative
Memphis, TN positive positive positive negative

Note: Boldface type indicates trends significant at the 10 percent level.

46

CLIMATE TRENDS IN ILLINOIS

# of Freezing Rain/Glaze Days (per 5 Yr)

1901 1911 1921 1931 1941 1951 1961 1971 1981
First Year of 5-Year Period

—=— MOLINE —*— PEORIA -4#— CHICAGO

Figure 35. Average number of days per five years with glaze (freezing precipitation)
at Chicago, Moline, and Peoria, IL, 1901-1990.

BO Ferenc etter creer cceteecteneenetntnenmetantneescemmnnnrntsterntenanene fcaemnematy

oS
oO

# of Freezing Rain/Glaze Days (per 5 Yr)
8
|
|
i
i
|
i

0
1901 1911 1921 1931 1941 1951 1961 1971 1981
First Year of 5-Year Period

—#— SPRINGFIELD —*— ST.LOUIS —&— EVANSVILLE

Figure 36. Average number of days per five years with glaze (freezing precipitation)
at Springfield, IL; St. Louis, MO; and Evansville, IL, 1901-1990.

47

CLIMATE TRENDS IN ILLINOIS

280
5
SY 260) 7a
Ww
[ad
jaa}
Ay
n
>
<
: 220
fe)
B 200: cecccejencnccececeqpenccececcscs Mannsenennceneccecessnncenserennapieassnascssccsecsecenesessnesas facecssnscncescssscsoenalenennasseee
ad
ea}
= 180 nucencceccecccencecencecscsecccsescecsccccscssesneececsesceennenenennan| ooensaghe\eanasneecncsapacnecsersnsserensanssecersnnssesensseeees
rs Ta Peoria
Be

160

1901 1911 1921 1931 1941 1951 1961 1971 1981
YEARS
Figure 37. Average number of days with thunderstorms per five years
(plotted on first of the five years) for Chicago and Peoria.

%
ao)
a4
ea}
Oy
”n
m4
<
a)
é
=
Yn
[ad
=

1

0
1901 1911 1921 1931 1941 1951 1961 1971 1981
YEARS

Figure 38. Average number of days with thunderstorms per five years (plotted on first of the five years)
for Moline, Springfield, and St. Louis, MO.

CLIMATE TRENDS IN ILLINOIS

120

100

60

# of Tornadoes

40

20

N W

ia4e 1865 1885 1905 1925 mae 1965 1985
Year

Figure 39. Number of reported tornadoes in Illinois (figure 7 from Wendland and Guinan, 1988), 1845-1990.

49

CLIMATE TRENDS IN ILLINOIS

The annual record of the number of days with tornadoes in
the state since 1960 (figure 40) exhibits several features:

1. Annual frequencies hover about a mean of about
11 tornado days per year from 1962 to 1971.

2. About 35 tornado days occurred in five consecu-
tive years, 1973 through 1977.

3. About 25 tornado days occurred per year thereafter.

As to areas of the state that might seem to “favor” tor-
nado activity, raw tornado frequencies clearly peak in
those counties with greater population densities (Wend-
land and Guinan, 1988). The authors believe, however,
that this relationship results merely from the fact that
more people are likely to see and report isolated, small-
scale events as tornadoes, and does not represent an
actual urban enhancement of tornado frequency.

Trends in the Frequency of Severe Snowstorms

Severe snowstorms in Illinois are defined as those
where 6 inches or more snow or freezing precipitation
of any intensity falls over at least 10 percent of the area
of the state within a 48-hour period. These storms exert
tremendous hardship on those within the area of impact
in terms of disruption to commerce, communication,
comfort, and economics. As opposed to the historical
tornado record, that for severe winter storms is believed
to be rather well defined since the turn of the century
because each such storm generally impacts an area of
some 15,000 square miles and would therefore be
recorded by observations from 50 or more NWS Coop
sites, even in areas of less population density than
represented today (Changnon, 1969, 1978). Newspa-
pers have proven to be a reliable source for a relatively
complete record of severe winter storms, whereas
tornadoes are not necessarily so reported, due to their
relatively small area of impact (on the order of a few
tens of square miles).

From the turn of the century to the early 1960s, severe
winter storms have occurred about five times per
winter (figure 41). After the early 1960s, the frequency
has declined to about three such storms per winter.
Winters with greatest severe storm frequencies include
1911-1912 with 12 storms, and 10 each in 1925-1926,
1943-1944, and 1950-1951. There were no severe
winter storms in ten of the last 30 winters, including
1961-1962, 1962-1963, 1965-1966, 1967-1968, and
1968-1969, 1970-1971, 1980-1981, 1981-1982, and
1982-1983, and 1991-1992.

The record of severe winter storm frequencies in
Illinois somewhat parallels that of temperature, sug-

50

gesting increased winter storm frequency with increased
mean temperatures. This relationship may be due to a
number of factors, including greater moisture supplies
during warmer winters, a more northerly storm track
during the middle of winter, etc. However, this study
did not examine specific factors for this relationship.

Trends in Illinois Droughts

Drought can be measured by several techniques. In this
instance we restrict our discussion to meteorological
drought, i.e., a shortfall of precipitation. We present the
percentage of the state experiencing 50 percent or less
of the 30-year average precipitation during July and
August from 1901 through 1991. The second uses the
same threshold but for the full growing season, April
through August, also from 1901 through 1991.

Five of the July-August droughts (figure 42) impacted
at least 50 percent of the area of Illinois during the
period 1901-1992, notably 1991 (impacting 79 percent
of the state), 1930 (70 percent), 1936 (51 percent),
1971 (66 percent), and 1983 (55 percent). The recent
1988 drought impacted only about 30 percent of the
state by this criterion.

According to the more stringent criterion, SO percent or
less than average precipitation from April through
August (figure 43), a somewhat different distribution
emerges, with extraordinary droughts in 1914 and 1936
(impacting about 60 percent of the state) and 1988 (54
percent). Neither 1914 nor 1988 were particularly note-
worthy according to the different criterion, 50 percent
or less precipitation during July and August. However,
those years (particularly 1988) were characterized by
extraordinary dryness in late spring and early summer.

Temporal Changes in Visibility

There has been and continues to be concern as to whether
the atmosphere is becoming more turbid with time,
perhaps due to industrialization, increased plowing and
cultivation frequencies etc. Such a change could lead to
reduced insolation, and related reduction in surface
temperature, atmospheric clarity, and visibility. Although
measurements of atmospheric turbidity are not gener-
ally available, we use several proxy records, including
visibility and frequency of days with smoke or haze.

Wendland and Bryson (1970) discussed a simple rela-
tionship between mean temperature, carbon dioxide
(CO,) concentration, atmospheric turbidity and sun-
spots from 1880 to 1960. Using dust concentrations
obtained in high snowfields of the Caucasus (Davitaia,

CLIMATE TRENDS IN ILLINOIS

i¢e) is)
oO oO

# of Tornado Days
De)
oO

@)
1960 1965 1970 1975 1980 1985
Year

Figure 40. Number of tornado days in Illinois per year (Wendland and Guinan, 1988), 1960-1985.

51

CLIMATE TRENDS IN ILLINOIS

2

i
eo)

# of Severe Winter Storms
O)

0)
1900 1910 1920 1930 1940 1950 1960 1970 1980 1990
Year

Figure 41. Number of severe winter storms per year in Illinois (defined as an area with at least 6 inches of snow
or any amount of glaze within 48 hours, over 10 percent or more of the state), 1900-1992.

52

CLIMATE TRENDS IN ILLINOIS

Areal Coverage (% of state)
w
Se ort it. cre tose - te

mack
oO

0
1901 1911 1921 1931 1941 1951 1961 1971 1981 1991
Year

Figure 42. Percent of Illinois impacted by 50 percent or less average precipitation
during July-August, 1901-1992.

53

CLIMATE TRENDS IN ILLINOIS

60

50

£
Oo

Areal Coverage (% of state)
ree)
oO

10

0)
1901 1911 1921 1931 1941 1951 1961 1971 1981 1991
Year

Figure 43. Percent of Illinois impacted by 50 percent or less average precipitation
during April-August, 1901-1992.

54

CLIMATE TRENDS IN ILLINOIS

1965) which they suggest reflect integrated hemi-
spheric loadings, they demonstrated that global mean
temperature changes during those eight decades could
be statistically explained by those three parameters.
Specifically, the warming from 1880 through the mid
1900s was primarily related to increases in CO,
concentration, whereas the cooling thereafter was
primarily related to the dramatic increase in dust load-
ings, noted during the 1940s and 1950s.

Atmospheric clarity can be defined and measured
several different ways. The concept of visibility, how-
ever, is straightforward, the measurement technique has
changed little over time, and hence visibility measure-
ments of decades ago are generally comparable to
those of today. Vinzani & Lamb (1985) showed declin-
ing visibility at NWS sites in and around Illinois since
1950. We analyzed hourly observations of visibility
since 1948 at Chicago-O’ Hare, Moline, and Peoria.
Frequencies of occurrences of visibilities in various
categories were accumulated for each year. Following
Vinzani and Lamb (1985), we estimated the values that
were exceeded 60 percent and 90 percent of the hours.
Trends in these values are shown in figures 44 and 45.
The trends in the 60 percent and 90 percent exceedance
values are upward at Chicago, indicating improving
visibility. Decreasing trends in the 90 percent value are
seen at Moline and Rockford. Decreasing trends in the
60 percent value are seen at Springfield. The increasing
visibility at Chicago, in contrast to the negative or
insignificant trends at the other sites, may be due to
more stringent pollution controls on industry.

Using yet another approach, Changnon (1987) showed
that the number of smoke and/or haze days at six NWS
First Order Stations in and around Illinois showed a
dramatic increase from a few tens of days per year prior to
the 1930s, to 100 or more days per year thereafter
through 1980! We have updated the frequency of
smoke and/or haze data through 1990 (figures 46 and
47), and find that the increased frequencies which
abruptly began in the 1930s essentially continue to the
present, though the highest frequencies at all sites were
noted in the 1930s and 1940s. The values of the last
few pentads continue at several times greater than
pre-1930 frequencies, but about half the maximum-
ever values.

ANOMALOUS EPISODES
FROM THE ILLINOIS CLIMATE RECORD

Global climate models (GCMs) suggest a future warm-
ing in Illinois and redistribution of precipitation during

the year under greenhouse-induced effects of additional
trace gases into the atmosphere. What would the im-
pacts of such climate changes be for Illinois? The
model outputs provide only average conditions, although
State resources vulnerable to climate are often influ-
enced by interannual or multiyear anomalies or by
several conditions not defined by GCMs. For example,
the range of certain species of insects or vegetation in
Illinois depends on such conditions as the number of
frost-free days, the timing of precipitation events, and
changes in diurnal temperature ranges, conditions not
available from GCMs.

The variability of a potential future climate may also
be documented by studying the climates of the past.
Identifying extreme seasons or years from the past that
lie within the average conditions specified by the
GCMs allows one to reconstruct variabilities and use
past analogs to estimate future climates.

Over the past century, notable months, seasons, and
years stand out as having been particularly warm, cold,
wet, or dry. Indeed, it is often thecombination of tem-
perature and precipitation extremes that stand out and
impact Illinois resources. For example, the Dust Bowl
years of the 1930s were prominent for their extreme
drought and warm temperatures as were the 1988
drought and 1993 floods.

To understand how climate has varied over the past
century, it is useful to isolate those episodes classified
as 1) warm and dry, 2) cold and wet, 3) warm and wet,
and 4) cold and dry. This characterization is also useful
in grappling with the impacts of potential future cli-
mates in Illinois.

To identify combined temperature and precipitation
extremes from the Illinois climate record, individual
mean monthly temperature and precipitation values
were recorded for each year between 1895 and 1991
and normalized relative to the 97-year average. The
index for temperature is calculated as:
I > (T, iT ie / Ths
where: I, = normalized monthly temperature
T, = average temperature for month in question
Tos = 97-year monthly mean temperature
T,,, = 97-year monthly temperature standard
deviation

For precipitation, the calculation is:

I ™ (P, + st / Pas

55

CLIMATE TRENDS IN ILLINOIS

56

20

18

167° sdonseonncossasesacsnessessesesancsonnfucnennaaman a a ee

meee tee eemeee eV = eee

VISIBILITY EXCEEDED 60% OF TIME

gt
1950: 1955-91 960) 1965 1970 975 “4 980" ASSES SS0
YEAR

Figure 44. Visibility (miles) exceeded 60 percent of the time for Moline, Chicago, and Peoria, 1950-1991].

8

st hHieaAgo t= Peoria

~

(op)

VISIBILITY EXCEEDED 90% OF TIME
1

2
1950 1955 1960» 1965 1970 1975 1980'°4985 ~1990
YEAR

Figure 45. Visibility (miles) exceeded 90 percent of the time for Moline, Chicago, and Peoria, 1950-1991.

CLIMATE TRENDS IN ILLINOIS

3007~
200

# of Smoke/Haze Days (per 5 Yrs)
b

100+

Year

—#— Moline ~+— Peoria --&-- Springfield

Figure 46. Number of days per five years when smoke and/or haze was reported
at Moline, Peoria, and Springfield (abscissa date in the first year of the five-year periods).

8

8

4000;—_—_—_+--__—__---——_—__ ++

:

b
8

# of Smoke/Haze Days (per 5 Yrs)
i) ro)
8 8

Figure 47. Number of days per five years when smoke and/or haze was reported
at Chicago, St. Louis, and Evansville.

57

CLIMATE TRENDS IN ILLINOIS

where: I, = normalized monthly precipitation
P, = total precipitation for month in question
PJ = 97-year monthly mean precipitation

P = 97-year monthly precipitation standard
deviation

A normalized monthly value near zero indicates a tem-
perature or precipitation value similar to that of the 97-
year average. Values above zero represent positive
deviations (warmer or wetter than the average). Values
below zero represent a negative deviation (cooler or
drier than the average). The magnitude of the deviation
is expressed in standard deviation units, i.e., +1.0 indi-
cates that the anomaly is one standard deviation greater
than the period average. A seasonal normalized value
was calculated for each season.

Warm and dry, and cold and wet seasons were distin-
guished by subtracting the normalized seasonal precip-
itation from normalized temperature: large positive
numbers represent the former, and large negative
numbers represent the latter. Warm and wet, and cold
and dry seasons were distinguished by adding the
normalized seasonal precipitation and temperature
values: large positive results represent warm and wet
seasons, whereas large negative numbers represent cold
and dry seasons.

When adding the normalized values, results near
zero indicate that either temperature or precipitation
is greater than the mean by about the same amount
that the remaining parameter is less than the mean.
When subtracting normalized precipitation from
normalized temperature, results near zero indicate
near-equal anomalies for each parameter.

An inherent problem occurs when interpreting the
result of either adding or subtracting normalized values
of temperature and precipitation. One relatively large
normalized value will dominate the result of either
process if the other normalized value is a very small
number, i.e., very close to the long-term mean. When
results from either addition or subtraction were near
zero, they had to be individually studied to determine
the significance of the number. In this study, small
normalized values, those between -0.25 and +0.25,
were set to zero. (Therefore some years in the follow-
ing figures exhibit no difference from the long-term
mean.) Normalized precipitation and temperature
values were added and subtracted to identify warm and
dry, cold and wet, warm and wet, and cold and dry
statewide conditions for each of four seasons between
1895 and 1991.

58

Statewide Summer Index

The combined indices for summer are plotted in figures
48 and 49. They show a high frequency (over three
times as likely) of warm and dry, and cold and wet
summers, as opposed to summers that were either
warm and wet, or cold and dry. In addition, the magni-
tude of the latter index is much less than that for warm
and dry, and cold and wet summers. As expected in a
continental climate, heat waves are more likely to be
associated with moisture deficiencies, and cool sum-
mers are more likely associated with excess moisture.

The combined extremes of warm and dry, and cold and
wet summers are more pronounced than warm and wet,
and cold and dry summers. A second feature is the
persistent nature of the occurrence of warm and dry,
and cold and wet summers; i.e., summers of a given
character tend to occur in succession. For example, the
period between 1899 and 1926 is characterized by a
variety of combined temperature and precipitation
extremes. The period between 1930 and 1944 is almost
exclusively warm and dry, whereas the period from
1961 to 1981 is almost exclusively cold and wet. The
period between 1950 and 1959 is characterized by a
return to a variety of extremes, as is the period between
1981 and 1991. All but one of the cold and dry sum-
mers occurred prior to 1920.

Individual summers that stand out as warm and dry are
1901, 1913, 1914, 1930, 1933, 1936, 1983, 1988, and
1991; whereas cold and wet are 1902, 1907, 1915,
1924, 1958, and 1981; warm and wet are 1980 and
1987; and cold and dry are 1920 and 1976. For the
summers with identified extremes, seasonal tempera-
ture and precipitation values and deviations from the
97-year average are presented in table 7.

Statewide Winter Index

The combined indices for the winter months are
plotted in figures 50 and 51 as a function of time. In
contrast to the summer months, warm and dry, and cold
and wet winters are Jess frequent than warm and wet,
and cold and dry conditions over the past 97 years.
Cold and dry winters were three times more likely than
cold and wet winters. The occurrence of cold and wet
winters is restricted to the years after 1962, and three
out of the four occurred after 1978. Individual winters
that stand out as warm and dry are 1921, 1931, 1953,
1954, and 1987; whereas 1979 and 1985 were cold and
wet; 1949, 1950, and 1983 were warm and wet; and
1963, 1977, and 1978 were cold and dry. For the win-
ters with identified extremes, seasonal temperature and

Index

CLIMATE TRENDS IN ILLINOIS

Warm and Dry

Fir |
ee yyy

Cold and Wet

-4
1895 1905 1915 1925 1935 1945 1955 1965 1975 1985
Year

Figure 48. Identification of statewide warm and dry, and cold and wet summers (1895-1991).

Index

See text for definition of index.

A A.A A ial
rw y

-2

Cold and Dry

-4
1895 1905 1915 1925 1935 1945 1955 1965 1975 1985
Year

Figure 49. Identification of statewide warm and wet, and cold and dry summers (1895-1991).

See text for definition of index.

59

CLIMATE TRENDS IN ILLINOIS

Table 7. Statewide Summer Seasonal Combined Temperature and Precipitation Extremes

Temperature Precipitation

Year Fahrenheit Deviation Inches __ Percent deviation
1901 76.7 +2.8 7.79 70
1902 71.9 -2.0 17.49 157
1907 71.9 -2.0 15.82 142
1913 76.4 +2.5 7.02 63
1914 76.4 +2.5 7.58 68
1915 69.3 -4.6 17.82 160
1920 72.5 -1.4 7.59 68
1924 71.6 -2.3 15.54 140
1933 76.0 +2.1 6.34 57
1936 78.6 +4.7 5.63 50
1958 71.7 -2.2 17.44 156
1976 72.3 -1.6 8.19 73
1980 TAS) +1.6 12.69 114
1981 73.1 -0.8 17.75 160
1983 76.6 +2.7 8.30 74
1988 76.2 +2.3 6.14 55

Note: Deviation and percent deviation calculated from the 97-year statewide summer-season average.

60

CLIMATE TRENDS IN ILLINOIS

Warm and Dry

Index

Lilli,

Cold and Wet

1925 1935 1945 1955 1965 1975 1985

-4
1895 1905 1915
Year

Figure 50. Identification of statewide warm and dry, and cold and wet winters (1895-1991)
See text for definition of index.

Warm and Wet

re ag desseds
sie, ee bos 58 a

Cold and Dry

Index

-4
1895 1905 1945 1955 1965 1975 1985
Year

1915 1925 1935

Figure 51. Identification of statewide warm and wet, and cold and dry winters (1895-1991). See text for

definition of index.

61

CLIMATE TRENDS IN ILLINOIS

precipitation values and deviations from the 97-year
average are presented in table 8.

Statewide Spring Index

The combined index for the spring months are plotted
in figures 52 and 53 as a function of time. In contrast
to both the summer and winter seasons, springtime
combined temperature and precipitation extremes are
more equally distributed among the four possible com-
binations, reflecting the relatively large day-to-day
temperature differences during transition seasons.
However, the warm and dry springs were for the most
part restricted to the years between the 1920s and
1930s and the late 1980s.

The spring climate record also shows active and inac-
tive intervals. The period between the 1920s and 1930s
experienced numerous deviations from average condi-
tions (usually warm and dry, or cold and dry), whereas
the late 1940s to the mid-1970s was a somewhat
benign period with only minor variations from the 96-
year average. Individual springs that stand out as warm
and dry are 1930, 1934, 1936, 1986, 1987, and 1988;
whereas cold and wet are 1983 and 1984; warm and
wet are 1921, 1922, 1945, and 1991; and cold and dry
are 1901, 1926, and 1971. For the springs with identi-
fied extremes, seasonal temperature and precipitation
values and deviations from the 97-year average are
presented in table 9.

Statewide Autumn Index

The combined indices for the autumn months are
plotted in figures 54 and 55 as a function of time.
Warm and dry autumns have occurred more often than
the other three possible combinations, with the largest
cluster of occurrences confined to the period between
the late 1950s and mid-1960s. A significant warm and
dry autumn has not occurred since 1971. The warm and
wet autumns are clustered in the period between the
early 1920s and the late 1930s. Individual autumns that
stand out as warm and dry are 1938, 1939, 1953, 1963,
and 1971; whereas cold and wet are 1911 and 1926;
warm and wet are 1927, 1931, and 1941; and cold and
dry are 1917 and 1976. For the autumns with identified
extremes, seasonal temperature and precipitation
values and deviations from the 97-year average are
presented in table 10.

Statewide Annual Index

While warm and dry, cold and wet, warm and wet, and
cold and dry seasons have been identified from Illinois’

62

97-year climate record, the succession of the seasons
and their climate has the ultimate impact. For example,
the impact of a warm and dry summer would likely be
greater if the previous spring and winter were also warm
and dry in relation to their respective averages. Thus,
efforts to group the climates of consecutive seasons are
useful. For example, an examination of calendar years
(a grouping of four seasons) identified 1914, 1930,
1934, 1936, 1953, and 1987 as warm and dry; the years
1926, 1950, 1951, and 1972 as cold and wet; the years
1973 and 1983 as warm and wet; and the years 1895,
1917, 1920, and 1976 as cold and dry. The results of
several GCMs suggest that the trend of the Illinois climate
of the future will be toward warm and dry summers and
warm and wet winters. Analog years with similar
summer and winter trends are 1930, 1933, and 1983.

Regional Index

The state of Illinois measures approximately 400 miles
north to south, and the climates of northern, central,
and southern Illinois are not homogeneous. For exam-
ple, a cold and wet summer in northern Illinois may
occur in concert with a warm and dry summer in the
southern third of the state. The National Weather
Service divides Illinois into nine climate regions (see
figure 1, p.9). For this comparison, the northern sector
of the state was represented by region 1, the central
district by region 5, and the south by region 9. Warm
and dry, cold and wet, warm and wet, and cold and dry
seasons were identified in a fashion similar to the
statewide calculations. Comparison of the combined
index for each of the three regions and the statewide
calculation shows general agreement across the region
for each of the four seasons.

A number of identified years overlap (especially those
identified as extreme) and the trends over time are
similar to the statewide trends. However, some year-to-
year differences occur when an identified season for a
particular region is not similarly identified as a
statewide occurrence and vice versa. As illustration,
warm and dry, cold and wet, warm and wet, and cold
and dry summer seasons are identified for each of three
regions (figures 56 and 57).

CONCLUSIONS

Although long-term trends can be identified in various
climate parameters, the dominant characteristic of
Illinois’s climate is the presence of large variability on
time scales of a few years or less. That is, the magni-
tude of long-term trends is in general considerably less

CLIMATE TRENDS IN ILLINOIS

Table 8. Statewide Winter Seasonal Combined Temperature and Precipitation Extremes

Temperature Precipitation

Year Fahrenheit Deviation Inches Percent deviation
1921 33.6 +4.9 5.10 719
1931 33.2 +4.5 2.47 38
1949 31.0 +2.3 11.41 177
1950 32.5 +3.8 15.51 240
1953 33.0 +4.3 5.48 85
1954 33.4 +4.7 5.25 81
1963 21.9 -6.8 2.37 37
1977 20.8 -7.9 2.95 46
1978 19.6 -9.1 4.56 71
1979 19.9 -8.8 7.95 123
1983 33:7 +5.0 9.01 140
1985 26.1 -2.6 8.93 138
1987 31.4 +2.7 4.03 62

Note: Deviation and percent deviation alculated from the 97-year statewide winter season average.

63

CLIMATE TRENDS IN ILLINOIS

Index

Figure 52.

Index

-2

Warm and Dry

ht
rye

Cold and Wet

“{a08 1905 1915 1925 1935 1945 1955 1965 1975 1985
Year

Identification of statewide warm and dry, and cold and wet springs (1895-1991). See text for
definition of index.

Warm and Wet

rt Fat ate

Cold and Dry

{808 1905 1915 1925 1935 1945 1955 1965 1975 1985
Year

-2

Figure 53. Identification of statewide warm and wet, and cold and dry springs (1895-1991).

CLIMATE TRENDS IN ILLINOIS

Table 9. Statewide Spring Seasonal Temperature and Precipitation Extremes

Temperature Precipitation

Year Fahrenheit Deviation Inches Percent deviation
1901 49.4 -2.1 7.07 65
1921 56.0 +4.5 11.97 110
1922 53.6 +2.1 14.97 140
1926 47.8 -3.7 7.93 73
1930 52.4 +0.9 6.07 56
1934 52.0 +0.5 5.16 47
1936 527 +1.2 6.01 35)
1945 53.6 +2.1 17.32 158
1971 49.6 -1.9 6.27 57
1983 48.8 -2.7 15.08 138
1984 47.3 -4.2 14.13 129
1986 54.6 +3.1 8.01 53
1987 55.3 +3.8 7.14 65
1988 $2.2 +0.7 6.91 63
1991 56.0 +4.5 12.34 113

Note: Deviation and percent deviation calculated from the 97-year statewide spring season average.

65

CLIMATE TRENDS IN ILLINOIS

Warm and Dry

Walt wl Ja
ret! pelt ae

Cold and Wet

Index

1925 1935 1945 1955 1965 1975 1985

-4
1895 1905 1915
Year

Figure 54. Identification of statewide warm and dry, and cold and wet autumns (1895-1991)
See text for definition of index.

Warm and Wet

Index

Cold and Dry

1945 1955 1965 1975 1985
Year

-4
1895 1905 1915 1925 1935

Figure 55. Identification of statewide warm and wet, and cold and dry autumns (1895-1991)
See text for definition of index.

66

CLIMATE TRENDS IN ILLINOIS

Table 10. Statewide Autumn Seasonal Temperature and Precipitation Extremes

Year

1911
1917
1926
1927
1931
1938
1939
1941
1953
1963
1971
1976

Temperature
Fahrenheit

SPA)
50.7
52.6
57.7
60.3
57.0
56.4
56.9
56.7
58.5
57.9
48.6

Deviation

-2.0
-3.8
-1.9
+3.2
+5.8
+2.5
+1.9
+2.4
+2.2
+4.0
+3.4
-5.9

Precipitation

Inches

14.99
6.19
17.28
13.62
13.46
6.72
4.26
17.08
4.05
4.66
6.66
6.05

Percent deviation

166
68
191
150
149
74

Note: Deviation and percent deviation calculated from the 97-year statewide autumn season average.

67

CLIMATE TRENDS IN ILLINOIS

Warm and Dry

Index

Cold and Wet

1935 1945 1955 1965 1975 1985
Year

—=- Region 1 (north) —+— Region 5 (central) —*— Region 9 (south)

Figure 56. Identification of regional (north, central, and south) warm and dry, and cold and wet summers
(1895-1991). See text for definition of index.

-4
1895 1905 1915 1925

Index

Cold and Dry

1945 1955 1965 1975 1985
Year

-4
1895 1905 1915 1925 1935

—S- Region 1 (north) —— Region 5 (central) —*— Region 9 (south)

Figure 57. Identification of regional (north, central, and south) warm and wet, and cold and dry summers
(1895-1991). See text for definition of index.

68

than the changes that can occur from one year or a few
years to the next. With that being said, however, some
identifiable longer-term features can be noted.

The most persistent and extreme summertime high
temperatures occurred primarily during the 1930s, the
Dust Bowl era. Indeed, average temperatures in Illinois
clearly increased by 4 to S°F from the mid- to late
1800s to the 1930s, and then cooled by about half that
amount to the present. All of the temperature parame-
ters assessed herein support those trends, including
average temperatures and frequency of days with
extreme temperatures, whether warm or cold. These
trends are found in records of the continent, the
hemisphere, and the world, although the magnitudes of
change decrease with increasing areas of integration.

During the peak of Illinois temperatures during the
1930s, the frequency and intensity of hot days was
unprecedented in Illinois and much of the upper
Midwest. The many severe impacts of the heat and
dryness of that period are well known. Since then, the
frequency and intensity of summertime heat has been
substantially less, particularly during the 1960s and
1970s. However, some resurgence of extreme summer-
time heat was noted during the 1980s, with accompa-
nying impacts on crop yields. The extent of this trend
toward higher frequencies of hot and dry summers
cannot be predicted at this time.

In general, the frequency of extreme cold events in-
creased from 1930 to the present. This upward trend
reached its peak in the late 1970s and early 1980s, with
an unprecedented string of extremely cold winters.
They had major negative impacts on the Illinois econ-
omy and environment, affecting transportation, health,
energy consumption, etc. During this cooling episode,
precipitation may have increased marginally, but the
major change in precipitation was the increase in
annual snowfall, peaking in the late 1970s. Also nota-
ble was the abrupt decline in the annual number of
severe winter storms in the state, dropping from an
average of five per year prior to about 1980, to about
three during recent years. The 1992-1993 winter was
perceived by many to be a severe winter, but it was
only near-average when compared to the situation of
the last two or three decades.

Generally benign, moderate summers occurred during
the 1960s and 1970s. As noted above, the 1980s were
characterized by more frequent hot and dry but quite
variable summers, similar to the earlier half of the cen-
tury. It should be noted that midwestern agriculture
experienced rapid technological advances during this

CLIMATE TRENDS IN ILLINOIS

relatively benign period. In addition, virtually all
farmers active today have spent their entire careers
during that benign period. The severity of the 1988
growing-season drought in Illinois had only been
equaled in two other years since the turn of the century.
The most recent Illinois drought of equal intensity
occurred in 1936, long before virtually all present-day
farmers were in the business.

The impact of climate variability in the state is
exemplified by the cool summer of 1992. Even though
overall corn yields were the highest ever in the United
States, the northernmost 100 miles of the Corn Belt
accumulated so few GDDs (sixth fewest since 1878)
that the corn crop did not mature at its usual rate in that
area. Indeed, it was not yet ripe at the time of the first
frost in southern Minnesota and Wisconsin, which was
within about one week of the long-term average.

The degree of cold of the 1992 summer was also demon-

strated by the low total CDDs for the state. Total CDDs
in 1992 were the seventh lowest since 1878. This was a
boon for the consumer and a bane to the electric utility.

The tornado frequency record in Illinois exhibits no
trends, rather an average of some 25 tornadoes per
year, but varying from 6 to 107 per year! Of interest
was the relatively high frequency recorded during the
early 1880s (essentially equal to that of today), even
though the tornado record for that time is known to
be incomplete.

Cloud cover and the frequency of ice/glaze storms have
both increased in the state. These changes do not ap-
pear to be due to changes in observing procedure, nor
are they related to other parameters that might assist in
their explanation.

It is clear that climate has not been stable in Illinois
during the last 150 years, nor should it have been so
anticipated. Many of the changes, although abrupt,
were of relatively small magnitude. Yet those relatively
small-scale changes levied a substantial toll on the
inhabitants of the state through discomfort; lost
income; increased costs; and impediments to com-
merce and agricultural production, in spite of major
strides in hybridization and field management prac-
tices. Since climate has always changed, it will likely
continue to do so. The past gives little hint as to the
future direction and magnitude of these changes, but it
does suggest limits within which climate may be
constrained over the immediate future. The current
increase to atmospheric carbon dioxide by the burning
of fossil fuels, however, is suspected to change some

69

CLIMATE TRENDS IN ILLINOIS

climate parameters at a faster rate than during the past
centuries, thereby possibly rendering past climate a
poor tool for evaluating future variability.

ACKNOWLEDGMENTS

We acknowledge the many hours of programming by
Julie Dian, who produced the scores of statistical calcu-
lations and figures initially produced for this study.
Robin Shealy performed many of the statistical cal-
culations. James Angel assisted with the figures, and
Jean Dennison managed the manuscript.

REFERENCES

Changnon, S.A. 1969. Climatology of Severe Winter
Storms in Illinois. Ilinois State Water Survey Bulletin
53, Champaign.

Changnon, S.A. 1978. Record Severe Winter Storms in
Illinois, 1977-78. Mllinois State Water Survey Report of
Investigation 88, Champaign.

Changnon, S.A., R.G. Semonin, and W.M. Wendland.
1980. Effect of Contrail Cirrus on Surface Weather
Conditions in the Midwest - Phase I. Final report to
NSF grant ATM 78-09568.

Changnon, S.A. 1984. Climate Fluctuations in Illinois:
1901-1980. Illinois State Water Survey Bulletin 68,
Champaign.

Changnon, S.A. 1987. Historical Atmospheric Trans-
mission Changes and Changes in Midwestern Air
Pollution. Bulletin American Meteorological Society,
68(5):477-480.

Davitaia, F.F. 1965. Possible Influence of the Atmo-
spheric Dust on the Regression of Glaciers and the
Warming of Climate. Trans. Soviet Acad. Sci., Geogr.
Ser. (2):3-22.

Draper, N.R., and H. Smith. 1981. Applied Regression
Analysis. John Wiley & Sons, Inc., New York.

70

Folland, C.K., T. Karl, and K.Ya. Vinnikov. 1990.
Observed Climate Variations and Change. In J.T.

Houghton, G.J. Jenkins, and J.J. Ephraums (eds.),
Climate Change: The IPCC Scientific Assessment,
Cambridge University Press, Cambridge, pp.195-238.

Hoff, F.A., and J.R. Angel. 1989. Frequency Distribu-
tion and Hydroclimatic Characteristics of Heavy Rain-
storms in Illinois. Ulinois State Water Survey Bulletin

70, Champaign.

Lamb, H.H. 1966. The Changing Climate. Methune &
Co.

Petersen, M.S. 1990. Implementation of a Semi-Physical
Model for Examining Solar Radiation in the Midwest.
Midwestern Climate Center, Illinois State Water Sur-
vey Miscellaneous Publication 123, Champaign.

Vinzani, P. and P.J. Lamb. 1985. Temporal and Spatial
Visibility Variations in the Illinois Vicinity during
1940-1980. Journal of Climate and Applied Meteorol-
ogy, 25:435-451.

Wendland, W.M. 1990. A History of the Weather
Observations in Illinois. Trans., Ill. Academy of Sci-
ence, 83 (1 and 2):43-56.

Wendland, W.M., and R.A. Bryson. 1970. Atmo-
spheric Dustiness, Man, and Climatic Change. Biologi-
cal Conservation, 2(2):125-128.

Wendland, W.M., and P. Guinan. 1988. A Tornado and
Severe Windstorm Climatology for Illinois: 1955-
1986. Trans., Ill. Academy of Science, 81 (1 and 2):
131-146.

Wendland, W.M., and H. Hoffman. 1993. Illinois
Tornadoes Prior to 1916. Trans., Ill. Academy of
Science (in press).

Wigley, T.M.L. and T.P. Barnett. 1990. Detection of the
Greenhouse Effect in the Observations. In T.J.
Houghton, G.J. Jenkins, and J.J. Ephraums (eds.),
Climate Change, the IPCC Scientific Assessment,
Cambridge University Press, Cambridge, pp. 239-255.

CLIMATE TRENDS IN ILLINOIS

APPENDIX A

The following tables present the results of the trend analysis for individual long-term stations. The values given
are the magnitude of the trend for those stations where the trend is statistically significant at the 10 percent level.

If the trend is not statistically significant, an "NT" is shown.

71

CLIMATE TRENDS IN ILLINOIS

Table A.1. Trends in Annual Precipitation Parameters (Asterisk Indicates Trends of Statistical Significance)

Number of
Number of days days with Total Number of
Total with measurable precipitation Snowfall days with
precipitation precipitation > one inch (inches/ measurable
City (inches/decade) (days/decade) (days/decade) decade) snowfall
(days/decade)
Northwest
Aledo 0.11 -0.59 0.01 0.97* 0.16
Dixon 1 NW 0.21 0.93 0.07 1.49* 0.73*
Galva 0.64* 0.73 0.35* -0.29 -0.38*
Mount Carroll 0.28 136% 0.07 0.52 -0.06
Walnut 27 -3.05* 0.03 0.70* -0.74*
Northeast
Aurora 0.26 4.12* 0.07 0.38 -1.23*
Marengo 0.61* 0.25 0.22* 1.41* -0.49*
Ottawa 4 SW 0.80* LS 7= 0.38* 0.40 0.37
West
La Harpe 0.37 1.54* 0.14 -0.10 -0.10
Monmouth 0.27 0.54 0.10 0.36 0.00
Rushville 0.97* 0.58 0.43* 0.08 -0.04
Central
Decatur 0.11 0.22 0.01 -0.08 -0.32
Lincoln 0.14 0.28 0.15 -0.16 -0.23
Minonk 0)55* 1.44* 0.17 0.61 0.41*
East
Danville 2s 4.18* 0.21 0.62 0.26
Hoopeston -0.01 0.63 -0.07 -1.50* 0.23
Pontiac 0.27 -0.55 0.15 -0.86* 0.04
Urbana 0.74* 2326 0.25* 0.72* 1311*
West-Southwest
Carlinville 0.02 1.03* -0.12 0.04 0.03
Greenville 1 E -0.16 -0.30 0.06 -0.37 -0.56*
Griggsville 0.16 2.63* 0.03 1.92* 0.83*
Hillsboro 0.27 -0.67 0.24* -0.36 -0.48*
Jacksonville 0.62* -0.60 0.37* 0.41 -0.73*
Pana 0.40 3.12* 0.14 1.32* 1.07*
White Hall 1 E -0.14 1.89* -0.30* -0.73* -0.50*
Fast-Southeast
Charleston 0.28 -0.49 0.14 0.20 -0.43*
Effingham 0.45 1.84* 0.14 1.16* 0.47*
Flora 0.58* 1.07 0.27* -0.90* -0.59*
Olney 0.09 0.83 0.15 -1.06* -0.72*
Palestine 0.15 1.85* 0.01 0.94* 0.62*
Paris Waterworks 0.72* 4.69* 0.10 2.03* 1.07*
Windsor -0.28 0.54 -0.12 0.08 -0.32

72

CLIMATE TRENDS IN ILLINOIS

Table A.1. Concluded

Southwest

Anna 1 E 0.51 2.23* -0.02 0.83* 0.77*

Carbondale -0.68 -2.15* -0.27 -0.61 0.66*

Du Quoin 4 SE 0.32 1.18* 0.10 -0.32 -0.10

Olive Branch 0.13 2.06* 0.07 0.21 -0.01

Sparta 0.20 -0.66 0.21 -0.19 0.01
4.12* 0.25 0.28 0.13

Southeast

Fairfield 0.91* 0.99 -0.13 -0.43 -0.26

Harrisburg -0.06 pare fs 0.02 0.11 0.17

McLeansboro 0.46 -0.07 0.09 0.17 -0.41

Mt. Vernon 0.16

All stations 0.35* 1.19* 0.11 0.37 0.12

73

CLIMATE TRENDS IN ILLINOIS

Table A.2. Trends in Annual Temperature Parameters (Asterisk Indicates Trends of Statistical Significance)

Number of days

with mean Highest daily Lowest daily
Mean temperature maximum minimum
temperature > SOF temperature temperature
City (°F/decade) (days/decade) (°F/decade) (°F/decade)
Northwest
Aledo -0.23 -1.24 NT 0.10
Dixon -0.01 0.52 NT 0.11
Galva -0.02 0.37 -0.36* 0.32
Mount Carroll -0.02 -0.06 NT 0.09
Walnut -0.29* 09.19 -0.29* 0.08
Northeast
Aurora -0.22* -0.89 NT -0.15
Marengo 0.03 0.28 NT -0.20
Ottawa 4 SW 0.53* 2.89* -0.31* -0.25
West
La Harpe -0.08 0.26 NT 0.17
Monmouth 0.06 0.84* -0.34* 0.25
Rushville 0.07 0.02 NT -0.03
Central
Decatur -0.29* -0.44 -0.27* -0.01
Lincoln -0.21 0.03 -0.32* 0.54*
Minonk -0.31 -1.35 -0.53* 0.22
East
Danville 0.49* 2.68* -0.58* -0.04
Hoopeston 0.44* 238% -0.33* 0.06
Pontiac -0.25* -0.59 -0.40* -0.07
Urbana 0.06 0.70* NT 0.40*
West-Southwest
Carlinville -0.09 0.41 -0.41* 0.10
Greenville 1 E -0.11 -2.00 NT 0.18
Griggsville -0.15 -0.68 NT 0.04
Hillsboro -0.07 0.11 NT 0.17
Jacksonville -0.16* -0.54 -0.36* -0.03
Pana 0.11 1.25* -0.42* 0.17
White Hall 1 E 0.25* 1.95* -0.38* -0.01
East-Southeast
Charleston 0.33 1.92 -0.35* 0.26
Effingham 0.17 1.70* -0.57* -0.04
Flora 0.34 2.76* NT 0.07
Olney -0.24 -0.20 -0.45* 0.08
Palestine -0.15 0.36 NT -0.10
Paris Waterworks 0.19* 2.00* -0.31* 0.32*
Windsor 0.02 1.04* -0.34* 0.31

74

Table A.2. Concluded

Southwest

Anna 1 E
Carbondale

Du Quoin 4 SE
Olive Branch
Sparta

Southeast

Fairfield
Harrisburg
McLeansboro
Mt. Vernon

All stations

-0.09
-1.06*
-0.19
-0.25*
0.02

0.04
-0,.34*
-0.14*
-0.13

-0.06

0.43
-4,.94*
-0.54
-0.87

0.90*

0.93
-0.11
-0.23
-0.31

0.26

-0.14

0.07
-0.05
-0.07
-0.04

-0.05
-0.10
-0.05

0.20

0.15

CLIMATE TRENDS IN ILLINOIS

75

CLIMATE TRENDS IN ILLINOIS

Table A.3. Trends in Annual Temperature Parameters (Asterisk Indicates Trends of Statistical Significance)

Number of days

Mean with minimum
minimum temperature
temperature > 70°F
City (°F/decade) (days/decade)
Northwest
Aledo -0.17 -0.06
Dixon 1 NW 0.04 0.22
Galva 0.05 0.24
Mount Carroll -0.06 -0.15
Walnut -0.32* -0.52*
Northeast
Aurora -0.14 -0.01
Marengo -0.18 0.01
Ottawa 4 SW 0.57* 0.78*
West
La Harpe -0.13* -0.15
Monmouth 0.11 0.20
Rushville -0.04 -0.45
Central
Decatur -0.22* -0.02
Lincoln -0.10 -0.05
Minonk -0.24 -0.21
East
Danville 0.45* -0.02
Hoopeston 0.45* 0.41*
Pontiac -0.17* -0.32
Urbana 0.08 -0.14
West-Southwest
Carlinville 0.06 0.65*
Greenville 1 E 0.01 0.86
Griggsville -0.13 -0.41
Hillsboro -0.06 -0.23
Jacksonville -0.11* -0.28
Pana 0.16* 0.09
White Hall 1 E 0.31* 0.53
East-Southeast
Charleston 0.39* 0.82*
Effingham 0.23 0.27
Flora 0.28 0.24
Palestine -0.04 0.60
Olney -0.16 -0.09
Paris Waterworks 0.31* 0.92*
Windsor 0.18* 0.69*

76

Number of days
with minimum
temperature
< OF
(days/decade)

0.14
0.66*
0.44
0.43
0.78*

0.21
0.77*
0.33

0.43
0.12
0.28

0.18
0.23
0.45

0.34
0.41
0.39
-0.04

0.03
-0.74*
0.14*
0.06
0.30
0.05
0.38

0.10
0.57*
0.20
0.20
0.19
0.20
0.00

Number of days
with minimum
temperature
<32°F
(days/decade)

-0.85*
0.79

-0.54
0.73
L237

-1.10*
Lae
-0.49

0.55
-0.83*
0.46

-1.12*
-1.31*
-0.09

1.07

0.72

0.40
-0.75*

-1.00*
-2.58*
0.60
-0.16
0.57
-0.55
0.57

0.76

1.20

0.26
-0.50
-0.46
-1.23*
-1.90*

CLIMATE TRENDS IN ILLINOIS

Table A.3. Concluded

Southwest

Anna 1 E -0.06 -0.71* 0.08 0.61
Carbondale -0.91* -1.93* 0.28 1.58*
Du Quoin 4 SE -0.10 0.24 0.25 -0.14
Olive Branch -0.15* -0.09 0.28* 0.35
Sparta 0.10** 1.04* -0.10 -0.76
Southeast

Fairfield 0.13 -0.35 0.19 0.91*
Harrisburg -0.30* -0.45 0.06 0.41
McLeansboro -0.15* -0.99* 0.16 1.36*
Mt. Vernon -0.22* -0.17 0.84* 0.78*
All stations -0.02 0.00 0.26 0.01

77

CLIMATE TRENDS IN ILLINOIS

Table A.4a. Trends in Annual Temperature Parameters (Asterisk Indicates Trends of Statistical Significance)

Number of days

Mean with maximum
maximum temperature
temperature >86°F
City (°F/year) (days/year)
Northwest
Aledo -0.11 0.25
Dixon 1 NW -0.08 -1.07*
Galva -0.09 -1.43*
Mount Carroll 0.00 0.03
Walnut -0.27* -0.61
Northeast
Aurora -0.40* -0.17
Marengo 0.22 0.87
Ottawa 4 SW 0.40* -0.72
West
La Harpe 0.00 -0.12
Monmouth 0.01 -0.97
Rushville 0.17 0.89
Central
Decatur -0.37* -0.86
Lincoln -0.29 -1.54*
Minonk -0.38 -1.07
East
Danville 0.52 -1.19
Hoopeston 0.44* -0.97
Pontiac -0.33* -2.34*
Urbana 0.03 0.28
West-Southwest
Carlinville -0.24* -1.24*
Greenville 1 E -0.22 -0.84
Griggsville -0.23 =o"
Hillsboro -1.10 -0.62
Jacksonville -0.21* -1.84*
Pana 0.01 -1.46*
White Hall 1 E 0.15 -1.16

78

Number of days

with maximum
temperature
>90°F
(days/year)

Number of days
with temperatures

>100°F
(days/year)

-0.16
-0.12
-0.21*
-0.16
-0.11

-0.10
-0.07
-0.34*

-0.18
-0.23
-0.13

-0.26
-0.16
-0.27*

-0.34*
-0.22*
-0.27*
-0.03

-0.52*
-0.45
-0.24
-0.31
-0.35*
-0.23
-0.43*

CLIMATE TRENDS IN ILLINOIS

Table A.4a. Concluded

East-Southeast

Charleston 0.26 -1.93* -2.35* -0.40*
Effingham 0.20 -2,31* -2.51* -0.66*
Flora 0.46 0.81 -0.14 -0.38
Olney -0.30 -1.07 -1.71* -0.41*
Palestine -0.26* -1.08 -1.25 -0.15
Paris Waterworks 0.08 -1.48* -1.54* -0.24
Windsor -0.13* -1.44* <2. one -0.37*
Southwest

Anna 1 E -0.11 -1.63* -1.95* -0.42*
Carbondale -1.20* -4,11* -4.68* -1.12*
Du Quoin 4 SE -0.29* -2.21* -2.32* -0.36
Olive Branch -0.34* -2.45* -3.01* -0.61*
Sparta -0.07 0.04 -0.73 -0.50*
Southeast

Fairfield -0.19 -3.13* -3.59* -0.52*
Harrisburg -0.39* -0.70 -1.57* -0.58*
McLeansboro -0.14 -1.12 -1.51* -0.57*
Mt. Vernon -0.05 -1.61* -1.42* -0.20*
All stations -0.10 -1.10* -1.55* -0.33*

79

CLIMATE TRENDS IN ILLINOIS

Table A.4b. Trends in Annual Temperature Parameters (Asterisk Indicates Trends of Statistical Significance)

Number of days Number of days
with maximum with maximum
City temperature < 0°F temperature < 32°F

Northwest
Aledo 0.02 0.38
Dixon 1 NW 0.06 1.33*
Galva 0.07* 0.62
Mount Carroll 0.05 0.61
Walnut 0.04 0.62
Northeast
Aurora 0.03 -0.40
Marengo 0.04 -0.48
Ottawa 4 SW 0.06* 0.75*
West
La Harpe 0.01 0.39
Monmouth 0.00 -0.21
Rushville 0.01 0.47
Central
Decatur 0.02* -0.05
Lincoln 0.01 0.53
Minonk 0.07* 0.85
East
Danville 0.02* 0.81*
Hoopeston 0.04* 1.30*
Pontiac 0.03 0.87*
Urbana 0.04* 0.45
West-Southwest
Carlinville 0.01 0.67*
Greenville 1 E -0.01 -1.93*
Griggsville 0.02 1.10*
Hillsboro 0.01* 0.12
Jacksonville 0.04* 0.93*
Pana 0.02* 0.59
White Hall 1 E 0.01 0.83*
East-Southeast
Charleston 0.03* 0.45
Effingham 0.04* 1.81*
Flora 0.01* 0.65*
Olney 0.01 0.55
Palestine 0.01 0.69*
Paris Waterworks 0.03* 0.07
Windsor 0.03* 0.76*

80

Table A.4b. Concluded

Southwest

Anna 1 E
Carbondale

Du Quoin 4 SE
Olive Branch
Sparta

Southeast

Fairfield
Harrisburg
McLeansboro
Mt. Vernon

All stations

0.00
0.01
0.01
0.02*
0.01

0.02*
0.00
0.01*
0.07

0.03*

CLIMATE TRENDS IN ILLINOIS

0.53*
0.74*
0.56*
0.64*
0.32

O07
0.32
0.88*
0.51

0.53

81

CLIMATE TRENDS IN ILLINOIS

Table A.5. Trends in Autumn Climate Parameters (Asterisk Indicates Trends of Statistical Significance)

City

Northwest

Aledo

Dixon 1 NW
Galva

Mount Carroll
Walnut

Northeast

Aurora
Marengo
Ottawa 4 SW

West

La Harpe
Monmouth

Rushville
Central

Decatur
Lincoln
Minonk

East

Danville
Hoopeston
Pontiac
Urbana

West-Southwest

Carlinville
Greenville 1 E
Griggsville
Hillsboro
Jacksonville
Pana

White Hall 1 E

East-Southeast

Charleston
Effingham

Flora

Olney

Palestine

Paris Waterworks
Windsor

82

Mean
temperature
(°F/decade)

-0.31*
-0.22*
-0.15*
-0.14

-0.19*

-0.16
0.02
0.38

-0.16*
0.00
0.14

-0.24
0.27
-0.24

0.11

0.50*
-0.17*

0.02

-0.11
0.02
-0.21*

-0.14
-0.18*
0.04
0.23

0.44
-0.17*
0.27
0.02
-0.14
0.24
-0.04

Mean minimum
temperature
(°F/decade)

-0.27*
-0.18
-0.06
-0.18**
-0.30*

-0.10
-0.27*
0.43*

-0.20*
0.06
0.00

-0.16
0.27*
-0.23

0.20

0.50*
-0.14

0.05

0.05
0.06
-0.19*

-0.10
-0.16*
0.11
0.30*

0.51*
0.00
0.24
0.07
-0.04
0,39*
0.17

Mean maximum
temperature
(°F/decade)

-0.22
-0.26*
-0.23*
-0.15
-0.08

-0.50*
0.31
0.47

-0.11
-0.06
0.24

-0.32
0.35
-0.25

0.03
0.29
-0.21*
0.00

-0.28
-0.01
-0.22*
-0.27
-0.21*
-0.20*
0.12

0.38
-0.35*
0.31

-0.03
-0.25*
0.10

-0.26

Total
precipitation
(inches/decade)

-0.04
0.02
0.16
0.04
0.03

0.02
0.12
0.21

0.23
0.02
0.34*

-0.06
0.00
0.27*

0.13
0.09
0.07
0.19

0.02
0.12
-0.06
0.09
0.13
0.12
-0.18

0.14
0.06
0.13
0.11
0.12
0.08
0.00

Table A.5. Concluded

Southwest

Anna 1E
Carbondale

Du Quoin 4 SE
Olive Branch
Sparta

Southeast

Fairfield
Harrisburg
McLeansboro
Mt. Vernon

All stations

-0.25*
-0.80*
-0.18*
-0.30*
-0.03

-0.26
-0.30
-0.16
-0.32*

-0.11

CLIMATE TRENDS IN ILLINOIS

-0.29*
-0.82*
-0.30*
-0.45*
-0.15

-0.48*
-0.37
-0.15
-0.08

-0.15*

0.02
-0.29
0.23
0.18
0.14

0.25*

0.02

0.16
-0.03

0.10

83

CLIMATE TRENDS IN ILLINOIS

Table A.6. Trends in Winter Climate Parameters (Asterisk Indicates Trends of Statistical Significance)

Mean Mean minimum Mean maximum Total
temperature temperature temperature precipitation
City (°F/decade) (°F/decade) (°F/decade) (inches/decade)
Northwest
Aledo -0.06 0.00 -0.11 -0.02
Dixon 1 NW 0.08 0.03 0.13 0.09
Galva -0.20 -0.14 -0.22 0.08
Mount Carroll -0.02 -0.09 -0.01 0.03
Walnut -0.31* -0.42* -0.21 -0.06
Northeast
Aurora -0.16 -0.17 -0.06 0.06
Marengo -0.04 -0.26 0.18 0.05
Ottawa 4 SW 0.10 0.09 0.11 0.05
West
La Harpe -0.18 -0.26* -0.10 -0.03
Monmouth 0.06 0.08 0.05 0.07
Rushville -0.20 -0.24* -0.16 0.13
Central
Decatur -0.21 -0.18 -0.24 0.04
Lincoln -0.76* -0.65* -0.87* -0.07
Minonk -0.18 -0.15 -0.21 0.15
East
Danville -0.06 -0.05 -0.06 0.11
Hoopeston 0.16 0.14 0.18 -0.16
Pontiac -0.43* -0.39* -0.47* -0.12
Urbana -0.09 -0.04 -0.14 0.10
West-Southwest
Carlinville -0.16 -0.03 -0.29 0.02
Greenville 1 E 0.37 0.41 0.34 -0.06
Griggsville -0.30* -0.27* -0.34* 0.10
Hillsboro -0.06 -0.12 0.01 0.10
Jacksonville -0.19 -0.20 -0.23 0.07
Pana -0.04 0.01 -0.10 0.16
White Hall 1 E -0.09 -0.11 -0.08 -0.04
East-Southeast
Charleston -0.07 -0.06 -0.08 0.05
Effingham -0.09 -0.15 -0.03 0.21
Flora 0.15 0.12 0.19 0.15
Olney -0.13 -0.10 -0.22 0.02
Palestine 0.01 0.05 -0.03 -0.05
Paris Waterworks 0.05 0.11 -0.01 0.17
Windsor -0.15 -0.03 -0.27* 0.01

84

CLIMATE TRENDS IN ILLINOIS

Table A.6. Concluded

Southwest

Anna 1 E -0.18 -0.17 -0.14 0.14
Carbondale -0.82* -0.84* -0.80* 0.00
Du Quoin 4 SE -0.25* -0.22 -0.29* 0.03
Olive Branch -0.23 -0.23 -0.22 -0.07
Sparta -0.21 -0.12 -0.31 -0.03
Southeast

Fairfield -0.10 -0.03 -0.17 0.10
Harrisburg -0.41* -0.39* -0.43* -0.13
McLeansboro -0.17 -0.22 -0.11 -0.07
Mt. Vernon -0.08 -0.21 0.10 0.05
All stations -0.14 -0.14 -0.14 0.04

85

CLIMATE TRENDS IN ILLINOIS

Table A.7. Trends in Spring Climate Parameters (Asterisk Indicates Trends of Statistical Significance)

Mean Mean minimum Mean maximum Total
temperature temperature temperature precipitation
City (°F/decade) (°F/decade) (°F/decade) (inches/decade)
Northwest
Aledo 0.04 0.05 0.14 0.07
Dixon 1 NW 0.06 0.15 -0.12 0.05
Galva 0.08 0.12 0.04 0.18
Mount Carroll -0.08 -0.04 -0.18 0.12
Walnut -0.03 -0.05 -0.01 0.24*
Northeast
Aurora 0.03 0.08 -0.06 0.12
Marengo 0.01 -0.19* 0.21 0.10
Ottawa 4 SW 0.26* 0.37* 0.16 0.22*
West
La Harpe 0.01 -0.05 0.07 0.13
Monmouth 0.15 0.17* 0.13 0.15
Rushville -0.12 -0.18 -0.06 0.24
Central
Decatur 0.14 0.11 0.16 0.03
Lincoln 0.30 0.29* 0.31 0.03
Minonk -0.20 -0.10 -0.30 0.24
East
Danville 0.12 0.11 0.13 0.14
Hoopeston 0.08 0.19* -0.03 -0.03
Pontiac -0.11 -0.07 -0.16 -0.05
Urbana 0.19* 0.19* 0.19* 0.05
West-Southwest
Carlinville -0.03 0.08 -0.14 0.03
Greenville 1 E -0.13 -0.08 -0.18 -0.05
Griggsville -0.13 -0.09 -0.17 0.08
Hillsboro 0.06 0.04 0.08 0.06
Jacksonville -0.09 -0.04 -0.15 0.28*
Pana 0.19* 0.21* 0.18 0.11
East-Southeast
Charleston 0.21 0.26* 0.17 -0.01
Effingham -0.08 0.00 0.03 0.05
Flora 0.26 0.19 0.44 0.16
Olney -0.41* -0.33 -0.49* 0.07
Palestine 0.24 0.31* 0.07 0.13
Paris Waterworks 0.09 0.19 0.00 0.05
White Hall 1 E 0.21 0.28* 0.13 0.12
Windsor 0.21* 0.30* 0.16 -0.23

86

CLIMATE TRENDS IN ILLINOIS

Table A.7. Concluded

Southwest

Anna 1 E 0.06 0.04 0.17 0.32
Carbondale -0.62* -0,53* -0.71* -0.03
Du Quoin 4 SE 0.12 0.15 0.09 0.24
Olive Branch -0.18 -0.09 -0.23* 0.10
Sparta 0:23* 0.23* 0.23 0.06
Southeast

Fairfield 0.01 0.09 -0.09 0.45*
Harrisburg 0.21 0.10 0.24 0.27
McLeansboro -0.09 -0.06 -0.12 0.27
Mt. Vernon -0.02 -0.13 0.08 0.19*
All stations 0.04 0.06 0.02 0.13

87

CLIMATE TRENDS IN ILLINOIS

Table A.8. Trends in Summer Climate Parameters (Asterisk Indicates Trends of Statistical Significance)

City

Northwest

Aledo

Dixon 1 NW
Galva

Mount Carroll
Walnut

Northeast

Aurora
Marengo
Ottawa 4 SW

West

La Harpe
Monmouth

Rushville
Central

Decatur
Lincoln
Minonk

East

Danville
Hoopeston
Pontiac
Urbana

West-Southwest

Carlinville
Greenville 1 E
Griggsville
Hillsboro
Jacksonville
Pana

White Hall 1 E

East-Southeast

Charleston
Effingham

Flora

Olney

Palestine

Paris Waterworks
Windsor

88

Mean
temperature
(°F/decade)

-0.21
0.04
0.18

-0,.22

-0.36*

-0.27
0.00
0.64*

0.02
0.01
-0.27

-0.53*
-0.40*
0.16

0.59*

0.31*
-0.29*

0.10

-0.04
-0.55
0.03
-0.15
-0.15*
0.22
0.63*

0.16
0.37
0.49
-0.34
-0.34
0.38*
0.05

Mean minimum
temperature
(°F/decade)

-0.17
0.16*
0.29*

-0.20

-0.29*

-0.12
-0.11
0.78*

-0.01
0.12*
-0.36*

-0.39*
-0.14
0.18

0.65*

0.42*
-0.07

0.14*

0.15
-0.27
0.04
-0.06
-0.05
0.29
0.75*

0.40*
0.55*
0.39
-0,24
-0.20
0.54*
0.27*

Mean maximum
temperature
(°F/decade)

-0.24
-0.08
0.-06
-0.09
-0.42*

-0.42*
0.02
0.20

0.14
-0.09
-0.18

-0.66*
-0.65*
0.15

0.52
0.19
-0.50*
0.07

-0.22
-0.83
-0.22
-0.23
-0.26*
0.15
0.43

-0.08
0.39
0.67

-0.34

-0.42
0.23

-0.17

Total
precipitation
(inches/decade)

0.10
0.05
0.22
0.00
0.12

0.11
0.32*
0.22

0.14
0.05
0.16

0.17
0.25
0.03

0.53*
0.01

0.37*
0.40*

-0.06
-0.13
0.02
0.03
0.14
0.02
-0.04

0.01
0.01
0.09
-0.06
0.02
0.36*
-0.06

Table A.8. Concluded

Southwest

Anna 1 E
Carbondale

Du Quoin 4 SE
Olive Branch
Sparta

Southeast

Fairfield
Harrisburg
McLeansboro
Mt. Vernon

All stations

0.03
-0.35*
-0.18
-0.28*

0.08

-0.03
-0.43*
-0.15*
-0.10

-0.03

CLIMATE TRENDS IN ILLINOIS

-0.19*
-0.49*
-0.32
-0.44*
-0.04

-0.26
-0.51*
-0.18
-0.30

-0.13

89

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AIR QUALITY TRENDS IN ILLINOIS

AIR QUALITY TRENDS
IN ILLINOIS

Donald F. Gatz
Illinois State Water Survey

INTRODUCTION
Purpose

There are three main purposes for including a compo-
nent on air quality in a comprehensive assessment of
the Illinois environment. The first purpose is simply to
document the current status of Illinois air quality and
any recent trends. Specifically, we will look for
changes (trends) in pollutant concentrations over time
within specific geographic regions, as well as state-
wide, and also show spatial variations of pollutant
concentrations in the Chicago area, the only area of the
state where there are enough sampling sites to permit
plots of spatial variability.

These analyses will reveal whether concentrations of
specific pollutants in Illinois air are getting better, or
getting worse, or staying the same. They may also indi-
cate which areas of the state experience the highest and
lowest pollutant concentrations, and whether such local
or regional conditions are improving or worsening.

The second purpose is to provide the information
necessary for assessments of human and ecosystem
exposure to airborne pollutants. These assessments will
appear in other reports in this series.

The final purpose is to identify gaps and needs in
Illinois air quality monitoring and research.

Scope

This analysis, for the most part, relies on data gener-
ated by routine air quality measurements carried out by
the Illinois Environmental Protection Agency (IEPA),
the state agency charged with monitoring compliance
with state and national air quality standards. It should

be noted that IEPA’s main purpose in making these
measurements is to monitor compliance, not to
document the state’s air quality. Thus, to some extent,
this examination of IEPA data for descriptions of past
and current air quality goes beyond the purpose for
which the data were originally collected. Consequently,
the goal of characterizing air quality over the whole
state may not be completely attainable.

Pollutants Analyzed

The air pollutants to be examined for temporal and
spatial trends include, first of all, the seven criteria
pollutants, i.e., those for which national or state air
quality standards have been set. These include four
gaseous pollutants: ozone (O,), sulfur dioxide (SO,),
nitrogen dioxide (NO,), and carbon monoxide (CO).
There are also standards for three pollutants that occur
as particles: These are lead (Pb); total suspended
particulate matter (TSP), for which there is only a state
standard; and particulate matter with aerodynamic
diameters of 10 micrometers (Um) or less (PM,,), for
which there is only a national standard.

In addition, we will examine measured concentrations
of several additional pollutants for which standards
have not been set. This group includes sulfate and
nitrate ions and the metals arsenic (As), cadmium (Cd),
chromium (Cr), iron (Fe), manganese (Mn), and nickel
(Ni). These pollutants all occur as particles in the
atmosphere, and they are measured by chemically
analyzing the same filters used to collect the TSP
samples. Several additional metals, such as beryllium,
copper, selenium, and vanadium, are currently being
measured or have been measured in the past by IEPA.
These additional metals are not included in this
assessment because their concentrations are mostly
smaller than the detection limit.

Finally, we will summarize occasional (nonroutine)
measurements of organic compounds. These measure-
ments were made by IEPA or published in the scien-
tific literature.

Period of Record

The Illinois data to be analyzed represent, at the
most, the years between 1978 and 1990. Some pol-
lutants were measured only during the later years of
this time period.

91

AIR QUALITY TRENDS IN ILLINOIS

DATA QUALITY

Methods of air pollutant sampling and analysis used by
IEPA conform to U.S. EPA standards. However, it
should be noted that artifacts can occur on certain types
of filters possibly used by IEPA for high volume sam-
pling during the data period. Positive sulfate artifacts
(formation of sulfate on the filter from gaseous pre-
cursors) have been reported (Appel et al., 1984). Both
positive and negative (evaporative loss) artifacts can
occur for nitrate (ibid.).

METHODS
Data Source

The data used in this report were taken from annual
summary reports published by IEPA for the years
1978-1990 (IEPA, 1979-1991). Questionable values
were checked with IEPA personnel (Swinford, personal
communications, 1992 and 1993) and corrected if
necessary.

Creation of Computer Files

Data were entered by hand into computer spreadsheet
and database files from IEPA reports. Sampling site
locations were specified by latitude and longitude.
Spreadsheet data files were also converted to Geo-
graphic Information System (GIS) files using existing
data conversion software. Site locations in the Chicago
area were verified by comparison of locations plotted
on GIS maps against road atlas maps, and corrected
when necessary.

Statistical Methods

Box Plots. Box plots were used to convey information
about the distribution of a particular pollutant and its
time-averaged concentrations (e.g., annual mean,
highest 24-hr average, or highest 1-hr average) over
sampling sites in a specific geographical region for a
specific year. For a given year, a box plot shows the
10th, 25th, 50th, 75th, and 90th percentiles, respec-
tively, as the low “whisker”, the bottom of the “box”,
the line across the box, the top of the box, and the
upper whisker. Individual values outside the 10th and
90th percentiles were also plotted as individual points.
Thus, a plot of a series of boxes representing a series of

92

years visually shows how various percentiles of the
distribution of pollutant concentration change over
time. In addition to showing plots to give a visual im-
pression of time trends, we have assessed the statis-
tical significance, or lack thereof, of changes in
pollutant concentrations over time.

Trend Analysis. Time sequences of pollutant con-
centrations were tested for statistical significance of
time trends over the entire period of record using the
nonparametric Spearman Rank Correlation Coefficient
as described by Snedecor and Cochran (1980). This is
the same trend test used by IEPA to detect the statisti-
cal significance of time trends in pollutant concentra-
tions. Please note that the critical values of the rank
correlation coefficient for significance at various
confidence levels given by IEPA (e.g., IEPA, 1991)
differ somewhat from critical values given by tables
A11(i) and A11(ii) in the appendices of Snedecor and
Cochran (1980), which were used in this study. The test
was applied to the series of medians of the annual
distributions over sites within each geographic area.

Where significant time trends were found, average an-
nual percent changes were calculated by dividing the
slope by the concentration computed for 1984 (the mid-
dle year of the 1978-1990 series). The 1984 concentra-
tion was computed from the linear regression line fitted
to the time series of median regional concentrations.

Regional Differences. One-way analysis of variance
was used to test for differences in mean pollutant
concentrations between independent regions; i.e., the
Chicago area, the Metro East area, and the remainder
of the state. Any data sets that failed the Bartlett’s test
for homogeneity of variance were also analyzed by the
distribution-free Kruskal-Wallace one-way analysis of
variance. Differences between individual regions were
evaluated from pairwise comparison probabilities
computed from the Tukey HSD multiple comparisons
procedure. These tests were all carried out using
SYSTAT (Wilkinson, 1990).

TIME TRENDS
IN POLLUTANT CONCENTRATIONS

Trends in pollutant concentrations over time are the
major emphasis of this part of the Air Resources volume.
Table 1 presents computed median pollutant concentra-
tions from the distribution of site values for each
geographical region and year. Table 2 presents results
of statistical tests for significance of time trend for all

Table 1. Air Pollutant Concentration Medians, from the Distribution of Individual Sampling Site Values, for Specific Geographic
Regions and Years

Chicago Metro East Remainder of
Variable Year Whole state area area State
1-hr maximum CO, ppm 1978 11.10 10.85 ---" 12.25
1979 12.10 12.10 --- 16.00
1980 12.75 12.95 --- 12.75
1981 16.10 12.65 --- 18.40
1982 13.50 10.40 --- 14.50
1983 14.15 14.40 --- 12.50
1984 14.00 13.00 --- 14.85
1985 9.95 9.65 --- 10.30
1986 8.90 8.45 --- 11.30
1987 9.75 8.80 --- 11.60
1988 9.30 9.10 --- 11.35
1989 8.60 7.30 --- 12.90
1990 9.50 8.90 --- 9.95
8-hr maximum CO, ppm 1978 7.65 7.15 --- TIS
1979 8.70 8.60 --- 9.40
1980 7.95 7.35 --- 8.10
1981 9.00 8.80 --- 9.50
1982 6.90 6.10 --- 7.20
1983 9.30 10.90 --- 6.30
1984 8.90 8.90 --- 9.10
1985 6.15 5.55 --- 6.35
1986 5.80 5.45 --- 5.90
1987 5.30 5.30 --- 5.30
1988 5.30 5.30 --- 5.75
1989 5.10 4.80 --- 7.35
1990 5.30 3.70 --- 5.60
Annual mean Pb, pg/M? 1979 0.430 0.425 0.650 0.340
1980 0.320 0.370 0.390 0.225
1981 0.260 0.285 0.345 0.160
1982 0.290 0.290 0.710 0.160
1983 0.285 0.280 0.400 0.150
1984 0.250 0.240 0.320 0.130
1985 0.150 0.130 0.265 0.070
1986 0.080 0.080 0.220 0.050
1987 0.060 0.060 0.260 0.035
1988 0.050 0.050 0.170 0.020
1989 0.040 0.040 0.165 0.025
1990 0.050 0.055 0.160 0.020

Note:

*--- Insufficient data at enough sampling sites to justify computing a median.

93

AIR QUALITY TRENDS IN ILLINOIS

Table 1. (Continued)

Chicago Metro East Remainder of
Variable Year Whole state area area State
1-hr maximum NO,, ppm 1978 0.1760 0.2250 --- ee
1979 0.1380 0.1460 --- oe
1980 0.1480 0.1480 --- ws
1981 0.2015 0.2015 --- nee
1982 0.1320 0.1320 = sof
1983 0.1320 0.1470 --- —s
1984 0.1160 0.1290 -—- ei
1985 0.1330 0.1340 -—- pad
1986 0.1060 0.0990 --- ates
1987 0.1150 0.1150 --- ae
1988 0.1255 0.1290 --- ann
1989 0.1125 0.1130 --- bose
1990 0.0885 0.0890 --- =
24-hr maximum NO,, ppm 1978 0.0970 0.0970 --- =
1979 0.0940 0.0950 --- =
1980 0.0965 0.0965 --- =
1981 0.0740 0.0740 -—- —_
1982 0.0850 0.0850 --- ons
1983 0.0800 0.0900 o-- o
1984 0.0570 0.0570 —_ a3
1985 0.0640 0.0650 --- &.2
1986 0.0550 0.0560 --- =
1987 0.0590 0.0595 --- ---
1988 0.0625 0.0650 --- BA
1989 0.0615 0.0630 --- ud
1990 0.0480 0.0495 --- ee
Annual mean NO,, ppm 1978 0.0450 0.0450 --- —
1979 0.0510 0.0510 --- Pvt
1980 0.0470 0.0470 --- =e
1981 0.0420 0.0420 --- me
1982 0.0385 0.0385 --- =
1983 0.0290 0.0300 --- ae
1984 0.0280 0.0290 --- ---
1985 0.0270 0.0280 --- aes
1986 0.0240 0.0250 --- ae
1987 0.0240 0.0260 -- =
1988 0.0230 0.0220 --- —_
1989 0.0270 0.0270 --- =e
1990 0.0240 0.0250 --- =

Note:

*.-- Insufficient data at enough sampling sites to justify computing a median.

94

AIR QUALITY TRENDS IN ILLINOIS

Table 1. (Continued)

Chicago Metro East Remainder of
Variable Year Whole state area area State
1-hr maximum O,, ppm 1978 0.1370 0.1510 0.1710 0.1175
1979 0.1180 0.1215 0.1265 0.1030
1980 0.1210 0.1255 0.1420 0.1050
1981 0.1145 0.1415 0.1230 0.0980
1982 0.1100 0.1170 0.1180 0.0940
1983 0.1370 0.1580 0.1540 0.1065
1984 0.1255 0.1270 0.1380 0.1035
1985 0.1110 0.1145 0.1270 0.1005
1986 0.1110 0.1110 0.1310 0.0985
1987 0.1215 0.1390 0.1260 0.1060
1988 0.1270 0.1275 0.1465 0.1130
1989 0.1100 0.1130 0.1165 0.1030
1990 0.0930 0.0930 0.1180 0.0900
3-hr maximum SO,, ppm 1978 0.1460 0.1125 --- 0.1815
1979 0.1320 0.1310 --- 0.1340
1980 0.1355 0.1450 --- 0.1170
1981 0.1350 0.1150 --- 0.1505
1982 0.1240 0.1160 --- 0.1450
1983 0.0980 0.0900 --- 0.1510
1984 0.1195 0.0955 --- 0.1515
1985 0.1195 0.1090 --- 0.1365
1986 0.1210 0.1020 --- 0.1990
1987 0.1530 0.1310 --- 0.1880
1988 0.1360 0.0880 --- 0.1860
1989 0.1245 0.0870 --- 0.2410
1990 0.1115 0.0660 --- 0.1340
24-hr maximum SO,, ppm 1978 0.0495 0.0435 --- 0.0720
1979 0.0795 0.0820 --- 0.0630
1980 0.0465 0.0400 --- 0.0600
1981 0.0640 0.0650 --- 0.0550
1982 0.0560 0.0615 --- 0.0530
1983 0.0420 0.0410 --- 0.0645
1984 0.0530 0.0460 --- 0.0670
1985 0.0505 0.0450 --- 0.0690
1986 0.0505 0.0440 --- 0.0850
1987 0.0510 0.0440 --- 0.0565
1988 0.0515 0.0420 --- 0.0850
1989 0.0455 0.0310 --- 0.0640
1990 0.0400 0.0260 --- 0.0480

Note:

*.-- Insufficient data at enough sampling sites to justify computing a median.

AIR QUALITY TRENDS IN ILLINOIS

Table 1. (Continued)

Chicago Metro East Remainder of
Variable Year Whole state area area State
Annual mean SO,, ppm 1978 0.0090 0.0075 --- 0.0150
1979 0.0120 0.0120 --- 0.0110
1980 0.0080 0.0080 --- 0.0095
1981 0.0090 0.0090 --- 0.0075
1982 0.0070 0.0080 --- 0.0070
1983 0.0080 0.0090 --- 0.0080
1984 0.0080 0.0080 --- 0.0080
1985 0.0080 0.0080 --- 0.0085
1986 0.0080 0.0060 --- 0.0085
1987 0.0080 0.0080 --- 0.0090
1988 0.0080 0.0070 --- 0.0080
1989 0.0070 0.0070 --- 0.0075
1990 0.0070 0.0070 0.0070
24-hr maximum TSP, pg/M’ 1978 189.0 189.0 209.0 183.5
1979 167.5 170.0 217.0 145.0
1980 155.0 156.0 202.0 137.0
1981 161.5 143.5 234.0 163.0
1982 143.5 146.5 166.5 134.5
1983 438.0 495.5 218.0 319.0
1984 139.0 137.0 167.0 139.0
1985 253.0 297.0 179.0 208.0
1986 119.0 123.0 218.0 107.0
1987 141.0 146.0 183.5 131.5
1988 169.0 174.0 188.0 138.0
1989 188.0 188.0 231.0 137.0
1990 220.0 220.0 158.0 237.0
Annual mean TSP, pg/M? 1978 64.0 63.0 85.5 63.0
1979 68.0 68.0 89.5 62.0
1980 68.0 66.0 84.0 65.0
1981 63.0 60.0 84.0 61.0
1982 51.0 52:5 60.0 46.5
1983 53.0 55.0 63.0 50.0
1984 49.0 51.0 63.0 43.0
1985 48.0 50.0 58.5 43.0
1986 48.0 50.0 68.0 44.0
1987 52.0 53.0 76.0 48.0
1988 56.0 60.0 84.0 51.0
1989 70.5 67.0 89.5 50.0
1990 63.0 65.0 78.0 52.0

Note:

*.-- Insufficient data at enough sampling sites to justify computing a median.

96

AIR QUALITY TRENDS IN ILLINOIS

Table 1. (Continued)

Chicago Metro East Remainder of
Variable Year Whole state area area State
Annual mean nitrate, 1g/M* 1978 5.50 5.50 5.60 5.60
1979 5.40 5.80 5.20 5.20
1980 5.40 4.75 5.50 5.50
1981 5.50 5.80 5.30 5.30
1982 5.15 5.40 4.50 4.50
1983 6.00 6.30 4.90 4.90
1984 5.80 6.00 4.20 4.20
1985 5.00 5.30 3.85 3.85
1986 5.25 5.50 4.10 4.10
1987 5.50 5.50 5.70 5.70
1988 5.30 5.45 4.80 4.80
1989 5.40 5.80 5.05 5.05
1990 4.60 4.80 4.55 4.55
Annual mean sulfate, g/M° 1978 11.90 12.10 13.40 10.50
1979 11.90 11.85 13.50 10.75
1980 12.30 12.55 12.90 11.05
1981 12.00 12.00 12.90 11.60
1982 10.45 10.05 11.55 10.30
1983 12725 12.20 13.80 10.95
1984 10.80 10.85 11.20 9.10
1985 10.20 10.20 11.65 9.00
1986 10.10 10.05 10.80 8.80
1987 10.20 10.00 14.20 10.00
1988 11.00 10.80 12.10 9.60
1989 10.80 10.50 12.60 9.95
1990 10.70 10.30 12.45 9.50
Annual mean As, yg/M? 1978 0.0030 0.0030 0.0080 0.0020
1979 0.0020 0.0020 0.0040 0.0020
1980 0.0020 0.0020 0.0050 0.0020
1981 0.0020 0.0020 0.0040 0.0020
1982 0.0020 0.0015 0.0120 0.0015
1983 0.0020 0.0015 0.0060 0.0015
1984 0.0010 0.0010 0.0080 0.0010
1985 0.0000 0.0000 0.0040 0.0000
1986 0.0010 0.0010 0.0060 0.0000
1987 0.0010 0.0010 0.0060 0.0010
1988 0.0020 0.0010 0.0060 0.0010
1989 0.0010 0.0010 0.0040 0.0010
1990 0.0010 0.0010 0.0045 0.0010

Note:

*.-- Insufficient data at enough sampling sites to justify computing a median.

97

AIR QUALITY TRENDS IN ILLINOIS

Table 1. (Continued)

Chicago Metro East Remainder of
Variable Year Whole state area area State
Annual mean Cd, pg/M? 1978 0.00230 0.00230 0.00760 0.00190
1979 0.00190 0.00175 0.00480 0.00165
1980 0.00070 0.00070 0.00450 0.00045
1981 --- --- --- ---
1982 0.00125 0.00055 0.00900 0.00010
1983 0.00040 0.00035 0.00540 0.00015
1984 0.00200 0.00100 0.00500 0.00150
1985 0.00100 0.00000 0.00450 0.00100
1986 0.00300 0.00300 0.00900 0.00100
1987 0.00200 0.00200 0.01000 0.00100
1988 0.00300 0.00300 0.00500 0.00000
1989 0.00000 0.00000 0.00450 0.00000
1990 0.00200 0.00200 0.00400 0.00000
Annual mean Cr, pg/M? 1985 0.0010 0.0010 -- 0.0010
1986 0.0095 0.0100 -- 0.0035
1987 0.0060 0.0070 -- 0.0035
1988 0.0050 0.0050 -- 0.0040
1989 0.0040 0.0050 -- 0.0030
1990 0.0010 0.0040 -- 0.0000
Annual mean Fe, yg/M? 1978 0.850 0.845 1.410 0.740
1979 0.985 0.990 1.785 0.805
1980 0.680 0.710 1.110 0.540
1981 0.605 0.600 1.040 0.580
1982 0.600 0.600 1.365 0.510
1983 0.810 0.810 1.360 0.640
1984 0.690 0.690 1.270 0.500
1985 0.640 0.640 1.435 0.375
1986 0.720 0.720 2.230 0.440
1987 0.840 0.820 2.550 0.530
1988 0.830 0.855 2.700 0.570
1989 1.100 1.245 2.145 0.510
1990 0.980 1.265 2.150 0.600

Note:

*.-- Insufficient data at enough sampling sites to justify computing a median.

98

AIR QUALITY TRENDS IN ILLINOIS

Table 1. (Concluded)

Chicago Metro East Remainder of
Variable Year Whole state area area State
Annual mean Mn, ug/M? 1978 0.0435 0.0405 0.1080 0.0590
1979 0.0560 0.0540 0.1330 0.0580
1980 0.0380 0.0355 0.0830 0.0395
1981 0.0380 0.0345 0.0770 0.0410
1982 0.0345 0.0325 0.0850 0.0335
1983 0.0450 0.0460 0.0720 0.0325
1984 0.0430 0.0475 0.0700 0.0320
1985 0.0450 0.0450 0.1040 0.0295
1986 0.0415 0.0420 0.1970 0.0290
1987 0.0430 0.0430 0.2710 0.0370
1988 0.0440 0.0410 0.2760 0.0360
1989 0.0480 0.0480 0.2185 0.0320
1990 0.0470 0.0555 0.2130 0.0360
Annual mean Ni, pg/M? 1984 0.0020 0.0020 -- 0.0030
1985 0.0010 0.0010 -- 0.0020
1986 0.0070 0.0070 -- 0.0040
1987 0.0050 0.0050 -- 0.0040
1988 0.0050 0.0070 -- 0.0030
1989 0.0030 0.0040 -- 0.0025
1990 0.0030 0.0045 -- 0.0020

99

AIR QUALITY TRENDS IN ILLINOIS

Table 2. Computed Mean Annual Percent Change in Regional Concentrations, Assuming a Linear Trend.
Significance of Time Trend was Based on Spearman Rank Correlation Coefficients.

Chicago Metro East Remainder
Species Observation Whole state area area of state
(oe) 1-hr maximum =e iS eee --- 2) Ff
(oe) 8-hr maximum Alaa =5'n) =< e/a
Pb Annual mean =20,5.~ -21.2° =12:6) -24.8°"°
NO, 1-hr maximum EAA Si --- ---
NO, 24-hr maximum Sy 5.2 --- ---
NO, Annual mean 7.1L. =6:80 -- —
OF 1-hr maximum n.s.(D”’) n.s.(D"’) n.s Is.
so, 3-hr maximum n.s. 3.5" --- ns.
SO, 24-hr maximum n.s. n.s. --- N.S.
so, Annual mean -2.6— Oh --- ns.
TSP 24-hr maximum nS. nS. ns. ns.
TSP Annual Geo mean ns. ns. ns. ns.
NO, Annual mean n.s. nS. ns. -2.1°
SOm Annual mean aii -1.6 ns. =1k4>
As Annual mean epee itil Gry Is. 95
Cd Annual mean Is. ns. Is. 27
Cr Annual mean ns. ns. --- ns.
Fe Annual mean ns. ns. +6.0" ns.
Mn Annual mean ns. ns. +10.0° 44°
Ni Annual mean ns. ns. --- Ns.

Notes:

n.s. = not significant.

D = downward trend (not quantifiable by method used).

Significance levels: “= 5 percent, “ = 2 percent, and “* = 1 percent.

--- = not enough sites to estimate trends. (When there were not enough Metro East sites for a trend estimate, the Metro East
sites are included with the remainder of the state.)

' O, data were tested for time trend both before and after accounting for surface temperature influences. Results in
parentheses show trend and significance after accounting for surface temperature.

100

pollutants examined. Time trends for each pollutant are
graphed and the text discusses the results of the related
statistical tests in table 2. The graphs show concentra-
tions at several percentile points on the distribution of
particular pollutant concentration statistics (i.e., annual
means or 24-hr maxima) over all sampling sites within
a given geographic area for each of a series of years,
and how these percentiles of concentration change over
the period 1978-1990. The box plot is a convenient
graphical device to use for this purpose, because it
shows several concentration percentiles simultane-
ously. The concentrations plotted in a single graph are
averages over a particular time period, such as one
hour, 24 hours, or an entire year. For the criteria
pollutants, these averaging times correspond to those
for which the standards are written for each particular
pollutant, so they vary from one pollutant to the next,
and a given pollutant may have two or more graphs,
corresponding to two or more averaging times.

It will become apparent later that the number of sam-
pling sites for a given pollutant in any geographical
area changed from year to year as new sites were
installed or previously used sites were taken out of
service. In recent years the number of sampling sites
has dropped markedly for most pollutants. This sug-
gests an alternative approach to the analysis of time
trends: analysis of time trends only at sites with long-
term records. This approach was rejected in favor of
the analysis of time trends in regional medians because
very few sites had long-term records, and such sites
could not be inferred to represent broad geographic areas.

Data used in the following statistical tests are shown in
table 1.

Results of the statistical tests for the significance of
trends are shown in table 2, and the time sequences of
box plots in figures 1-30. Each figure shows results for
up to four geographical areas, depending on whether
the number of sampling sites in the area was sufficient
to justify a separate plot. Each figure shows results for
the entire state and the Chicago area, which is the
portion of the U.S. EPA’s Air Quality Control Region
(AQCR) 67 in Illinois. Many figures also show results
for the Metro East area on the Illinois side of the
Mississippi River across from St. Louis. This is the
Illinois portion of AQCR 70. The fourth geographic
region shown is the remainder of the state, excluding
the Chicago area and the Metro East area (if shown
separately). If the Metro East area is not shown
separately, then the region labeled as “remainder of
state” also includes any data from the Metro East area.

AIR QUALITY TRENDS IN ILLINOIS

Criteria Pollutants

Carbon Monoxide (CO). Results for CO are shown in
table 2 and figures | and 2. The table shows trends
toward decreasing concentrations significant at the 5
percent level, or better, for both 1-hr (figure 1) and 8-hr
(figure 2) maxima during the 1979-1990 period, in all
three geographic areas with sufficient data to test. The
overall linear trends range from -2.7 percent to -5.3
percent. Figure 1 suggests generally higher and slightly
increasing 1-hr maximum concentrations at all per-
centile levels represented by the box plots during the
1978-1984 period, followed by lower and decreasing
concentrations from 1985 onward. This pattern appears
in all three geographic areas shown in the figure. The
1-hr CO standard, 35 parts per million (ppm), appears
not to have been exceeded anywhere in the state during
the 13 years of record.

On the other hand, the 8-hr maximum concentrations of
CO (figure 2) exceeded the standard, 9 ppm, repeatedly
until very late in the 1979-1990 period. Figure 2 shows
a pattern of higher concentrations at all percentiles
during the 1978-1984 period, followed by lower and
decreasing concentrations from 1985-1990, in all geo-
graphic areas.

The statistical comparison of CO concentrations
between geographical regions found a significant
difference (5 percent) between the Chicago area and
the remainder of the state for the 1-hr (figure 1), but
not for the 8-hr (figure 2) averaging time. For both
averaging times, however, it appears that early in the
1979-1990 period the Chicago area contributed the
highest individual concentrations, whereas the highest
values during the later years of the period came from
elsewhere in the state.

For both CO averaging times, the number of sampling
sites (N) remained relatively constant, with about 15
sites in the entire state, 6 to 11 sites in the Chicago
area, and 6 or 7 sites elsewhere.

Lead (Pb). Airborne Pb concentrations decreased dra-
matically (figure 3) during the 1979-1990 period, due
in part to the phase-out of leaded gasoline, beginning in
about 1975 (IEPA, 1991). Table 2 shows decreasing
linear trends between -12.6 percent and -24.8 percent
per year in the four geographical regions. While the
trends are significant at the 1 percent level in all four
regions, the rate of decrease in the Metro East area is
considerably smaller than the others. Figure 3 graph-
ically shows the marked drops in all the concentrations

101

AIR QUALITY TRENDS IN ILLINOIS

Chicago area 4

fe)

Oo

bhag

6 11 #11 11

Concentration, ppm

Note: The national and state
primary standards for 1-hr
average CO are both 35 ppm.
These are not to be exceeded
more than once per year.

N = number of samples.

78 79 80 81 82 83 84 85 86 87 88 89 90
Year

Figure 1. Trends in 1-hr maximum CO concentrations in Illinois.

Chicago area

Whole state

78 79 80 81 82 83 84 85 86 87 88 89 90

Note: The Federal and State
Primary Standards for 8-hr
average CO are both 9 ppm.
These are not to be exceeded
more than once per year.

N= number of samples.

Concentration, ppm

78 79 80 81 82 83 84 85 86 87 88 89 90

Year

Figure 2. Trends in 8-hr maximum CO concentrations in Illinois.

102

3

Concentration, wg/M

Concentration, ppm

AIR QUALITY TRENDS IN ILLINOIS

Whole state Chicago area

N:52 60 60 35 33 37 27 21 Th Ed 8

Remainder of state

Metro-East area

N:25 26. 21

79 80 81 82 83 84 85 86 87 88 89 90 79 80 81 82 83 84 85 86 87 88 89 90
Year

Note: Standards are based on the quarterly arithmetic mean. See text.
N = number of samples.

Figure 3. Trends in annual mean Pb concentrations in Illinois.

~o Whole state Chicago area
0.30

0.25

0.20

0.15 at nge ata th ine

ep eao eGand ae BTOgSe.
0.05 a7 ae a ee oa soe ee Te H 4 5 9 9 10 9 8 13 13 14
sic 78 79 80 81 82 83 84 85 86 87 88 89 90 78 79 80 81 82 83 84 85 86 87 88 89 90

Year

Note: There are no national or state standards for 1-hr average NO,,.

Figure 4. Trends in 1-hr maximum NO, concentrations in Illinois.

AIR QUALITY TRENDS IN ILLINOIS

& Whole state Chicago area
a.

[Si

S
fe

w

i

Se

8)

cS)

S

fe)
oO

78 79 80 81 82 83 84 85 86 87 88 89 90 78 79 80 81 82 83 84 85 86 87 88 89 90
Year
Note: There are no national or state standards for 24-hr average NO,,.
N = number of samples.
Figure 5. Trends in 24-hr maximum NO, concentrations in Illinois.

e Whole state Chicago area
roe

rol

S 6 Standard zs

y=] a 5 =)

w

= H

O

3) fe)

) g

fe)

O 36.32 21) (21 78419) 18°42 49)) (a) a7 sy 8

78 79 80 81 82 83 84 85 86 87 88 89 90 78 79 80 81 82 83 84 85 86 87 88 89 90
Year

Note: The national and state standards for annual mean NO, are 0.053 ppm.
N= number of samples.

Figure 6. Trends in annual mean NO, concentrations in Illinois.

104

Concentration, ppm

Concentration, ppm

0.30

0.25

0.20

0.15

0.10

0.05

0.00
0.30

0.25

0.20

0.15

0.10

0.05

0.00

0.8
0.7

AIR QUALITY TRENDS IN ILLINOIS

Whole state Chicago area

78 79 80 81 82 83 84 85 86 87 88 89 90 78 79 80 81 82 83 84 85 86 87 88 89 90
Year

Note: The national and state primary standards for 1-hr O, are 0.12 ppm.
N = number of samples.

Figure 7. Trends in 1-hr maximum O, concentrations in Illinois.

Whole state “Chicago area

1@)
ie)

Lebeagiudiges

78 79 80 81 82 83 84 85 86 87 88 89 90

0.8
0.7
: 0.6
Note: There are no national or
state primary standards for 3-hr a
maximum SO, concentration. 0.4
The secondary national standard 0.3
for 3-hr maximum SO, concen- 0.2
tration is 0.5 ppm. N = number 04
of samples. ;

78 79 80 81 82 83 84 85 86 87 88 89 90
Year

Figure 8. Trends in 3-hr maximum SO, concentrations in Illinois.

AIR QUALITY TRENDS IN ILLINOIS

106

Concentration, ppm

Concentration, ppm

0.30

Whole state Chicago area

0.25

0.20
0.15 80

O s.
0.10 if 2
0.05 A 4 ale :
42°29 31°31 30

78 79 80 81 82 83 84 85 86 87 88

0.00
30 30 27

Note: The national and state
primary standards for 24-hr
maximum SO, concentration
are both 0.14 ppm. N = number

of samples.
fe)

17 16 17 16 16 16 16 16

78 79 80 81 82 83 84 85 86 87 88 89 90
Year

Figure 9. Trends in 24-hr maximum SO, concentrations in Illinois.

0.05

Whole state Chicago area

0.04
0.03
0.02

0.01

0.00 N:52 52 39 25 27 29 27 29 27 26 26 27 26 N:38 36 25 11 11 13 11 13 11 11°11 #11 «11

78 79 80 81 82 83 84 85 86 87 88 89 909 5

Remainder of state
0.04
Note: The national and state 0.03
primary standards for annual
arithmetic mean SO, concen- 0.02
ration are 0.03 ppm. N = number

of samples. 0.01

0.00 N:14 16 14 14 16 16 16 16 16 15 15 16 15

78 79 80 81 82 83 84 85 86 87 88 89 90
Year

Figure 10. Trends in annual mean SO, concentrations in Illinois.

3

Concentration, g/M

3

Concentration, wg/M

AIR QUALITY TRENDS IN ILLINOIS

Whole state Chicago area

oO
O

78 79 80 81 82 83 84 85 86 87 88 89 90 78 79 80 81 82 83 84 85 86 87 88 89 90
Year

Note: The state primary standard for 24-hr maximum TSP concentration is 260 gM’.
N = number of samples.

Figure 11. Trends in 24-hr maximum TSP concentrations in Illinois.

Whole state Chicago area

Remainder of state

r 3
78 79 80 81 82 83 84 85 86 87 88 89 90 78 79 80 81 82 83 84 85 86 87 88 89 90
Year

Note: The state primary standard for annual geometric mean TSP is 75 pg/M°
N = number of samples.

Figure 12. Trends in annual geometric mean TSP concentrations in Illinois.

107

AIR QUALITY TRENDS IN ILLINOIS

108

3

Concentration, ug/M

Concentration, wg/M

Whole state

Chicago area
fe)

1987 1988 1989 1990

Note: The national primary
standard for 24-hr.average
PM,, is 150 yg/M . It is not
to be exceeded more than

once per year. N = number
of samples.

-50
Year

1987 1988 1989 1990

Figure 13. Trends in 24-hr maximum PM_,, concentrations in Illinois.

Whole state Chicago area

Standard

1987 1988 1989 1990 100
80
Note: The national primary
standard for annyal average 60
PM,, is 50 wg/M . It is not to

be exceeded more than once 40
per year. N = number of samples.

20

1987 1988 1989 1990

0
Year

Figure 14. Trends in annual mean PM, concentrations in Illinois.

AIR QUALITY TRENDS IN ILLINOIS

Whole state

Chicago area

8

age

ie)
=
ee
fe)
2
= N:0 0 0 0 0 0 O 51 40 41 36 18 19 N:0 0 0 0 0 0 O 39 209 3 25 9 8
©
= Metro-East area Remainder of state
5)
=
fe)
, ;
"G
Mo. oO. G.. 0. (0 0) 0" 6" 5. bo! 4:8
78 79 80 81 82 83 84 85 86 87 88 89 90 78 79 80 81 82 83 84 85 86 87 88 89 90
Year
Note: There are no national or state standards for NO,. N = number of samples.
Figure 15. Trends in 24-hr maximum NO, concentrations in Illinois.
1 .
Whole state Chicago area
fe) fe) é =
: ye petaned a
2 e) j Gul 50
[ole) 3}

N:110 12793 86 30 46 47 49 36 39 27 15 17 N:72 80 48 45 20 35 36 37 26 29 18 7 8

Remainder of state

zoe!

Metro-East area

2286 = =o

Concentration, wg/M

=| aaa

ae fag e

5

3.17 17 15 46 &§ 6 &§ 56 & 4 4

O- NWA HAMDNWO OOH NWA UAAHMHN WOO

78 79 80 81 82 83 84 85 86 87 88 89 90 78 79 80 81 82 83 84 85 86 87 88 89 90
Year :
Note: There are no national or state standards for NO, . N = number of samples.

Figure 16. Trends in annual mean NO, concentrations in Illinois.

109

AIR QUALITY TRENDS IN ILLINOIS

Concentration, wg/M>

3

Concentration, ~g/M

110

Whole state Chicago area

N:0 0 0 0 0 O O 51 40 41 36 18 19 N:0 0 0 0 0 0 0 39 29 30 25 9 8

Metro-East area Remainder of state

spat

N:G. 0, 6. 0, 0. 0.0, 6. 6.6.6. 5 5

78 79 80 81 82 83 84 85 86 87 88 89 90 78 79 80 81 82 83 84 85 86 87 88 89 90
Year

Note: There are no national or state standards for SO i N = number of samples.

Figure 17. Trends in 24-hr maximum SO,? concentrations in Illinois.

Whole state

sisigeabees

N:124 127 93 86 30 46 47 49 36 39 27 15 17

Chicago area

isSegestes

N:80 80 48 45 20 35 36 37 26 29 18 7 8

Metro-East area Remainder of state

a Pe babs eeec ido.

NiS@17*17"16" 4" 6: Ge 6 Ce Orel « 4 N:29 30 28 26 6 6 66 5 5 4 4 §

78 79 80 81 82 83 84 85 86 87 88 89 90 78 79 80 81 82 83 84 85 86 87 88 89 90

Year
Note: There are no national or state standards for SO bx N = number of samples.

Figure 18. Trends in annual mean SO? concentrations in Illinois.

Concentration, yg/M°

Concentration, 4.g/M°

AIR QUALITY TRENDS IN ILLINOIS

0.7
0.6 ; Whole state re) Chicago area
0.5

0.15

Analysis of individual
filters for metals
began in 1985.

WO 00.20 0.0 0. 0. 8.6 "6 6's. 5

78 79 80 81 82 83 84 85 86 87 88 89 90 78 79 80 81 82 83 84 85 86 87 88 89 90
Year

Note: There are no national or state standards for As. N = number of samples.

Figure 19. Trends in 24-hr maximum As concentrations in Illinois.

0.05
Whole state Chicago area
0.04
0.03
0.02 i ae
1e) fe) fe)
ry ES BE a ae: 8
Zo
0.00 5 AS fmm Lefora.
N:104 86 73 66 12 15 15 17 21 22 20 14 17 N:60 39 28 25 2 4 4 5 11 12 11 6 8
0.05
Metro-East area Remainder of state
0.04
fo)
0.0
. fe)
0.02 -
oor = Hada &
ell
0.00 ae Be8 =2988ee%o1h002
N18 17 17 18 4 85 6 6 5 6 & 4 4 N:29 30 28 26 6 6 6 6 &6§ 6 44 &
78 79 80 81 82 83 84 85 86 87 88 89 ee 78 79 80 81 82 83 84 85 86 87 88 89 90
ear

Note: There are no national or state standards for As. N = number of samples.

Figure 20. Trends in annual mean As concentrations in Illinois.

111

AIR QUALITY TRENDS IN ILLINOIS

112

3

Concentration, wg/M

3

Concentration, 4.g/M

Whole state

Chicago area

Analysis of individual
filters for metals
began in 1985.

Remainder of state

NiO: 0) 0° 0! JOO) O56: So 5) Siege is

78 79 80 81 82 83 84 85 86 87 88 89 90 78 79 80 81 82 83 84 85 86 87 88 89 90
Year

Note: There are no national or state standards for Cd. N = number of samples.

Figure 21. Trends in 24-hr maximum Cd concentrations in Illinois.

0.05
Whole state Chicago area
0.04
0.03
0.02+ °
Bie _ fo)
0.01 8 fe)
(be! Oaagergad $ge_____288a

N:104 85 73 0 12 15 15 17 36 39 27 15 17 N:60 38 28 0 2 4 4 5 26 29 18 7

0.05
Metro-East area Remainder of state

0.04

0.03

0.02

fe)

“tege -tb,0 00g] [f

0.00 = 23 §aga— e——=o
NSS 17-717 0 4 8" Sie Gieeb eS) GS) Fe) 54 N:29 30 28 O eye. "Ss ere 6
78 79 80 81 82 83 84 B5 BG B7 BB 89 90 78 79 80 81 82 83 84 85 86 87 88 89 90

Year

Note: There are no national or state standards for Cd. N = number of samples.

Figure 22. Trends in annual mean Cd concentrations in Illinois.

AIR QUALITY TRENDS IN ILLINOIS

0.30
Whole state Chicago area
0.25
0.20
0.15
o 0.10
=
G 0.05
*
- 0.00 :
= 40 1 36 30
2
ow
5 ,
c Remainder of state
Q fe)
fe) Note: Cr measurements began
O in 1985. There are no national
or state standards for Cr. fo) ra)
N = number of samples. |
N=12 11 11 W 9 11
85 86 87 88 89 90
Year
Figure 23. Trends in 24-hr maximum Cr concentrations in Illinois.
0.06
Whole state i
dan Chicago area
0.04 °
0.03 oo
% 0.02 O° O
0.00 Be
c N=17 36 39 27 15 17 29 18 if 8
fe)
= 85 86 87 88 89 90 oo,
= inder o
= nae Remain if state
cS)
5 Note: Cr measurements began 0.04
(3 in 1985. There are no national

or state standards for Cr. ae

fe)
fo)
N = number of samples. 0.02 O fe) O
N=12 11 11 11 9 11

85%..1860....872...86.2..69.4 (90

0.00

Year

Figure 24. Trends in annual mean Cr concentrations in Illinois.

AIR QUALITY TRENDS IN ILLINOIS

50
Whole state Chicago area
40
30
20 ie I 33
Lt) Analysis of individual
= 10 filters for metals
oO) began in 1985.
ay
S| 4 N:0 0 0 0 0 O O 36 26 29 18 8 9
=
—
= Metro-East area Remainder of state
S)
e
fo)
O
NO”. (OF 30" \OrcO" GneO) 1GaeS) 5: $5), 4. 6
78 79 80 81 82 83 84 85 86 87 88 89 90 78 79 80 81 82 83 84 85 86 87 88 89 90
Year
Note: There are no national or state standards for Fe. N = number of samples.
Figure 25. Trends in 24-hr maximum Fe concentrations in Illinois.
Whole state Chicago area
0 O fo)
o 2 Oo 0°
= 5 ie) [olne)
BS oo, 0
< esesesee88iae
5 N:124104 93 86 30 46 47 48 36 39 27 16 17
=
= :
5 Metro-East area Remainder of state
é)
e
fe)
O

dda H8

$Seeendque-—=

NSS 167 15 ares 8 es Ses 64 N:20.30 28 26 6 6 G6’ 6 5 8.4 4.5

78 79 80 81 82 83 84 85 86 87 88 89 90 78 79 80 81 82 83 84 85 86 87 88 89 90
Year

Note: There are no national or state standards for Fe. N = number of samples.

Figure 26. Trends in annual mean Fe concentrations in Illinois.

3

Concentration, 4g/M

3

Concentration, g/M

AIR QUALITY TRENDS IN ILLINOIS

Whole state

Chicago area

Analysis of individual
filters for metals
began in 1985.

=~ NO FA HDN OW

oO

NEO -Oite0 or OO 0-6 Gp-2

8

7} Metro-East area Remainder of state
6

5

4

3

2

1

Oieae cca mes Seow ee 5.5.5! <4 ©

78 79 80 81 82 83 84 85 86 87 88 89 90 78 79 80 81 82 83 84 85 86 87 88 89 90
Year

Note: There are no national or state standards for Mn. N = number of samples.

Figure 27. Trends in 24-hr maximum Mn concentrations in Illinois.

Whole state Chicago area

39 28 16 17

Remainder of state

Metro-East area

absgasegSubed

NIG T1716 £°6 66 Cre 6 4 4

78 79 80 81 82 83 84 85 86 87 88 89 90 78 79 80 81 82 83 84 85 86 87 88 89 90
Year

Note: There are no national or state standards for Mn. N = number of samples.

Figure 28. Trends in annual mean Mn concentrations in Illinois.

115

AIR QUALITY TRENDS IN ILLINOIS

Whole state Chicago area

3

Remainder of state

Note: Ni measurements began
in 1984. Analysis of individual
filters for metals began in 1985.
There are no national or state
standards for Ni. N = number
of samples.

Concentration, g/M

84 85 86 87 88 89 90

Figure 29. Trends in 24-hr maximum Ni concentrations in Illinois.

Whole state Chicago area

3

Note: Ni measurements began
in 1984. There are no national
or state standards for Ni.
N = number of samples.

Concentration, wg/M

84 85 86 87 88 89 90

Year

Figure 30. Trends in annual mean Ni concentrations in Illinois.

116

corresponding to the percentiles depicted by the
box plots.

There has clearly also been a drop in the highest annual
Pb concentration in each region, as depicted by the
uppermost open circle. This dropoff was quite dramatic
in the entire state, where numerous concentrations
exceeded 1 microgram per cubic meter ([1g/M*) oc-
curred between 1979 and 1982, but none thereafter.
Comparison of the “whole state” panel with the other
three panels clearly shows that high individual concen-
trations during the 1979-1982 period occurred in the
Metro East area. The marked improvement in maxi-
mum and median Metro East airborne Pb concentrations
after 1982 is likely to have been strongly influenced by
the closure and subsequent cleanup of a secondary lead
smelter in Granite City (Cooper and Frazier, 1983).

The national and state standard for Pb is a quarterly
arithmetic mean of 1.5 ug/M?. No violations of
this standard have occurred in Illinois since 1982
(IEPA, 1991).

Inspection of figure 3 indicates systematically higher
individual Pb concentrations (open circles) in the
Metro East area than in the Chicago area or the remain-
der of the state, especially during the 1979-1984 per-
iod. The statistical test for differences between regional
means found only the Metro East Pb concentrations
higher than the remainder of the state at the 5 percent
level. Median Pb concentrations in the Metro East area
were also higher than those in the Chicago area, but
the differences were not quite significant at the 5
percent level.

As Pb concentrations have decreased, and violations of
the standard have dropped to zero, the number of sites
measuring airborne Pb concentrations in Illinois has
also dropped sharply. Statewide, the number of sam-
pling sites with valid annual means decreased from a
high of 101 in 1980 to 17 in 1990. In the Chicago area
the corresponding numbers are 60 and 8; in the Metro East
area, 16 and 4; and in the remainder of the state, 26 and 5.

Nitrogen Dioxide (NO,). Concentrations of NO,
decreased substantially in Illinois during the 1980s.
Table 2 shows decreasing trends for 1-hr and 24-hr
maxima, as well as annual mean NO, concentrations,
both statewide and in the Chicago area. The trends
range from 4.4 percent to -7.1 percent per year, and all
are significant at the 1 percent level. There were not
enough sampling sites in the other areas of the state to
justify testing. Box plots for 1-hr maximum, 24-hr

AIR QUALITY TRENDS IN ILLINOIS

maximum, and annual mean NO, are shown in figures
4-6, respectively. They illustrate the decreases, during
the 1979-1990 period, in NO, concentrations both
statewide, and in the Chicago area, over all three averag-
ing times. Although only the trend of the median has
been tested statistically (table 2), it is apparent that
concentrations associated with all the percentiles de-
picted by the box plots have decreased. The same is
true for the highest individual values each year (plotted
as open circles).

No national or state standards exist for 1-hr or 24-hr
maximum NO, concentrations. Numerous concentra-
tions above the annual mean NO, standard (0.053 ppm)
occurred in the late 1970s, but none have occurred
since 1980.

Almost all of the NO, measurement sites are in the
Chicago area, so it is impossible to look for differences
in concentrations between geographic regions.

The number of sites measuring valid 1-hr NO, concen-
trations has increased recently, both statewide and in
the Chicago area, from a low of four sites in the early
1980s to about 15 sites in 1990. The number of sites
with valid 24-hr and annual mean NO, concentrations
has decreased from 36-47 sites in the late 1970s to 10-
15 sites in 1990.

Ozone (O,). All pollutants are affected to some extent
by weather conditions. Poor dispersion conditions
caused by low wind speeds or calm conditions can
allow pollutant concentrations to build up in local areas
near emission sources, and long-range transport can
carry pollutants from their sources for deposit at distant
locations. For O,, which is produced in the atmosphere
by reactions between hydrocarbons and nitrogen
oxides, however, weather conditions—especially tem-
perature—affect how much of the pollutant is pro-
duced. Thus, it is desirable to remove the effect of
weather conditions when looking for trends in O,. We
first discuss O, concentrations as observed, and then
use a Statistical regression approach to look for trends
in O, after the effect of temperature has been removed.

Table 2 indicates that no significant trend could be
detected in the median of 1-hr maximum O, concentra-
tions in any geographic region of Illinois. The box plots
of 1-hr maximum values by geographic area are shown
in figure 7. Year-to-year variations are apparent,
including higher concentrations statewide and in the
Chicago area in 1983 and 1988, when abnormally high
temperatures and little rain occurred during the summer

117

AIR QUALITY TRENDS IN ILLINOIS

ozone season, but overall the visual impression of no
trend toward higher or lower concentrations agrees
with table 2.

The Chicago and Metro East areas clearly had higher
median concentrations during the data period, com-
pared to the remainder of the state, and these two urban
centers also account for all the the highest individual
site values statewide (open circles).

The national and state standard for 1-hr maximum O,
concentration is 0.12 ppm. It is apparent that numerous
violations of the standard occur most years in Illinois,
although only two sites exceeded the standard (one
time each) in 1990. Weather conditions over much

of Illinois, and especially in the Chicago area, during
the summer of 1990 were cooler, cloudier, rainier, and
windier than normal (IEPA, 1991). These conditions
undoubtedly contributed to the lower O, concentra-
tions observed.

The test for differences between regions showed that
mean concentrations in the Chicago and Metro East
areas were each significantly (5 percent level) greater
than that in the remainder of the state.

A number of attempts to account for effects of atmo-
spheric conditions on O, concentrations have been
published (e.g., Korsog and Wolff, 1991; Shively, 1991).
Some of the past work has been quite sophisticated in
searching for the best meteorological parameters to use
in predicting O, concentrations, and in the use of sta-
tistical methods to characterize the relationship. In
contrast, the following attempt to account for the effects of
meteorological conditions on O, in Illinois is relatively
simple. It uses only mean surface temperature during
the O, season (April-October) in a regression equation
involving a constant, a temperature term, and a time
term. The model is:

[O,] = Constant + B, - (Time) +B,- (Temperature) (1)

where [O,] is ozone concentration, and B, and f, are
coefficients of time and temperature, respectively.

The procedure is to evaluate the coefficients of the
time and temperature terms using observed O, and
temperature data, and to observe whether the coeffi-
cient of the time term is significantly different from
zero. If it is, then there is a significant trend with time
after the effect of temperature on O, concentrations has
been removed. The model was evaluated for each of

118

the four geographical areas defined above. The O, data
were the medians of the annual distribution of 1-hr
maximum O, concentrations over all measurement sites
in each geographical region. Corresponding April-
October mean temperatures were obtained for each
region from the Midwestern Climate Information Sys-
tem (Kunkel et al., 1990). Time was sequential year
number, beginning with 1 in 1978. The regression
analyses were carried out using the Multiple General
Linear Hypothesis (MGLH) regression routine in the
SYSTAT statistical software (Wilkinson, 1990).
Results are shown in table 3.

The first thing to notice about the results is that the
temperature coefficient is positive, indicating a positive
correlation between O, and temperature, as expected,
in all four geographical regions. The temperature coef-
ficient is significantly different from zero (5 percent
level) over the whole state, and also in the “remainder”
region, significant at the 10 percent level in the Metro
East region, and just barely misses the 10 percent sig-
nificance level (P = 0.1011) in the Chicago area.

The coefficient of the time term is negative, indicating
a downward trend in concentrations, in all four regions.
The downward trend, after accounting for the effects of
surface temperature, is significant at the 5 percent level
over the whole state and in the Chicago area, and at the
10 percent level in the other two regions. (Note: al-
though not shown in table 2, a -1.9 percent per year
downward linear trend in O, significant at the 10 percent
level was detected in the O, concentrations as observed.)

A further indication of the importance of the time term
is the improvement in the squared multiple correlation
(R? or the percent of the total variance explained by the
regression) as the time term is added to the regression
model. The model was evaluated separately without the
time term to obtain R? values with and without the time
term in each region. Results are shown in table 4. There is
a substantial improvement in the percent variance,
explained with the addition of the time term in each
region—from 26 to 58 percent for the whole state, from
5 to 46 percent in the Chicago area; from 16 to 42 per-
cent in the Metro East area, and from 40 percent to 55
percent in the remainder of the state.

The results regarding ozone time trends may be sum-
marized as follows. Without accounting for temper-
ature effects, no trends were found at a 5 percent
significance level, but a 1.7 percent per year downward
trend in concentrations, significant at the 10 percent
level, was detected in the Chicago area (not shown in

AIR QUALITY TRENDS IN ILLINOIS

Table 3. Results of Regression Analysis To Test Significance of O, Time Trend after Accounting for Effects of Average

Surface Temperature

Geographic region

Whole state

Chicago area

MetroEast area

Remainder of state

Note:

“Temperature = mean surface temperature, April through October, in the region.

Table 4. Comparison of R* Values (Explained Variance) in Regression Equations Evaluated with and without Time Terms

Geographic region

Whole state
Chicago area
Metro East area
Remainder of state

Variable”

Constant
Time
Temperature

Constant
Time
Temperature

Constant
Time
Temperature

Constant
Time
Temperature

Coefficient

-0.30400
-0.00177
0.00565

-0.36658
-0.03108
0.00707

-0.28673
-0.00212
0.00548

-0.20043
-0.00073
0.00401

R? (without time)

0.260
0.048
0.165
0.399

Me
(2-tail)

0.0774
0.0196
0.0186

0.2236
0.0194
0.1011

0.2287
0.0605
0.0811

0.0651
0.0958
0.0098

R? (with time)

0.582
0.464
0.423
0.551

119

AIR QUALITY TRENDS IN ILLINOIS

table 3). After accounting roughly for temperature
effects, downward trends were detected at the 5 percent
level statewide and in the Chicago area, and downward
trends significant at the 10 percent level were detected
in the other two regions. For evaluating potential effects of
ozone on agricultural crops, concentrations in the non-
urban “remainder” region would be the most relevant.

Sulfur Dioxide (SO,). Table 2 shows downward trends
significant at the 1 percent level for 3-hr maximum SO,
in the Chicago area (-3.5 percent per year), and for
annual mean SO, over the whole state (-2.6 percent per
year). The table also shows a downward trend signifi-
cant at the 5 percent level for annual mean SO, in the
Chicago area (-5.2 percent per year). No trends signif-
icant at 5 percent or better were detected for any of the
three types of observations in any region of the state;
note, however, that the number of observing sites in the
Metro East area was insufficient to estimate a trend.

Box plots for 3-hr maximum SO, are shown in figure 8.
Any trends are difficult to perceive by the naked eye,
despite the highly significant trend detected in the
Chicago area (table 2). Comparison of SO, concentra-
tions between regions shows that the highest 3-hr
maximum SO, values observed statewide occurred in
areas other than the Chicago area. There is no national
or state primary standard for 3-hr maximum SO,. The
secondary national standard for 3-hr maximum SO, is
0.5 ppm. Figure 8 shows that this standard has been
exceeded on a few occasions during the 1978-1990
period. The test for differences between regions found
concentrations in the Chicago area significantly lower
than those in the rest of the state. The number of
sampling sites with valid data for 3-hr maximum SO,
has remained very constant during the period.

Box plots for 24-hr maximum SO, are shown in figure
9. No trends are apparent to the naked eye, and no
significant trends were detected in table 2. Again, the
Chicago area experienced somewhat lower median
concentrations than the remainder of the state, which in
this case includes the Metro East area. The difference
was signif-icant at better than the 1 percent level. The
national and state primary standard for 24-hr maximum
SO,, 0.14 ppm, has been exceeded a few times state-
wide in most years between 1978 and 1990; but only
four of these occurred in the Chicago area. Numbers of
sites with valid data fell by a factor of about 2 state-
wide over the 1978-1990 period. Most of this reduction
oc-curred in the Chicago area, which had 47 sites with
valid data in 1979, but only 11 sites in 1990. Most of
this reduction is the result of closing sampling sites,
rather than active sites not meeting completeness criteria.

120

Figure 10 shows box plots for annual mean SO, con-
centrations. Decreases in median concentrations
statewide are significant at the 1 percent level and in
the Chicago area at the 5 percent level (table 2).
Regional maximum concentrations (highest open
circle) also appear to be on downward trends, at least
for the “whole state” and “remainder” regions. There
were no significant (5 percent) differences in concen-
tration between geographic regions. Numbers of sites
with valid data have also decreased over the 1978-1990
period, statewide by about 50 percent, and by about 70
percent in the Chicago area.

Particulate Mass (TSP, PM,,)- The state standard
for particulate mass is written for total suspended
particulate matter (TSP), while the national standard
is for PM,,, or particulate matter up to 10 um in
aerodynamic diameter.

Box plots for 24-hr maximum TSP concentrations are
shown in figure 11. Median values have remained
relatively constant over the 1978-1990 period with
occasional anomaly years such as 1983, when spring
dust storms in east central and northeastern Illinois
(IEPA, 1984) caused numerous measurements > 500
g/M?. No significant trends in median concentrations
were detected (table 2) in any of the four geographic
regions for the entire 1978-1990 period, but careful
examination of the individual regional plots suggests a
decreasing trend for the first half of the 1980s followed
by an increasing trend in the later half, at least in
some regions.

It is important to know whether these recent concentra-
tion increases reflect actual regional environmental
conditions, or whether they are an artifact of the year-
to-year changes in the measurement network. Numbers
of sites with valid samples have dropped sharply in all
regions in recent years, and if the sites removed from
the network were systematically in cleaner areas (which
might be expected), then average concentrations at the
remaining sites would be higher. This question could
be investigated by comparing records of concentrations
at removed sites and at remaining sites.

The test for differences between regions found none (5
percent level), but some violations of the standard have
occurred each year in all regions.

Trends in annual geometric mean TSP concentrations
are shown in figure 12. No overall significant trends
were detected (table 2), but the pattern of decreasing
concentrations up to about 1985, followed by increases

AIR QUALITY TRENDS IN ILLINOIS

through 1989, is clear in all four regions. Again, how-
ever, the reality of the increase in the late-1980s is in
doubt because the number of sites with valid data
declined sharply during the 1986-1990 period. Com-
parison of concentrations between regions showed that
concentrations in the Metro East area were significantly
(1 percent level) higher than those in both the Chicago
area and the remainder of the state. In addition, visual
examination suggests that the highest individual site
values occurred in the Metro East region.

The first published data for PM,, are for 1987 (IEPA,
1988), after the U.S. EPA changed its particulate
matter standard from TSP to PM,,, although some
measurements were being made in anticipation of the
new national standard as early as 1984 (IEPA, 1988).
Because data were available for only four years, no
evaluation of PM,, trends was included in table 2.
However, box plots for 24-hr maximum and annual
mean PM, are shown in figures 13 and 14, respectively.

Figure 13 shows a number of violations of the 150 1g/
M? standard for 24-hr maximum PM, in both the Chi-
cago area and other areas of the state. The number of
sampling sites increased in all three geographic areas
shown in the figure during the 1987-1990 period.

Figure 14 also shows a few concentrations above the 50
g/M? standard statewide, none of which occurred in
the Chicago area.

This concludes the presentation of data on the criteria
pollutants. There are currently no standards for the
remaining pollutants, which all occur as particles in the
atmosphere, and are collected on high-volume filters.

Filter Analyses: Nitrate, Sulfate, and Metals

Through 1984, annual mean concentrations of nitrate,
sulfate, and metals were derived from analyses of
filters composited monthly. Thus, distributions of
annual means are available from 1978-1990, but dis-
tributions of 24-hr maximum concentrations are avail-
able only after 1985, when measurements of individual
24-hr filters began.

Since all of these analyses depend on the collection of
filter samples, it is appropriate to discuss changes in the
number of valid samples that apply to all of the follow-
ing analytes. Of course there are small individual
differences between pollutants in the number of valid
samples, but the major trends are determined primarily
by the number of active sampling sites.

Statewide, the number of sampling sites collecting
filter samples dropped from >100 in 1978 to <20 in
1990; the change from 1985 to 1990 was from about
50 sites to <20. In the Chicago area the drop in sites
was from >70 in 1978 to 35-40 in 1985 to <10 in
1990. In the Metro East area, there were >15 sites
up to 1981 and <5 after 1985. The numbers for the
remainder of the state were >25 sites up to 1981 and
4-6 thereafter.

Nitrate Ion (NO,). Box plots for 24-hr maximum
and annual mean NO, concentrations are shown in
figures 15 and 16, respectively. High year-to-year
variability in the median (and other box plot per-
centiles) 24-hr maximum NO, concentrations (figure
15) is apparent, particularly in the Metro East and
“remainder” areas, where the plots are based on

<10 sites.

For annual mean NO,, the only significant trend (1 per-
cent level) was a -2.1 percent per year decreasing trend
in the “remainder” region (table 2). Figure 16 shows a
relatively steady median concentration of about 5 [g/
M:’ statewide and in the Chicago area, and somewhat
lower concentrations, with more year-to-year variabil-
ity in the Metro East and remaining areas of the state.
The comparison between regions found the Chicago
area concentrations significantly (5 percent or better)
higher than those in the Metro East area and the re-
mainder of the state.

Both statewide and in the Chicago area, the varia-
bility within individual years, as measured by the
interquartile (75th-25th percentile) range, or the
height of the “box” in the box plot, appears to have
decreased in recent years. Again, however, this oc-
curred along with a steep decline in the number of
sites with valid data, so the smaller variability may
simply reflect the fact that measurements are occur-
ring at fewer locations.

Sulfate Ion (SO,”). Box plots for 24-hr maximum and
annual mean SO,” are shown in figures 17 and 18,
respectively. Here also relatively high year-to-year
variations in 24-hr maxima occurred, particularly in
years or geographic areas with <10 sites.

Table 2 shows that declines in annual mean SO,* con-
centration significant at the 5 percent (whole state and
Chicago area) or 2 percent (remainder of the state)
levels occurred in all areas other than the Metro East
area. The significant linear trends were all between -1.3
and -1.6 percent per year.

121

AIR QUALITY TRENDS IN ILLINOIS

Concentrations were significantly (1 percent level)
higher in the Metro East area than in the Chicago and
“remainder” regions of the state.

Arsenic (As). Box plots for 24-hr maximum and an-
nual mean As are shown in figures 19 and 20, respec-
tively. Except for the Metro East area, where median
24-hr maximum concentrations >0.01 pg/M? occurred
in four of the six years of record, the median 24-hr
maximum was <0.01 pg/M? statewide. One value >0.5
ug/M? was observed in the Metro East area in 1990.

Differences between geographical regions are the most
interesting feature of these plots. Comparison of the
plots for the various regions shows clearly that the
Metro East area accounts for the highest measured
values statewide; this is true for both maximum 24-hr
concentrations as well as mean annual concentrations.
The statistical test for differences between regions was
not run on the 24-hr maximum As data because of the
relatively few years of data. For annual mean As, the
test found that concentrations in the Metro East area
were higher than those in the Chicago and “remainder”
areas at the 0.1 percent significance level.

Table 2 shows declines in As concentrations of -8.4,
-11.1, and -9.5 percent per year in the whole state, the

Chicago area, and the “remainder” region, respectively.

All are significant at the 1 percent level.

Cadmium (Cd). Box plots for 24-hr maximum and
annual mean Cd are shown in figures 21 and 22,
respectively. Again, differences between geographical
regions are clear, and again the Metro East area
accounts for most of the highest values that were ob-
served statewide over both averaging times. The test
for regional differences in annual mean concentration
confirmed that the highest concentrations occurred in
the Metro East area at a significance level of 0.1 per-
cent. Table 2 shows that the decline of -12.7 percent
per year in annual mean Cd in the “remainder” region
was significant at the 2 percent level.

Chromium (Cr). Figures 23 and 24 show box plots of
24-hr maximum and annual mean Cr, respectively.
Note that Cr was not measured on filter samples until
1985. The number of sites in the Metro East area was
insufficient for a separate plot, so the Metro East sites
are included in the “remainder” region. The marked
differences between regions that were apparent for As
and Cd are smaller or absent here. Rather than the
Metro East area having the highest concentrations, Cr
appears to be somewhat more prominent in the Chi-

122

cago area, although the differences were not statisti-
cally (5 percent) significant. No significant trends in
the median annual mean Cr were detected (table 2).

Iron (Fe). Box plots for 24-hr maximum and annual
mean Fe are shown in figures 25 and 26, respectively.
For both averaging periods, it is again clear from the
figures that the Metro East area accounts for the
highest values statewide. The test for differences
between regions confirmed that the highest annual
mean Fe concentrations occurred in the Metro East
area (0.1 percent level). Moreover, the trend test (table
2) de-tected an upward trend (6.0 percent per year) in
the Metro East area, significant at the 2 percent level.
While it is true that the two area sampling sites with
the lowest annual mean concentrations were closed
during the 1985-1990 period (one after 1985, and the
other after 1988), this is not the cause of the increasing
trend in concentrations. Concentrations at the remain-
ing sites also increased during these years.

Manganese (Mn). Figures 27 and 28 show box plots
for 24-hr maximum and annual mean Mn, respectively.
Both the Chicago area and the Metro East area contrib-
uted to the highest values over both averaging periods
observed statewide, but the Metro East area experi-
enced noticeably higher median concentrations (figures
27 and 28). This was confirmed for annual means by
the test for differences between regions, which found
higher concentrations in the Metro East area compared
to the Chicagto and “remainder” areas (0.1 percent
level). As in the case of Fe, the trend test (table 2)
detected an increasing trend (10 percent per year), sig-
nificant at the 5 percent level, in annual mean Mn in
the Metro East area. Also as in the case of Fe, the clos-
ing of sampling sites could have had only a minor
effect on the increasing Mn concentrations over the
1985-1990 period.

Nickel (Ni). Box plots for 24-hr maximum and annual
mean Ni are shown in figures 29 and 30, respectively.
Ni was not measured on filter samples until 1984.
Since the number of sites in the Metro East area was
insufficient for a separate plot, the Metro East sites are
included in the “remainder” region. For Ni, it appears
that the Chicago area contributed most of the highest
24-hr maximum concentrations (figure 29). Comparing
annual means between regions, it appears that the
Chicago area has recently experienced somewhat
higher median concentrations, but the statistical test
found no significant (5 percent level) differences. Ap-
parently, both regions contribute more or less equally
to the highest annual means. The trend test (table 2)

AIR QUALITY TRENDS IN ILLINOIS

found no significant trends in the median annual
mean Ni.

Metal Enrichments, Relative to Soil. Enrichment
factors are a common and convenient method of
distinguishing between natural and anthropogenic
sources of metals in the atmosphere. For many metals
the typical natural source examined is crustal or soil
aerosols. Enrichment, E, for element X is expressed as
a ratio of ratios:

E = (X/RE),,/CWRE),,, (2)

where RE is a reference element for which the soil or
earth’s crust is the dominant source in the atmosphere.
Enrichments are usually evaluated for measured con-
centrations of X in air, using abundances (mass frac-
tions) of X and RE in soils from compilations of global
mean soil or crustal composition. Figure 31 shows
element enrichments based on the median statewide
annual mean concentrations of As, Cd, Cr, Pb, Mn, and
Ni, using Fe as the reference element, for 1978-1990. If
the ratio of the subject element to Fe in air is the same
as the ratio of the same elements in soil, then the en-
richment is 1.0, and the element X is assumed to come
only from the soil. In practice, because of variations in
soil composition, elements with enrichments between 1
and 10 are usually assumed to have primarily soil
sources. Values of E >10 are assumed to indicate
anthropogenic sources.

Using these criteria, figure 31 shows that Pb and Cd,
with enrichments between 100 and 3000, have primar-
ily anthropogenic sources in Illinois. Note that the
enrichment of Pb has dropped from 1000-2000 in the
early 1980s to 100-200 in recent years as Pb has been
removed from automotive fuels. Nevertheless, the
concentration of Pb currently in the atmosphere is far
in excess of what would be there naturally from soil
wind erosion. Sources such as Pb smelters and fly ash
from coal burning may now be the major sources of Pb
in the atmosphere, although Pb recycled from previ-
ously deposited auto exhaust may also be important.

Cd also has a high enrichment factor of about 1000,
which has remained relatively constant during the
1980s. The absence of a Cd datapoint in 1981 is the
result of a one-year hiatus in Cd measurements. Those
in 1983 and 1989 were caused by median Cd concen-
trations of 0.000 1g/M®.

Arsenic enrichments are mostly >10 also, suggesting a
mostly anthropogenic source. They appear to be de-

creasing somewhat, in agreement with the decreasing
trends in concentration seen in table 2.

Mn enrichments are firmly in the 1-10 range generally
ascribed to natural sources, with very little year-to-year
variation. The majority of Mn in the Illinois atmo-
sphere may well be natural. Mn sources also tend to be
Fe sources, however, so the extent to which the refer-
ence element Fe is nonnatural, may also hold true for
Mn. For example, anthropogenic sources are very
likely to be the cause of the significant increase (at the
5 percent level, or better) in concentrations of both Fe
and Mn during the 1980s in the Metro East area.

The year-to-year variations in Cr and Ni enrichments
during the 1985-1990 period are similar to each other,
but unlike those of any of the other metals in figure 31.
The enrichments are mostly in the 1-10 range, suggest-
ing mainly natural sources for these metals, but the
sometimes large year-to-year variations suggest that
concentrations of both of these metals are enhanced by
anthropogenic sources at least during some years.

SPATIAL DISTRIBUTIONS
OF POLLUTANT CONCENTRATIONS

It was a very early goal of this summary of Illinois air
quality to draw spatial contours of pollutant concentra-
tions on a statewide scale. Examination of the data
quickly dashed that early hope, however, as it became
clear that sampling sites were concentrated in the two
major industrial and population centers, the Chicago
and the Metro East areas. The relatively few sampling
sites not in these major centers were clustered in
smaller cities and the environs of troublesome sources.
Data are not available from sparsely populated regions
with no major sources. For an agency such as the IEPA
with the mission of documenting problems and enforc-
ing standards, this sampling strategy is obvious. It does
not provide data needed to document statewide air
quality, however.

For this report, we propose that the data are adequate
to show spatial variations in that portion of the state
with the highest concentration of sampling sites, the
Chicago area. To provide a sense of the temporal
changes in Chicago-area spatial patterns, contours have
been drawn for 1980, 1985, and 1990, and are pre-
sented below. The rapid drop in the number of sam-
pling sites, even in the Chicago area, makes the 1990
contours very problematic. The 1990 contours are
meant to suggest only the most rudimentary spatial

AIR QUALITY TRENDS IN ILLINOIS

10000

1000

100

10

Enrichment (X/Fe)../ QVFe)..i

0.1

78 79 80 81 82 83 84 85 86 87 88 89 90
Year

Figure 31. Trends in airborne metal enrichment, relative to soil, in Illinois.

124

patterns. For most pollutants the data are not adequate
to do more.

The data and associated discussions for the several
pollutants appear in the same order as those of the
temporal trends above.

Criteria Pollutants

CO. Figures 32 and 33 show spatial distributions of 1-
hr maximum and 8-hr maximum CO concentrations in
the Chicago area in 1980, 1985, and 1990. The low
density of sampling sites, and the changes in site loca-
tions between years preclude identification of persistent
locations of high or low concentrations. In 1980 one
sampling site near the lake shore had a 1-hr maximum
>20 ppm. In 1985 three sites had 1-hr maxima between
10 and 20 ppm, but there were no measurements >20
ppm. In 1990, only one site recorded a 1-hr maximum
>10 ppm. This general decline in concentrations agrees
with the significant overall decline in Chicago-area
concentrations detected by the trend test (table 2). No
violations of the 35 ppm national and state standard for
1-hr maximum CO were observed.

Figure 33 shows spatial patterns of 8-hr maximum CO
concentrations. Again, temporal changes in the sam-
pling network limit discussion mostly to temporal
changes in concentrations. In 1980, one lakeshore site
exceeded the national and state standard for 8-hr maxi-
mum CO (9 ppm), but no violations were observed in
the region in 1985 or 1990. A general decline in con-
centrations is apparent from 1980 to 1985 to 1990 that
agrees with the significant decline detected in the
Chicago area from the full 1978-1990 dataset (table 2).

Pb. Spatial distributions of annual mean Pb concentra-
tions in the Chicago area for 1980, 1985, and 1990 are
shown in figure 34. Persistent concentration maxima
occurred in the central city and the southeast Chicago
industrial area; however, the well-known and very
significant decline (table 2) in atmospheric Pb concen-
trations during the 1980s is evident here as well. The
value of the highest contour line declined from 0.6 L1g/
M? in 1980 to 0.2 pg/M? in 1985 and 0.1 g/M? in
1990. Pb standards are written in terms of mean con-
centrations over calendar quarters, which cannot be
inferred from the data shown here. See the earlier
discussion related to temporal trends.

NO,,. Spatial patterns of 24-hr maximum and annual
mean NO, in the Chicago area are shown in figures 35
and 36, respectively. In 1980 the highest 24-hr maxima

AIR QUALITY TRENDS IN ILLINOIS

occurred in suburban northwestern (DesPlaines) and
southern (Flossmoor) Cook County, with single sites in
each area experiencing one or more days with mean
concentrations >0.14 ppm. By 1985, the two sites with
the highest values in 1980 were no longer active. The
data suggest an overall decline in maximum 24-hr con-
centrations, with the highest value (0.094 ppm) occur-
ring in the city center. The 1990 data show a further
overall decline in maximum 24-hr concentrations, with
the maximum (0.062 ppm) occurring in a cluster of
sampling sites near O’ Hare Airport. There are no
national or state standards for 24-hr maximum NO,,.

Figure 36 shows Chicago-area contours of annual mean
NO.,,. Shaded areas in the northwest suburbs (DesPlaines)
and the central city exceeded the national and state
primary standard (0.053 ppm) in 1980. Although no
violations of the standard were observed in 1985 and
1990, the available data suggest that the highest annual
mean concentrations still occurred in the central city
area near Lake Michigan. The apparent decreasing
concentrations agree with the significant trend in Chi-
cago area annual mean NO, discussed earlier (table 2).

O,. Figure 37 shows contours of maximum 1-hr O,
concentrations in the Chicago area for 1980, 1985, and
1990. Most of the Chicago area was in violation of the
0.12 ppm standard in 1980, with a conspicuous area of
lower values across the middle of Cook County and the
city of Chicago. In 1985 the violations of the standard
were confined to the northeastern and southeastern
portions of Cook County. No violations were observed
in 1990; however, there is evidence that this may have
occurred because summer weather conditions were
relatively unfavorable for O, formation, as discussed
earlier. Table 2 indicates no significant temporal trend
in O, as observed; however, as shown earlier, when
surface temperature is accounted for, there is a signifi-
cant trend (2 percent level) toward decreasing concen-
trations in the Chicago area over the 1978-1990 period.

SO,. Figures 38-40 show Chicago-area spatial contours
for SO,: 3-hr maximum, 24-hr maximum, and annual
mean concentrations, respectively. The contours drawn
from the available data are very difficult to interpret.
Overall, 3-hr maximum concentrations were primarily
>0.10 ppm in 1980 and 1985, and primarily <0.10 in
1990. This apparent trend toward decreasing concentra-
tions agrees with the significant trend for the Chicago
area detected earlier (table 2). A single high value
(0.446 ppm) was observed in the western suburbs
(Bedford Park) in 1985. There are no national or state
standards for 3-hr maximum SO.,.

AIR QUALITY TRENDS IN ILLINOIS

Figure 32. Spatial distributions of 1-hr
maximum CO (ppm) in the Chicago area for

Bs Bae 1980, 1985, and 1990.

126

AIR QUALITY TRENDS IN ILLINOIS

Figure 33. Spatial distributions of 8-hr
maximum CO (ppm) in the Chicago area for
1980, 1985, and 1990. The shaded area denotes
violation of an air quality standard.

127

AIR QUALITY TRENDS IN ILLINOIS

Figure 34. Spatial distributions of annual mean
Pb (| ug/M? ) in the Chicago area for 1980, 1985,
and 1990.

128

DULERAGE

AIR QUALITY TRENDS IN ILLIN'

Figure 35. Spatial distributions of 24-hr
maximum NO, (ppm) in the Chicago area for
1980, 1985, and 1990.

ols

129

AIR

UALITY TRENDS IN ILLINOIS

130

DU PAGE

0.02

Figure 36. Spatial distributions of annual
mean NO, (ppm) in the Chicago area for 1980,
1985, and 1990. Shaded areas denote
violations of an air quality standard.

AIR QUALITY TRENDS IN ILLINOIS

0.10

©/0.093

Figure 37. Spatial distributions of 1-hr
maximum O? (ppm) in the Chicago area for
1980, 1985, and 1990. Shaded areas denote
violations of an air quality standard.

DU PAGE

131

AIR QUALITY TRENDS IN ILLINOIS

Figure 38. Spatial distributions of 3-hr
maximum SO, (ppm) in the Chicago area for
1980, 1985, and 1990.

DU PAGE

© 0.052

132

AIR QUALITY TRENDS IN ILLINOIS

0.05 N* 0.021

:

2 063
0,021 0,023 \ Uy

0.049@ 0.05

/ 0.074
°

Figure 39. Spatial distributions of 24-hr
maximum SO, (ppm) in the Chicago area for
1980, 1985, and 1990. The shaded area
denotes violation of an air quality standard.

© 0.023

133

AIR QUALITY TRENDS IN ILLINOIS

0.01

Figure 40. Spatial distributions of annual
mean SO, (ppm) in the Chicago area for 1980,
1985, and 1990.

134

AIR QUALITY TRENDS IN ILLINOIS

Chicago-area spatial contours for 24-hr SO, are shown
in figure 39. One site in south-suburban Cook County
(Blue Island) exceeded the national and state standard
(0.14 ppm) in 1980. Aside from this extreme, concen-
trations at most locations were <0.05 ppm. The avail-
able data for 1985 and 1990 show maxima in the
western suburbs (Bedford Park) in both years, but again
most sites experienced 24-hr maximum concentrations
<0.05 ppm. There were no violations of the standard in
1985 or 1990. Table 2 shows no significant temporal
trends in 24-hr maximum SO, in the Chicago area.

Chicago-area spatial contours for annual mean SO,
concentrations are shown in figure 40. The figure
shows no strong spatial patterns in any of the three
years, but very different patterns of minimal high and
low concentrations over the three years. There were no
violations of the national and state standard (0.03 ppm)
for the annual mean in any of the three years.

TSP. Spatial patterns of 24-hr maximum and annual
mean TSP in the Chicago area are shown in figures 41
and 42, respectively. The spatial pattern for 1980
shows areas of high concentrations, including viola-
tions of the 260 j1g/M®? standard, in (1) the industrial
area of southeastern Chicago, (2) the western suburbs
of Cook County (River Forest) and eastern DuPage
County (Elmhurst), (3) northwestern Will County
(Romeoville, Joliet, and Rockdale), and (4) southwest-
ern Will County (Braidwood). In 1985 there were
widespread violations of the standard, over almost all
of Cook County, the southern half of DuPage County,
and southwestern Will County On the other hand, there
were no violations in the Joliet area. The widespread
extent of the violations of the 24-hr standard suggests
that a regional weather event might have been respon-
sible, and indeed, a dust storm that occurred on May
31, 1985, was the cause of most of the violations
(IEPA, 1986). In 1990 the much-reduced sampling
network detected only one violation of the standard at
Rockdale in Will County.

Figure 42 shows the spatial patterns for annual geomet-
ric mean TSP. In 1980 maximum concentrations above
the national standard (75 tig/M*) occurred in a broad
band from the central city area of Chicago to suburban
Summit, in the southeast Chicago industrial region, in
northeast DuPage County (Bensenville), and in the
Joliet area of Will County In 1985, only two sites in the
Chicago area violated the standard, one in southeast
Chicago and one in west suburban McCook. In 1990 only
one site, in downtown Chicago, was in violation; however,
the number of sampling sites was down considerably.

Filter Samples: Nitrate, Sulfate, and Metals

There are no standards for the remaining pollutants.

NO,. Spatial patterns of 24-hr maximum and annual
mean NO, in the Chicago area are shown in figures 43
and 44, respectively. Data on 24-hr maximum NO, are
not available before 1985. Maximum 24-hr NO, con-
centrations are relatively uniform over the area in both
1985 and 1990, varying by a factor of only about 2
from minimum to maximum. Thus, the importance of
the locations of relative high and low concentrations is
difficult to assess.

Figure 44 shows spatial patterns of mean annual NO,
in the Chicago area. Again, the values are spatially
very uniform. It is interesting to note that in 1980 the
Cook County concentrations were generally <5 pg/M?
while those in DuPage and Will Counties were >5 p1g/
M?’; in 1985 the pattern was reversed, although that
assessment is based on only four sites outside of Cook
County There were too few sites in 1990 to justify any
comment on spatial variations.

SO,”. Spatial patterns of 24-hr maximum and annual
mean SO,* in the Chicago area are shown in figures 45
and 46, respectively. Interpretation of these patterns is
limited by the same factors that affected the NO, data:
lack of data for 1980 and a large disparity between the
number of sampling sites in 1985 and 1990. Based on
only four sampling sites, the data provide a hint that
maximum 24-hr concentrations >40 g/M? affected
more of the Cook County area in 1990 than in 1985.
Further, the southeast Chicago industrial area had
maximum concentrations >40 tg/M? in both years.

Figure 46 shows Chicago-area spatial patterns of
annual mean SO,*. The areas where mean annual SO,”
exceeded 15 [1g/M? were scattered throughout Cook
County in 1985, but were reduced to one site in 1985,
and none at all in 1990.

Fe. Spatial distributions of 24-hr maximum and
annual mean Fe concentrations in the Chicago area
for 1985 and 1990 are shown in figures 47 and 48.
In 1985, maximum 24-hr concentrations varied be-
tween | and 4 g/M? over most of the three-county
Chicago area, but there was a small area where
concentrations exceeded 20 j1g/M? in the southeast
Chicago industrial area. In 1990 the typical range
was still 1-4 1g/M?, but the high concentrations in
southeast Chicago were down considerably.

AIR QUALITY TRENDS IN ILLINOIS

200
Figure 41. Spatial distributions of 24-hr

maximum TSP (\g/M?) in the Chicago area for
1980, 1985, and 1990. Shaded areas denote
violations of an air quality standard.

PAGE

DU

136

AIR QUALITY TRENDS IN ILLINOIS

Figure 42. Spatial distributions of annual
geometric mean TSP (1g/M?) in the Chicago
area for 1980, 1985, and 1990. Shaded areas
denote violations of an air quality standard.

AIR QUALITY TRENDS IN ILLINOIS

Figure 43. Spatial distributions of 24-hr maximum NO; ( ug/M?) in the Chicago area for 1985
and 1990.

138

AIR QUALITY TRENDS IN ILLINOIS

Figure 44. Spatial distributions of annual mean
NO; ( ug/M? ) in the Chicago area for 1980,
1985, and 1990.

139

AIR QUALITY TRENDS IN ILLINOIS

Figure 45. Spatial distributions of 24-hr maximum SO a ( ug/M3 ) in the Chicago area for 1985 and 1990.

140

AIR QUALITY TRENDS IN ILLINOIS

15

Figure 46. Spatial distributions of annual mean
SO? ( ug/M? ) in the Chicago area for 1980,
1985, and 1990.

141

AIR QUALITY TRENDS IN ILLINOIS

Figure 47. Spatial distributions of 24-hr maximum Fe (1g/M?) in the Chicago area for 1985
and 1990.

142

AIR QUALITY TRENDS IN ILLINOIS

-
GaN 952 Ja

Figure 48. Spatial distributions of annual
mean Fe (1g/M?) in the Chicago area for
1980, 1985, and 1990.

AIR QUALITY TRENDS IN ILLINOIS

Figure 48 shows spatial patterns of mean annual Fe
concentrations in the Chicago area. The main feature
of the 1980 pattern was a broad area of concentrations
>1.0 ug/M? in central and southeastern Cook County
Note, however, the lack of 1980 measurements in the
southeast Chicago industrial region, where mean values
>3 ug/M? were observed in 1985. Most of the few
sampling sites remaining in 1990 reported mean Fe
concentrations >1.0 j1g/M?, but the southeast Chicago
hot spot was gone.

Mn. Spatial patterns of 24-hr maximum and annual
mean NO, in the Chicago area are shown in figures
49 and 50, respectively. Notice the similarities be-
tween the spatial patterns of Mn (figures 49 and 50)
and Fe (figures 47 and 48), especially in 1985. This
is not surprising since baseline concentrations are
probably the result of wind erosion of soil dust, while
local peak concentrations are often associated with
industrial activity, especially steel-making. Peak 24-
hr maximum concentrations of both Mn and Fe in
southeast Chicago decreased by a factor of 4 or 5
from 1985 to 1990.

Because of very limited numbers of sampling sites in
the Chicago area through 1985, spatial data for the
remaining four metals, As, Cd, Cr, and Ni, is limited to
1990 annual means, all presented in figure 51.

As. Figure 51 shows annual mean concentrations of As
at eight sites in the Chicago area. Mean concentrations
at six of the eight sites were 0.001 pg/M°, and the other
two were 0.000 1g/M°. The figure presents only the
data. No contours are justified.

Cd. The 1990 spatial pattern of Cd in the Chicago
area is also shown in figure 51. Annual mean con-
centrations were 0.001 or 0.002 ug/M? at six of the
eight sites, with a maximum value (0.008 tg/M®°) in
southeast Chicago.

Cr. The 1990 spatial pattern of Cr in the Chicago area
is also shown in figure 51. Annual means ranged from
0.000 to 0.015 tg/M?. The maximum concentrations
occurred in southeast Chicago and the western suburb
of Maywood.

Ni. The Chicago-area pattern of mean annual Ni con-
centrations for 1990 are also shown in figure 51. There
was a broad concentration maximum (>0.005 [g/M?)
over central and southern Cook County, with the high-
est value at Maywood.

144

TABULATIONS OF LITERATURE DATA
ON OCCASIONAL MEASUREMENTS

Some pollutants, including some considered toxic, have
not been measured routinely in Illinois for reasons that
usually involve the cost of analysis, the ability to detect
the extremely low concentrations present in the atmo-
sphere, or both. Table 5 summarizes some recent
measurements of volatile organic compounds in IIli-
nois. Measurements in table 5 were made in Chicago,
in the Metro East area, and at a rural agricultural site
near Champaign between 1986 and 1990. Sweet and
Vermette (1992) found that the concentrations they
measured in Illinois urban areas were quite similar to
those observed in other urban areas of the United
States. Further, the concentrations they observed in an
industrial area were quite similar to those seen in other
cities without significant industry, which suggests that
the major types of sources were widespread small
sources, especially related to automotive emission and
fuels, rather than large point sources.

The available data are not adequate to address the issue
of temporal trends, except that there are a few substan-
tial differences in mean concentrations between years.
With regard to spatial variations, there are compounds
for which rural and urban concentrations are similar or
different, and some differences in concentration be-
tween urban areas.

As is true for many atmospheric parameters, concentra-
tions of many volatile organic compounds are highly
variable both spatially and temporally. These variations
depend on sample duration, proximity of sampling to
sources, variations in source strength, and especially on
weather conditions.

Variability between years is seen by comparing the
various measured mean concentrations in Chicago. In
general, the results of McAlister et al. (1989, 1991) for
1988 and 1990 agree quite well with those of Sweet
and Vermette (1992) measured over the 1986-1990
period. However, there are a few exceptions to this
rule, notably carbon tetrachloride and tetrachloroethy-
lene in 1990, and toluene and meta- plus para-xylene in
1988. The measurements of Wadden et al. (1992) were
made in a different Chicago location, near the city cen-
ter. There is good agreement between the Wadden et al.
measurements and those already mentioned for some com-
pounds, such as benzene, but poor agreement for others,
such as chloroform and ethylbenzene. In most cases the
Wadden et al. values are greater than the others,
probably because of spatial variations within the city.

AIR QUALITY TRENDS IN ILLINOIS

e
0.087° 9120

ONS 9242
.

Figure 49. Spatial distributions of 24-hr maximum Mn (1g/M?) in the Chicago area for 1985
and 1990.

AIR QUALITY TRENDS IN ILLINOIS

146

0.035
e

0.042
e

0.045
0.038
FF oofs 20.039

0.939 0.05

Figure 50. Spatial distributions of annual
mean Mn ( ug/M3 ) in the Chicago area for
1980, 1985, and 1990.

AIR QUALITY TRENDS IN ILLINOIS

Figure 51. Spatial distributions of annual mean As, Cd, Cr, and Ni ( ug/M3 ) in the Chicago area for 1990.

147

AIR QUALITY TRENDS IN ILLINOIS

Table 5. Summary of Measured Concentrations of Some Volatile Organic Compounds in Illinois (N = number of samples)

Chicago** Metro East** Bondville (rural)**
Compound Ref* Years N Mean SD Max N Mean SD Max N Mean SD Max
acetaldehyde 3 1990 28 3.8 2
acetylene 4 1990-91 60 SPP 2.7 13.4
acetone 3 1990 28 IE? 19
acetaldehyde 3 1990 28 3.8 9
benzene 1 1986-90 103 4.6 66 54 83 10.6 17.2 102 23 1.3 0.5 2.4
2 1988 34 2.8 23
3 1990 29 5.1 18
4 1990-91 78 7.6 68 38
bromodichloromethane 4 1990-91 56) 2057" Sito 552
butane 4 1990-91 71 13.0 29.2 241
i-butane 4 1990-91 64 TM. 12> “91
carbon tetrachloride 1 1986-90 103 0.7 0.2 iN 7/ 83 0.9 0.3 ae 23 0.8 0.2 1.2
3 1990 29 2.9 23
4 1990-91 63 7.4 95 34
chlorobenzene 1 1986-90 103 0.3 0.2 1.6 83 3.0 6.3 36 23 0.2 0.1 0.5
chloroform 1 1986-90 103 0.3 0.2 1.6 83 0.5 0.9 6.6 23 0.3 0.1 0.4
4 1990-91 66 4.6 7.2 40
cumene 4 1990-91 65 GitenS4:2. 27/7
n-decane 4 1990-91 53 8.6 38.8 282
dibromochloromethane 4 1990-91 63 81.8 189 1025
o-dichlorobenzene 4 1990-91 52) 20:4) 33:3) 150
2-2-dimethylbutane 4 1990-91 76 4.5 7.6 41
ethane 4 1990-91 60 9.9 33° «22:8
ethylbenzene 1 1986-90 103 1.4 1:2 7.6 83 69) ee 0 23 0.4 0.3 1.6
2 1988 34 3.0 26
3 1990 29 1.8 6.6
4 1990-91 78 8:8. 22.7 «195
ethylene 4 1990-91 60 Tks! Soe 16:7)
formaldehyde 3 1990 28 6.0 17
n-heptane 4 1990-91 56 6.4 8.3 45
hexane 4 1990-91 78 0:2) FS7.9 137
2-methylbutane 4 1990-91 Ti AG 168% 76
methylene chloride 4 1990-91 65: 319:5 34:2. 174
2-methylpentane 4 1990-91 77 Te 8.7 40
3-methylpentane 4 1990-91 77 6.4 69 39
n-octane 4 1990-91 39 13) 1.1 “by /
pentane 4 1990-91 78 86 24) 72
alpha-pinene 4 1990-91 60 0.1 0.2 1.4
n-propylbenzene 4 1990-91 74 85) 29:1 247
tetrachloroethylene 1 1986-90 103 1.8 1.6 9.1 83 1.4 1.3 6.1 23 0.4 0.3 1.2
3 1990 29) lO 41
4 1990-91 60 18. 1074, 83

148

AIR QUALITY TRENDS IN ILLINOIS

Table 5. (Concluded)

Compound

toluene

1,1,1-trichloroethane

trichloroethylene

1,3,5-trimethylbenzene

m-, p-xylene

o-xylene/styrene

Notes:

*References: 1. Sweet and Vermette (1992). 2. McAlister et al. (1989). 3. McAlister et al. (1991). 4. Wadden et al. (1992).

Ref*

PWR KWH WN

fhwWNeK HhWN

Years

1986-90
1988
1990

1990-91

1986-90
1988
1990

1990-91

1986-90
1990

1990-91

1990-91

1986-90
1988
1990

1990-91

1986-90
1988
1990

1990-91

N

103
34
29
78
103
34
29
73
103
29
76
77

103
34
29
78
103
34
29
78

Chicago**
Mean SD
8.9 8.9
61.1
11.8
231515 24:6
3.3 3.5
4.1
8.1
7 sper PSY
1.0 1.0
1.2
3.2 ppd
6.8 24.4
3.9 8.3
20.6
9.8
24.6 86.7
2.9 55
3:9
3H.
18.8 79.4

Max

83

83

83

83

Metro East**

Mean SD
8.5 ONS 4aS
3.9 i os} |
2.1 5.8 43

16 AZ 4 SZ

3.3 5:0. 3S

Max

N

23

23

23

23

23

Bondville (rural)**

Mean

3.0

1.1

0.6

1.2

SD

23

0.6

0.5

0.8

0.9

Max

9:5

3.9

4.3

**For Reference 1, sampling was carried out at several sites in southeastern Chicago and several sites in the Metro East area. The compounds
reported in Reference 1 were detected in 95 percent of the samples. For References 2 and 3, sampling was carried out at Carver High School in

Chicago and Sauget in the Metro East area. Sampling for Reference 4 was carried out near the Chicago city center.

The compounds listed in References 2 and 3 were detected in at least 50 percent of the samples. In all cases, for purposes of computing mean
values, sample concentrations reported as < the detection limit were assumed to have concentrations of one-half of the detection limit. The rural
samples were collected at an agricultural site in Champaign County, 8 km west of Champaign, near Bondville, Illinois.

149

AIR QUALITY TRENDS IN ILLINOIS

Variability between concentrations of organic com-
pounds between urban areas occurs for some com-
pounds, and not for others. For benzene, chlorobenzene,
ethyl benzene, trichloroethylene, and meta- plus para-
xylene, mean concentrations show differences ranging
from factors of 2 to 10. Note, however, that the variability
of the respective concentrations is very high, judging
from the standard deviation, so that the differences may
not be statistically significant. It is noteworthy that
the Metro East concentration was higher for each of
these compounds.

Urban/tural variability, or to be more precise, the lack
of differences in concentration between urban and rural
locations, is seen for a few compounds, notably carbon
tetrachloride and chloroform (Sweet and Vermette,
1992). Such uniform concentrations are evidence of a
well-mixed contaminant with a long atmospheric resi-
dence time and few current sources. Carbon tetrachlo-
ride is an example of a compound no longer used
commercially, so its current sources are very small.
However, its residence time in the atmosphere is long,
so over time it has become well mixed in the atmo-
sphere, and its concentrations are relatively uniform
regardless of whether the sampling site is in a rural or
urban area (Sweet and Vermette, 1992).

DISCUSSION

Trends over Time: Increasing, Decreasing,
and Level Concentrations

Criteria Pollutants. Six pollutant types are included
among the criteria pollutants: CO, Pb, NO,, O,, SO,,
and particulate matter. Some of these are measured
over multiple averaging times, but there is not neces-
sarily a standard for every averaging time. For exam-
ple, NO, is measured over averaging times of 1 hour,
24 hours, and the calendar year, but there are state and
national primary standards only for the annual mean.
Particulate matter is measured as TSP, for which there
are only state standards (for both annual geometric
mean and 24-hr maximum concentrations), and as
PM.,,, for which there are only national standards
(annual arithmetic mean and 24-hr maximum). Thus,
for the six pollutant types, measurements have been
made for 14 separate pollutant-averaging time combi-
nations, 11 of which have state or national primary
standards. The PM,, measurements have only been
made for a few years, not enough to make a trend test
meaningful, so of the 14 combinations, only 12 (at the

150

most) have been tested for time trends within individual
geographic regions. In some regions there were not
enough sites to carry out trend tests. The results of
the trend tests within regions are summarized in
table 2.

For the state as a whole, 12 pollutant/averaging time
datasets were tested for time trend. Seven of these
(table 2) showed trends toward decreasing concentra-
tions significant at the 1 percent or 2 percent level,
four had no significant trend (5 percent) (including
DO before accounting for temperature effects), and
none showed increasing trends. After accounting for
temperature effects, O, showed a decreasing trend (2
percent level, table 3), rather than no trend.

The same 12 kinds of datasets were tested for trend in
the Chicago area. Eight of these (table 2) showed
decreasing trends at the 5 percent level (all but one of
these were at the 1 or 2 percent level). Four showed no
significant trend at the 5 percent level, and none
showed an increasing trend. After accounting for tem-
perature, O, showed a decreasing trend (2 percent
level, table 3), rather than no trend.

In the Metro East area, only four pollutant/averaging
time datasets were measured at enough sites to warrant
testing for time trend. Only Pb showed a decreasing
trend (1 percent), while three other datasets showed no
significant trend (5 percent). After accounting for
temperature, O, showed a decreasing trend at the 6
percent level, but not at the 5 percent level. No sig-
nificant (5 percent) increasing trends were found for
criteria pollutants.

Nine datasets were tested for time trend in the “remain-
der” region, four of which included a few sites from the
Metro East region when there were insufficient sites for
separate tests there. Three of these datasets showed
decreasing trends at the 5 percent level (all but one at
the 1 percent level), and five datasets showed no sig-
nificant (5 percent level) trends. After accounting for
temperature, the decreasing trend of O, with time was
significant only at the 10 percent level. Again, there

were no significant increasing trends.

Aside from Pb, the largest decreasing linear trends
were found for annual mean NO,, at -7.1 percent per
year statewide and -6.8 percent per year in the Chi-
cago area. The greatest magnitudes of decreasing
linear trends were found for Pb: -12.6 percent per
year in the Metro East area, and -20 to -25 percent
per year statewide and in the other geographic areas.

AIR QUALITY TRENDS IN ILLINOIS

Noncriteria Pollutants. Only trends in annual mean
concentrations of the noncriteria pollutants measured—
NO,,, SO,, and the six metals (As, Cd, Cr, Fe, Mn,
and Ni)—were tested for trend over time. Currently,
24-hr average metal concentrations are also being
measured, but their record is too short to permit a
meaningful test of time trend. The eight noncriteria
pollutants were all tested for trend in all four regions,
except for the Metro East region, where the data were
not adequate to test Cr and Ni. Of the 30 trend tests, 20
showed no significant (5 percent) trend. Over the state
as a whole, and in the Chicago area, two species
showed significant decreases: SO,” at the 5 percent
level and As at the | percent level. In the Metro East
area, Fe and Mn showed significant increases (the only
increasing trends found in this study): Fe at the 2 per-
cent level, and Mn at the 5 percent level. In the “remain-
der” region, SO,” (2 percent), As (1 percent), Cd (2
percent), and Mn (5 percent) showed significant decreases.

Spatial Trends: Illinois Hot Spots

Comparison of Geographic Regions. Comparison of
median and maximum pollutant concentrations within
geographic regions, from the yearly box plots in figures
1-30, indicates which geographic areas of the state
experienced the highest concentrations of air pollutants.
Chicago generally had higher median regional values
of annual mean NO,, and of annual mean and 24-hr Cr.
It also experienced higher median annual mean Ni and
the highest individual 24-hr Ni concentrations. These
Cr and Ni results agree with the observations of Sweet
and Gatz (1988), based on measurements of fine and
coarse particles with dichotomous samplers. On the
other hand, the Chicago area generally experienced
lower concentrations of 3-hr and 24-hr SO, than the
rest of the state.

The Metro East area generally experienced higher
concentrations than the rest of the state for annual
mean Pb, annual mean TSP, and both 24-hr and annual
mean As, Cd, Fe, and Mn. Again, the As and Cd results
agree with those of Sweet and Gatz (1988), but for Pb
and Mn, Sweet and Gatz found higher concentrations
in Chicago.

The Chicago and Metro East areas experienced higher
concentrations than the rest of the state for 1-hr max-
imum O, and annual mean SO.,.

Chicago Area Concentration Contours. Based on the
contour plots in figures 32-51, statements about
preferred areas of relatively high pollutant concentra-

tions should be regarded as best guesses, based on
available data. But these statements are uncertain as to
magnitude of pollutant concentrations as well as to pre-
cise location. The available data are not adequate for
highly certain conclusions because: 1) sampling sites
are distributed nonuniformly over the area; 2) the
plotted data represent only three years (1980, 1985, and
1990); and 3) numbers of sampling sites, and their
locations, have changed from one plotted year to the
next, and they have recently dwindled to the extent that
the meaning of contours is questionable.

Only one locale in the Chicago area stands out for
being the location of high concentrations for multiple
pollutants: the industrial area of southeast Chicago
around Lake Calumet. This area has persistent, rela-
tively high concentrations of some metals, in terms of
both 24-hr maxima and annual means. The metals with
highest 24-hr maxima in the Lake Calumet area were
Fe and Mn. There was a persistent peak in 24-hr SO,>
in this area as well. Metals with local peaks in annual
mean concentrations include Pb, and possibly Cd, Cr,
and Mn, although the evidence for the latter three is
somewhat weaker.

Other locations in the Chicago area appeared as areas
of persistent high concentrations of only one or two
pollutants. One-hour maximum CO was persistently
high along the Lake Michigan shoreline north from
downtown Chicago. Annual mean NO, concentra-
tions were also persistently high near the Chicago
city center (the Loop). The plotted O, data (figure
37) suggest a possible persistent area of high 1-hr
maximum concentrations in northeast Cook County.
There is also some weak evidence for relatively high
1-hr maxima of O, and annual mean NO,. Southwest
suburban Bedford Park had persistently high concen-
trations of 3-hr maximum, 24-hr maximum, and
annual mean SO.. Finally, various locations in Will
County experienced persistently high 24-hr maxi-
mum TSP concentrations, perhaps related to wind
erosion of agricultural soils.

CONCLUSIONS

Temporal Trends

Criteria Pollutants. Of all the criteria pollutants tested
for time trend in any geographic region of the state,
results indicated only decreasing trends or no signifi-
cant (5 percent) trends. No increasing trends in criteria
pollutants were detected.

151

AIR QUALITY TRENDS IN ILLINOIS

For the state as a whole, seven of 12 pollutant/averag-
ing time datasets tested for time trend showed signifi-
cant trends (5 percent level or better) toward decreasing
concentrations over the 1978-1990 data period. Pb had
the largest decreasing mean linear trend statewide, -20.5
percent per year. After accounting for temperature
effects, OQ, showed a significant decreasing trend (2
percent level), rather than no trend.

In the Chicago area, eight of the 12 datasets showed
decreasing trends at the 5 percent level (all but one of
these’ were at the 1 or 2 percent level). Again, Pb had
the largest trend, -21.2 percent per year. After account-
ing for temperature, O, showed a decreasing trend (2
percent level, table 3), rather than no trend, in the
Chicago area.

In the Metro East area, only four pollutant/averaging
time datasets were measured at enough sites to warrant
testing for time trend. Only Pb showed a significant (5
percent) trend, a trend toward decreasing concentra-
tions. The Pb trend was sizable, -12.6 percent per year,
but much smaller than the Pb trend observed in all
other areas of the state. After accounting for tempera-
ture, OG; showed a decreasing trend at the 6 percent
level, but not the 5 percent level.

Three of nine datasets tested for time trend in the “re-
mainder’ region showed decreasing trends at the 5 per-
cent level or better. Again, Pb had the largest trend,
-24.8 percent per year.

Noncriteria Pollutants. The eight noncriteria pollut-
ants were all tested for trend in all four regions, except
for the Metro East region, where the data were not
adequate to test Cr and Ni. Of the 30 trend tests, 20
showed no significant (5 percent) trend. Over the state
as a whole, and in the Chicago area, two species
showed significant decreases—SO,” and As. In the
Metro East area, Fe and Mn showed significant
increases (the only increasing trends found in this
study). In the “remainder” region, SO,”, As, Cd, and
Mn showed significant (5 percent or better) decreases.

Spatial Trends

Comparison of median and maximum pollutant con-
centrations within geographic regions, from the yearly
box plots in figures 1-30, indicates which geographic
areas of the state experienced the highest concentra-
tions of air pollutants. Chicago generally had higher
median regional values of annual mean NO,, and of
annual mean and 24-hr Cr. It also experienced higher

152

median annual mean Ni and the highest individual 24-
hr Ni concentrations. On the other hand, the Chicago
area generally experienced lower concentrations of 3-hr
and 24-hr SO, than the rest of the state.

The Metro East area generally experienced higher
concentrations than the rest of the state for annual
mean Pb, annual mean TSP, and both 24-hr and annual
mean As, Cd, Fe, and Mn. The Chicago and Metro East
areas experienced higher concentrations than the rest of
the state for 1-hr maximum O, and annual mean SO.,,.

Within the Chicago area, only one location stands out
for its high concentrations of multiple pollutants. This
is the industrial area of southeast Chicago around Lake
Calumet. This area has persistent, relatively high an-
nual mean or 24-hr concentrations, or both, of SO/,
Fe, Mn, and Pb, and possibly Cd, Cr, and Mn. The
evidence for the latter three is somewhat weaker than
for the others, however. Other locations in the Chicago
area appeared as areas of persistent high concentrations
of only one or two pollutants.

ACKNOWLEDGMENTS

Bob Swinford and Dave Kolaz, of the Illinois EPA,
were very helpful in providing data, answering ques-
tions about the data, and discussing numerous issues.
Sherman Bauer entered most of the data into computer
files. Peter Scheff and Clyde Sweet provided data on
concentrations of organic pollutants. Bob Sinclair pro-
vided guidance for conversion of the data to GIS files.
Tom Heavisides supplied information on data sources.

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ATMOSPHERIC DEPOSITION
TRENDS IN ILLINOIS

Donald A. Dolske and Van C. Bowersox
Illinois State Water Survey

INTRODUCTION
Purpose

In the conceptual design of Illinois’ Critical Trends
Assessment Project or CTAP, (IENR, 1992), atmo-
spheric deposition is treated as an environmental
process that describes how pollutants from various
sources are delivered to receptors. The CTAP plan
addresses six natural environmental receptors: (1)
forests, (2) agroecosystems, (3) streams and rivers, (4)
lakes and impoundments, (5) prairies and savannas, and
(6) wetlands. Among these, atmospheric deposition is
considered for forest ecosystems and lakes and im-
poundments, because research has shown possible
damage to these receptors from certain kinds and
amounts of atmospheric deposition. For example, the
recently completed National Acid Precipitation Assess-
ment Program reported evidence that acidic deposition,
along with other stresses, affects some high-elevation
spruce forests in the eastern United States (Shriner et
al., 1990). Long-term changes in low-elevation forests
may be possible as well.

This section describes the characteristics of atmo-
spheric deposition in Illinois, which pollutants have the
highest concentrations, and how the concentrations
vary across the state and throughout the year. Where
there are data sufficient for an analysis, changes over
several years are calculated and trends are inferred, if
these changes are significant. Also shown are maps of
deposition fluxes, which together with the concentra-
tion data provide information necessary for assessments
of the exposure of Illinois’ natural environment to
atmospheric deposition. While an explicit description
of the source-receptor relationships for major pollut-
ants, such as sulfur dioxide or SO, and nitrogen oxides
or NO., is not considered in this section, the sources of
these pollutants in Illinois and surrounding states are
compared to their occurrence in atmospheric deposi-
tion. Finally, additional work is discussed that is
necessary to improve our assessment of atmospheric
deposition in Illinois and over Lake Michigan.

ATMOSPHERIC DEPOSITION TRENDS IN ILLINOIS

Scope

Atmospheric deposition includes gases and aerosols
(solid, liquid, or mixed phase). It includes primary
pollutants, which retain their chemical identity between
source and receptor; and it includes secondary pollut-
ants, which undergo physical or chemical transforma-
tion during transport in the atmosphere. Calcium is an
important chemical element in Illinois atmospheric
deposition. As a primary pollutant, calcium is found in
dust particles that become airborne during automobile
traffic on unpaved roads (Barnard et al., 1986). Dust
particles are relatively large (diameter >1 micrometer
or [m); and because of their size and density, the
primary mechanism for their removal from the atmo-
sphere is gravitational settling, a form of atmospheric
deposition. Sulfate is a secondary pollutant in the
Illinois atmosphere, because it is emitted mostly as a
gas, SO,, when fossil fuels are burned. In the air it is
transformed into relatively small sulfate aerosols
(diameter < 1 [1m), which are removed by deposition
processes other than gravitational settling.

Deposition of pollutants from the atmosphere is a
continuous process, though there are large temporal
variations in the deposition rate or flux. These varia-
tions relate to the kind of deposition that is occurring
and to surface and atmospheric conditions. There are
two kinds of atmospheric deposition, wet and dry. Wet
deposition is defined as the delivery of pollutants to the
surface by precipitation. Dry deposition is the delivery
of gases and aerosols to the surface by mass transfer
processes other than precipitation. By “surface,” here,
is meant soil or rock or water bodies or crops or forests
or whatever man-made structures cover these natural
surfaces (e.g., buildings, monuments, highways, park-
ing lots). In principle, dry deposition occurs continu-
ously, while wet deposition occurs episodically, e.g.,
when it rains.

Wet deposition includes precipitation-borne pollutants,
such as dissolved gases, dissolved particles, insoluble
particles, and dissolved materials that are leached from
the chemical matrix of particles that do not dissolve in
precipitation. An example of this latter type of wet
deposition is the potassium that rainwater leaches from
soil particles, even though the solid soil particles do not
themselves dissolve. Precipitation-borne pollutants can
be organic or inorganic. Wet deposition is measured by
chemically analyzing precipitation. To measure the
chemistry in samples of precipitation collected sepa-
rately from dry deposition requires a device that opens
only during precipitation. Such a device, a wet-only

155

ATMOSPHERIC DEPOSITION TRENDS IN ILLINOIS

deposition collector, involves some compromise, since
dry deposition occurs continuously even during precip-
itation. Under most conditions and for most chemicals,
however, this compromise is inconsequential, since the
total, wet plus dry, deposition during precipitation is
dominated by the wet part (wet > 10 x dry). An
example of an exception to this statement would be
rain that occurs during blowing dust, when the chemi-
cal deposition from the falling dust dominates that
from the rain.

For this project, wet deposition data from the national
network, the National Atmospheric Deposition Pro-
gram/National Trends Network (NADP/NTN) were
used. The NADP/NTN reports the concentrations of
dissolved calcium or Catt, magnesium or Mg**, sodium
or Na*, potassium or K*, ammonium or NH,’, sulfate or
SO,;, nitrate or NO,, chloride or Cl, orthophosphate or
PO ie and free hydrogen ion or H*, measured in pH
units. No trace metals or organic compounds are
measured by the NADP/NTN. Samples are filtered to
remove insoluble materials, so the NADP/NTN pro-
vides data for the soluble major inorganic ions found in
precipitation, i.e., for those chemicals that result in
acidic deposition or “acid rain,” which occurs over all
of Illinois.

Dry deposition includes the mass transfer of pollutants
to the surface by a variety of physicochemical pro-
cesses: turbulent diffusion, diffusion followed by
surface sorption of gases, gravitational settling of large
particles, impaction, and interception of solid and
liquid particles. Dew, fog, and mist are examples of
liquid particles that are considered dry, not wet, depo-
sition, because the mass transfer to the surface is by
interception, not precipitation.

Dry deposition fluxes are strongly affected by atmo-
spheric factors, which influence the rate at which
pollutants are delivered to a receptor surface, and by
surface factors, which influence the efficiency with
which pollutants “stick” to a receptor surface. Among
the atmospheric factors are wind speed and turbulence,
air temperature, solar radiation, and relative humidity.
Among the surface factors are roughness, wetness,
surface-to-air temperature difference, and type of sur-
face, which includes whether the surface is animate
(plant) or inanimate. The relative importance of these
factors in determining the dry deposition rate also
depends on the physical and chemical nature of the
pollutant. For example, factors that affect the mass
transfer of carbonaceous soot, an unreactive, insoluble
particle, are much different than the ones affecting
nitric acid, a highly reactive, soluble gas.

156

The dry deposition of gases and submicron aerosols
involves highly complex processes for which direct
measurements are intractable on a spatial domain of
the size and complexity of Illinois. Thus, an indirect
method was applied to infer, rather than measure, dry
deposition fluxes. This inferential method employs a
conceptual model. This computerized model esti-
mates an atmosphere-to-surface coupling parameter
known as the deposition velocity, V,. The dry
deposition flux is the product of V, and the measured
air concentration of a particular pollutant. Model
inputs include the atmospheric and surface factors
discussed earlier. These are measured at a network of
sites sponsored by the National Oceanic and Atmo-
spheric Administration’s Atmospheric Turbulence
and Diffusion Division (NOAA/ATDD). Land use
and vegetation type and status are also reported at
these sites, along with the airborne concentrations of
Cl, SO,, particulate NO,, nitric acid vapor or
HNO,, and SO,. The NOAA/ATDD sampling system
is especially designed to exclude large particles,
since the inferential method of calculating dry
deposition applies specifically to gases and sub-
micron aerosols.

The dry deposition of large particles, which have an
aerodynamic diameter > 1 jum, is typically estimated
from an analysis of the mass of a pollutant accumulated
on a surrogate surface (Dolske and Gatz, 1984). The
term “surrogate surface” is used, because it is impracti-
cal to measure dry deposition on every animate (plant)
and inanimate surface, and so a substitute surface is
selected. Since the physical process that dominates
large particle dry deposition is gravitational settling,
the surrogate surface must receive and retain large
particles in a manner equivalent to soils, rocks,
streams, building surfaces, plant surfaces, etc. In prin-
ciple, there is no universal surrogate for surfaces with
such diverse characteristics.

To estimate the dry deposition of large particles,
(i.e., sedimentation or dryfall) for the CTAP, data
from the NADP/NTN were used. The NADP/NTN
measures dryfall in separate samples collected by the
sampler also used for precipitation. This device, a
wet/dry collector, has two identical containers; it dis-
criminates between wet and dry conditions, exposing
the wet deposition container during precipitation

and the dryfall container at all other times. Dryfall
samples are sent for analysis of the same analytes
measured in precipitation.

Evaluations of dry deposition monitoring methods
(Hicks et al., 1986) have raised objections to this

method for monitoring dry deposition. One objection
is that contamination by foreign matter (plant parts,
insects, bird feces, etc.) compromises sample integrity.
A second objection is that gaseous and fine-particle
deposition are poorly characterized. For this study,
however, the data were screened to minimize the
effect of sample contamination from foreign matter.
As to the second objection, the data were used only
for dryfall or sedimentation estimates, not for the
deposition of gases and submicron aerosols.

DATA SOURCES
Wet Deposition

Data for assessing the precipitation quality and wet
deposition fluxes for Illinois’ CTAP were taken from
the NADP/NTN. This network began operations at
20 sites in 1978 and today operates at nearly 200
U.S. locations, with a site in American Samoa and
one in Puerto Rico. Sites were chosen according to
network design criteria (Robertson and Wilson,
1985), and equipment was installed to minimize
sampling artifacts from local point and area sources
(Bigelow, 1984). Based on these criteria, data from
network sites are expected to be representative of the
region where the sites are located. Using a wet/dry
collector of a specified type, one-week, wet-only,
deposition samples are collected on Tuesday morn-
ings in the NADP/NTN (Bigelow and Dossett, 1988).
All samples are sent to a single Central Analytical
Laboratory (CAL), located at the Illinois State Water
Survey. At the CAL, approved standard procedures
are followed for sample analysis (Peden et al., 1986)
and data validation (Bowersox, 1984).

Table 1 lists the locations, site types, and periods of
operation of NADP/NTN sites in Illinois. Seven sites
are currently active in the state; the site near Salem
(1L47) was closed in 1988 and moved to Stephen
Forbes State Park, near Omega (IL99), in 1989.
Figure 1 shows the locations of NADP/NTN sites in
and around Illinois. Data from outside the state are
included with Illinois data during all spatial objective
analyses of precipitation quality and deposition flux
fields. This approach precludes “edge effects” from
extrapolation of the data, by providing in-state and
out-of-state observations that can be used to interpo-
late data fields out to and beyond state borders. Edge
effects can produce spurious results in objective
analysis schemes.

ATMOSPHERIC DEPOSITION TRENDS IN ILLINOIS

Dry Deposition

Each of the NOAA/ATDD sites, Argonne, IL19, and
Bondville, IL11 (see figure 1 for locations), was
deliberately co-located with an existing NADP/NTN
site. The objective of the NOAA/ATDD program is to
develop and verify models for estimating dry deposi-
tion from easily measured surface conditions, micro-
meteorological parameters, and air quality data (Hicks
et al., 1991). Weekly average air concentration data
and a site-specific deposition velocity, V,, are used to
calculate dry deposition amounts for certain gases and
ions associated with small particles. Surface conditions
include local topography, land use, and vegetation type
and status. Each of these surface factors contributes to
V,, at varying length scales, and these factors vary only
slowly over time. Micrometeorological parameters
(wind speed and direction and their fluctuations, tem-
perature, humidity, solar radiation, surface wetness,
and precipitation rate) are monitored continuously and
digitally recorded every 15 minutes.

All of these factors are input variables for a computer
model that provides a set of V, values for a predeter-
mined time period (usually weekly). Each V, in the set
is specific to the chemical species and to the gas or
particle size range in which that species is found in the
atmosphere. The weekly-average airborne concen-
trations of particulate Cl, SO,-, and NO,,, and of gases,
HNO, and SO,, are reported for each site. The sampling
system excludes large particles, since the inferential
method of calculating dry deposition applies specifi-
cally to gases and submicron aerosol-associated materials.

The weekly accumulated flux of each chemical is then
calculated as the product of the average concentration
and the appropriate V . These calculated results are
periodically compared with directly measured dry
deposition data at the NOAA/ATDD sites (Meyers et
al., 1991), so that the model can be calibrated and
revised as needed. The current version of the NOAA/
ATDD model produces dry deposition estimates that
generally agree within 10 to 20 percent of the directly
measured values. Further model calculations are used
to “scale up” to larger areas and longer time scales.

In addition to the NOAA/ATDD data, longer term
(annual) and wider area concentration data for other
chemical species in Illinois’ atmosphere were provided
through the CTAP (see Air Quality Trends in Illinois
chapter). For each of these chemicals (arsenic or As,
cadmium or Cd, chromium or Cr, iron or Fe, manga-
nese or Mn, nickel or Ni, nitrogen dioxide or NO,,
NO,, ozone or O,, lead or Pb, SO,, and SO,", as well

157

ATMOSPHERIC DEPOSITION TRENDS IN ILLINOIS

Table 1. National Atmospheric Deposition Program/National Trends Network Sites in Illinois

Site designation PETE Boe IL 18 IL 19* IL 35 IL 47 IL 63 IL 78 IL 99
Site location :
Latitude 40°3'12". 41°50'29" = 41°42'4"—- 37°42'36" = 38°38'36" = 37°26'8" = 40°56'0" —- 338°42'36"
Longitude 88°22'19" 88°51'4" 87°59'43" = 89°16'8"—- 88°S58'1" — 88°40'19" 90°43'23" = 88°44'57"
Elevation (m)** 212 265 229 146 173 161 229 153
Nearest town Bondville Shabbona Argonne Carbondale Salem Glendale Monmouth Omega
Site type rural rural suburban rural suburban rural rural rural

Dates of operation
Wet deposition
Begin 27 Feb 79 26 May 81 11 Mar80 31 Jul79 15 Apr80 30Jan79 8Jan85 26 Sep 89
End continuing continuing continuing continuing 22 Nov 88 continuing continuing continuing

Dryfall deposition

Begin 27 Feb 79 26 May 81 1Apr80 20Jul79 15 Apr 80 30 Jan 79 none none
End continuing continuing continuing continuing 22 Nov 88 continuing none none
Notes:

*Also a NOAA/ATDD dry deposition site where airborne pollutant concentrations and atmospheric and surface factors are
measured so that deposition can be inferred.
**Elevations are reported in heights above mean sea level.

Site Type
+ NADP/NTN site in IL

* NADP/NTN and co-located
NOAA site in IL

0 NADP/NTN site outside IL;
used for spatial variation
analyses

Illinois Site ID

11 - Bondville
18 - Shabbona
19 - Argonne
35 - Carbondale
47 - Salem

63 - Glendale
78 - Monmouth
99 - Omega

Figure 1. Map of the Illinois region, showing locations of sites providing atmospheric deposition data
used in the CTAP.

158

as total suspended particulate matter or TSP), annual
Statistics from the Illinois Environmental Protection
Agency (IEPA) concentration data were combined with
the appropriate NOAA/ATDD V, to estimate the
median and range of dry deposition loadings.

Dryfall deposition samples are collected at a subset of
NADP/NTN sites, using the “dry side” of the wet/dry
collector. There are five dryfall sites in Illinois, listed
in table 1. While the wet deposition samples at NADP/
NTN sites are collected weekly, dryfall samples are left
to accumulate for eight weeks, and then are sent to the
CAL for analysis. At the CAL, 250 milliliters (mL) of
deionized water is added to the samples, the mixture is
swirled to remove dry deposited matter from all sur-
faces, and the samples are allowed to equilibrate over-
night. Next, the samples are filtered (0.45 um pore
diameter), handled, and analyzed the same way as rain
samples (Peden et al., 1986; Lockard, 1987). Results
are reported in mass flux units.

DATA ANALYSIS METHODS
Data Screening Procedures

Data from NADP/NTN collection sites in Illinois and
neighboring states were used to assess atmospheric
deposition for the CTAP. These data were used only
when standard network procedures for field, laboratory,
and data management operations were followed.
Several documents describe the NADP/NTN quality
assurance program (NADP/NTN, 1990), field site
sample collection and measurement methods (Bigelow
and Dossett, 1988), analytical laboratory operations
(Peden et al., 1986), and data verification and screening
procedures (Bowersox, 1984).

Adhering to standardized procedures ensures that data
comparisons among sites are not biased by differences
in equipment or procedures and that comparisons over
time at a site are not biased by changes in equipment or
procedures. Where NADP/NTN measurements of
atmospheric deposition were used for Illinois’ CTAP,
analyses were based on data that are comparable in
space and time.

In addition to the selection of samples collected
according to standard procedures, all wet deposition
data must have been for wet-only precipitation sam-
ples, uncontaminated by matter foreign to the forma-
tion and occurrence of precipitation. The requirement
for wet-only sample collection was described in the
introduction of this chapter. An example of foreign

ATMOSPHERIC DEPOSITION TRENDS IN ILLINOIS

matter is a leaf or a bug. A report of foreign matter
in a sample, while necessary, is not sufficient for
exclusion of a record from the analysis. An objective
system for identifying an NADP/NTN sample as
“contaminated” requires the co-occurrence of foreign
matter and an anomalous chemical composition
(Bowersox, 1984). “Contaminated” samples are ex-
cluded from the analysis.

Finally, to create chemical concentration maps and to
calculate interannual trends, the data must satisfy four
completeness criteria. Expressed as percentages, these
four criteria quantify the fraction of time and the frac-
tion of the precipitation amount for which there are
valid wet deposition data. In summary, these criteria
require that there be:

1. Precipitation amount measurements for 90 percent
or more of the time,

2. Valid chemistry measurements for 75 percent or
more of the time,

3. Valid chemistry measurements for 75 percent or
more of the measured precipitation, and

4. A ratio of total wet-only sample volumes to total
measured precipitation of 75 percent or more, tak-
ing data records where both values are available.

These same criteria are applied in the publication of the
NADP/NTN Annual Data Summary (NADP/NTN,
1991). Wet deposition data that pass these validation
tests have an acceptable level of comparability and
representativeness for making maps and performing
trend analyses for the CTAP.

Figure 2 displays the record of when wet deposition
measurements were made at each of the Illinois sites.
Also displayed are the periods when data passed the
validation tests for calculating trends. Whole years of
data were used to evaluate the four completeness cri-
teria. Consequently, a year either passed or failed the
tests and is included or excluded, respectively, from the
trends analyses. A meteorological year, 1 December
through 30 November (Trenberth, 1983), was used to
ensure proportionate representation from each season
in each year (i.e., December of year | and January and
February of year 2 comprise a winter season).

For making maps, the completeness criteria were
applied to the time period on which the maps were
based (multiple meteorological years). Notice in the
figure that valid data from the Monmouth site (IL78)
began in 1986. Given the importance of the data from
this site in defining east/west gradients in several of the
chemical species, it was decided that concentration

159

ATMOSPHERIC DEPOSITION TRENDS IN ILLINOIS

SS Fonvrait | L__Jowi*"®
Wii beens, WEED teens
ees ee RS eel 8 ew
M—DBORO* Lien
Fee beatae ch AMI let Beara cy
WAR AAAAANN

ILLINOIS SITES

RX SSSR
RE AR

1979 1981 1983 1985 1987 1989 1991
YEAR
Figure 2. Record of wet and dry depositio nts at Illinois sites (sé
for site locations). eae sition Be ive a Bi drafa hin mea Shi NADPINEN ites. Depositic
velocity, V n NOAA/ATDD site: » wie for long-term trends

eme re fror
‘viet HERO TRENDS) are also indicated.

160

ATMOSPHERIC DEPOSITION TRENDS IN ILLINOIS

maps would be based on valid data beginning in 1985
and ending in 1990. During this period the Salem site
(IL47) terminated operations and was replaced by the
Omega site (IL99), with a one-year (1989) hiatus. In
the interest of having maximum spatial coverage, valid
data from these two nearby sites were pooled in mak-
ing concentration maps. With only a year of valid data
at the Omega site, no statistical tests were done to
evaluate whether these two data sets were from the
same population and could be pooled without introduc-
ing a bias. This should be done when more data are
available for future analyses.

Figure 2 also displays when and where dryfall (i.e., dry
bucket) measurements are made at Illinois sites (see
also figure 1 and table 1). Unlike NADP/NTN wet
deposition data, dryfall data are not routinely screened
to ensure samples that are dry-only and uncontam-
inated. Gatz et al. (1988) examined dry bucket data
from the entire NADP/NTN to develop criteria that
could be applied to remove unrepresentative contami-
nated samples from the data. Their approach was to
examine the distribution of ionic concentrations, while
applying ever more stringent screening criteria, thus
eliminating more and more data. Samples were
removed in groups, first due to sample collection and
handling errors (i.e., nonstandard procedures), and-
successively due to contaminants: water, bird drop-
pings, plant or insect debris, and other contaminants.
Screening to the level where samples with visible
contaminants were removed resulted in a clear and
Statistically significant break in the NH,*, K*, and PO,’
distributions; this level sharply reduced a positive bias
in these constituents. Even H* and SO,*> showed some
effects from removal of these samples, though not as
marked. For these reasons, dryfall data from Illinois
sites were screened to this level for this report.

Recall from previous sections that the NADP/NTN dry
bucket sampling protocol is to collect an eight-week
integrated sample from the “dry side” of a wet/dry
collector. This results in either six or seven samples per
year. Removing samples to the screening level described
above, leaves from two to six samples per year at each
of the five Illinois sites. With so few samples in a year,
all of the data from each site were pooled to examine
spatial patterns in dryfall deposition, but the calculation
of interannual trends at a single site was impractical.

The dry deposition of gases and submicron aerosols in
Illinois is inferred from measurements at two NOAA/
ATDD sites (Argonne and Bondville), shown in figure
1 and listed in table 1. Figure 2 shows the period of
operation of these two stations. As previously de-

scribed, data from the NOAA/ATDD sites include the
chemistry of weekly-integrated air samples and 15-
minute-average micrometeorological measurements,
digitally recorded. These are input variables to a model
that infers dry deposition fluxes. These data are screened
for completeness by verifying that all micrometeoro-
logical measurements are continuous during a period
for which V,, is computed. Airborne concentrations also
must be available for this period. This requires uninter-
rupted field equipment operations and complete
laboratory analyses for each weekly period. Quality
assurance practices (Hicks et al., 1991) help to ensure
accurate and complete data from NOAA/ATDD sites.
These include:

1. Weekly site operator verification of instrumental
performance,

2. Monthly air quality system checks with field and

procedural blanks,

Quarterly flow rate calibrations, and

4. Semiannual calibration of the entire system by
NOAA/ATDD personnel.

v

Finally, the NOAA/ATDD model occasionally is tested
and verified against direct measurements of dry depo-
sition at “core” sites in the network. Argonne is one of
these sites. The Bondville site is a so-called “satellite”
site, where identical methods and instrumentation are
used, but no on-site model verification is done. For this
report, 45-51 weekly, quality-assured, data points per
site per year were available to calculate dry deposi-
tion fluxes, for an overall completeness better than
90 percent.

Annual median values from the IEPA concentration
data were used (see Air Quality Trends in Illinois
chapter) with distribution statistics from the
NOAA/ATDD V, results to calculate statewide
deposition estimates. Deposition amounts are re-
ported only for those years in which sufficient data
were available from both data sets to calculate an-
nual statistics.

Finally, annual estimates of total atmospheric deposi-
tion were computed for 1985-1990 for those chemical
species (calcium, sulfur, and nitrogen) for which all the
various components of wet and dry deposition were
available, according to each of the above criteria. The
total deposition values were based on summation of
annual means of NOAA/ATDD V, values for fine
particles and gases, the IEPA concentration data for
sulfate, sulfur dioxide, and nitrate, the NOAA/ATDD
nitric acid concentration data, and NADP/NTN dryfall
and wet deposition data.

161

ATMOSPHERIC DEPOSITION TRENDS IN ILLINOIS

Statistical Analyses

Ion concentrations in valid wet-only deposition sam-
ples from NADP/NTN sites were used to characterize
Illinois precipitation chemistry. An approach used by
Semonin and Bowersox (1983) to identify the ions that
dominate the stoichiometry of precipitation was ap-
plied to Illinois data. Briefly, ions were ranked in order
of their average concentrations and the subset that
comprised about 90 percent of the total ion strength
was selected. This subset was used to calculate a
charge balance for individual samples and to assess
whether the distribution of charge balances was near
zero. Zero is expected, if all ions are measured with
zero bias and precision, which requires perfect mea-
surements. In practice, ion balances are positive and
negative due to noise, or imprecision, in every
measurement; and so the distribution of ion balances
clusters around zero. When the distribution of ion
balances for the subset was within the distribution for
all measured ions, the process of adding ions to the
subset was stopped. This subset was assumed to be
characteristic of Illinois precipitation.

The distributions of ions that “characterize” Illinois
precipitation are summarized by site and by year in this
chapter. Ion concentration distributions are presented in
notched box-and-whisker plots, as in figure 3. A com-
mercial software package on a personal computer
created these plots by ranking the data in ascending
order from minimum to maximum. The middle half of
the data, defined as the 25th to the 75th percentile, is
enclosed in a box. In figure 3 this box begins at 0.23,
the 25th percentile, and ends at 0.61, the 75th percen-
tile. The box width is proportional to the square root of
the number of observations (N) in the data set it repre-
sents. Side-by-side boxes are scaled accordingly, so
that the reader can distinguish how much data is
represented by each box relative to its neighbors.

One generally puts more confidence in large rather
than small data sets, because they are less likely to
change significantly when new data are added, i.e.,
results are generally more robust as N grows. Along
the sides of the box in figure 3 is a V-shaped notch
with a horizontal line in its middle. This line marks
the median, at ~ 0.36 in figure 3, which is the value
in the ranked data with as many observations above
as below it. Notches are drawn according to formula
(1) and approximate a 95 percent confidence interval
around the median (AXUM, 1992). Notches are
drawn at the

median + (1.57 * height of box¥N) (1)

162

“Informally interpreted, if the notches for two boxes
don’t overlap, it is evidence that the medians are
different’ (AXUM, 1992). To facilitate the comparison
of medians, a shaded band that fills the notch for a pre-
selected reference site was added to some figures, e.g.,
figure 7. Vertical lines, or whiskers, are located above
and below the box and these mark the highest and
lowest 25 percent of data, respectively. Short horizon-
tal tick marks have been added to the whiskers to mark
the location of the 10th and 90th percentiles. In figure 3
the 90th percentile occurs at 0.88 and the 10th percen-
tile at 0.17. Altogether, the notched box-and-whisker
plots mark the location of the minimum and the 10th,
25th, 50th (median), 75th, and 90th percentiles, and
the maximum.

Statistical distributions of pollutant concentrations in
Illinois precipitation are presented as notched box-and-
whisker plots in figures 7-12. All valid data from each
site are summarized in these figures. Sites are shown in
order from north on the left to south on the right (see
table 1 and figure 1 for site names and locations). The
number of valid data points (N) for each site varies and
depends mostly on the duration of sampling at each site
(figure 2). For each site in these figures, N is 450
(IL190); 402 (IL18); 245 (1L78); 497 (IL11); 77
(IL99); 293 (IL47); 483 (IL35); and 502 (IL63).

All of the basic statistical analyses used to analyze dry
deposition estimate distributions to arrive at means,
medians, standard error, and ranges were computed
using SYSTAT 5.0 routines (Wilkinson, 1990). The
development of variables used to generate combined
data, e.g.,

as Tees :
dry deposition = V, * concentration
and

total deposition = wet + dry (gas)
+ dry (aerosol) + dryfall

was done using either SYSTAT data management
utilities or BASIC programs written specifically for
each task.

All graphical displays of the dry deposition data,
including box plots, bar graphs, stacked bars, and
scatterplots, were generated on an ARES 486-66 PC
using SYGRAPH 1.0 and SIGMAPLOT 5.0 (Jandel,
1992). SYGRAPH 1.0 was used to prepare the total
deposition plots, which were overlaid on a State of
Illinois map.

ATMOSPHERIC DEPOSITION TRENDS IN ILLINOIS

1.0
<_______—__ MAXIMUM
0.9
0.8 Sipe es 90TH PERCENTILE
07 75TH PERCENTILE
0.6 TOP OF NOTCH
05
0.4 MEDIAN
0.5 BOTTOM OF NOTCH
0.2 a pe SHADING BAND
agen ee PERCENTILE
0.1 nL Sen pate ay PERCENTILE
MINIMUM
0.0

Figure 3. Example of a notched box-and-whisker plot used to summarize the distributions of ion concentrations
in wet deposition.

163

ATMOSPHERIC DEPOSITION TRENDS IN ILLINOIS

GIS Use Disclaimer

Due to the extremely low spatial density of deposition
data, no geographic information system (GIS) cover-
ages have been developed for atmospheric deposition.
The assignment of wet deposition values to nine cli-
mate zones, and dry deposition to the “Chicago area”
vs. “Remainder of State” is the best spatial resolution
possible with currently available data. Furthermore, for
dry deposition in particular, large variations in the rate
of deposition can occur for a specific receptor system
(lake surface, forest, or agricultural crop) and even
elements within a system (for example, various parts of
a building exterior within a city).

RESULTS
Spatial Analysis

Wet Deposition. General characteristics of Illinois
precipitation chemistry. Wet-only data from the
NADP/NTN were used to examine the spatial varia-
tions in precipitation-borne pollutant deposition in
Illinois. The NADP/NTN reports mass concentrations
of ten ions, (Ca**, Mg**, Na‘, K*, NH,*, SO,, NO,,
Cl, PO,*, and H*) routinely measured in the filtrate of
precipitation collected at network sites. This set of ions
includes the major anionic and cationic species that
cause acidic precipitation in Illinois and elsewhere.
Except for orthophosphate and possibly potassium,
each of these ten ions is important in determining the
average ionic composition of precipitation somewhere
in the United States. In many coastal areas, the proxim-
ity of a major source of airborne sea salt results in
precipitation having a chemistry dominated by CI, Na’,
SO,;, and Mg** (Junge, 1963).

In other areas, the chemical signature of wet deposition
reflects the mix of sources that dominates the air quality in
the local area. Semonin and Bowersox (1983) used data
from North American precipitation chemistry networks
to identify these signatures, where possible. In the east-
ern United States and Canada, four different signatures
were reported. One four-ion signature (H*, SO,-, NO,,
and NH,,") occurred across a portion of Illinois, although it
was determined from a limited set of data, one to two
years of data at four sites. Data are now available from
eight different locations and for up to 12 years at some
of these locations (see figures 1 and 2 and table 1).

A procedure similar to the one used by Semonin and
Bowersox (1983) was applied to Illinois precipitation

164

data. Volume-weighted average concentrations were
calculated for each ion at every site. Site-specific total
inorganic ion concentrations were computed by sum-
ming up these values at the respective sites. Then,
ratios of the average concentrations of each anion and
cation to this total inorganic ion concentration were
calculated. These ratios are shown as percent of
inorganic ions for all eight Illinois sites in figure 4. In
this figure, anion and cation percentages are presented
in a stacked bar totaling 100 percent for each site.
Anion percentages are stacked on the bottom of the
figure, beginning with SO; then NO, and CI, which is
their order of importance. Average orthophosphate
concentrations were below the analytical limit of
detection and too small to show. Cation percentages are
stacked on the top of the figure. Except for free hydro-
gen ion, H*, the order in which cations are stacked is
generally from least important (K*) to most important
(NH,*). The H* percentages are shown numerically in
the clear bar that separates the anions from the other
cations. This arrangement facilitates the comparison of
the average precipitation chemistry among sites, and it
depicts H* as making up the cations needed to push the
total to 100 percent. Indeed, this is what happens
chemically in acidic precipitation: the total cation
concentration without H* must equal the total anion
concentration, and to the extent the cations are defi-
cient, H* makes up the difference.

Hydrogen ion percentages range from 17 percent at the
Monmouth site in western Illinois (IL78) to 28 percent
in the east-central and south-central parts of the state
(Bondville-IL11, Salem-IL47, and Omega-IL99). At all
sites, from IL19 in the north to IL63 in the south, there
is little variation in the acidic anion contributions of
sulfate, nitrate, and chloride. What changes most from
site to site is the contribution of basic cations, (NH,*,
Ca**, Mg**, Na‘, and K*) that counterbalance or neutra-
lize the acidic anions. The cations maximize at Monmouth
(IL78), where H* is lowest, and minimize at the Bond-
ville (IL11), Salem (IL47), and Omega (IL99) sites (see
figure 4). At all sites in figure 4, SO clearly is the
largest contributor to the inorganic ion strength, i.e.,
the sum of the ion concentrations. Hydrogen ion is the
next most important contributor, followed by NO,,
NH,;, and Ca**. Using volume-weighted average
concentrations, these five ions make up 89.4 to 93.1
percent (mean of 90.6 percent) of the inorganic ions in
Illinois precipitation.

Taking concentrations in individual samples instead of
averages, the sulfate concentration is highest in 65 to
82 percent of all Illinois samples. The order of impor-
tance of ions in individual samples is the same as for

ATMOSPHERIC DEPOSITION TRENDS IN ILLINOIS

100 NH4

@
oO

fo?)
oO

seq NOS
a ae Renae aa] Eted YZ] S04

[OO VID,
ROH
SKK
4

x

=x

K
eats

Soneee,
ERK
Sestetetee

aS
oO
EERE

PERCENT OF INORGANIC IONS

ie)
oO

Figure 4. Ratios (in percent) of volume-weighted-average anion and cation concentrations to the total measured
inorganic ion concentrations at Illinois sites (see figure 1 for site locations). Numbers in the clear stacked bar in
the middle are percentages for the hydrogen ion, H*. Average orthophosphate concentrations were below the
analytical detection limit and too small to show.

165

ATMOSPHERIC DEPOSITION TRENDS IN ILLINOIS

the average concentrations: SO," > H* > NO, > NH,”
> Ca‘. Notably, for all but 0.4 percent of the wet
deposition samples in Illinois, one of these five ions
has the highest concentration. Figure 5 presents the
percentage ratios of the sum of these five ions to the
sum of all ten ions in individual samples. These ratios
are ranked from lowest to highest, and the rankings are
depicted in this figure. For individual samples, 90
percent of the ratios are between 82.5 and 96.2 percent,
which is consistent with the ratios when average con-
centrations are used.

A test of whether these five ions are sufficient to char-
acterize the stoichiometry of Illinois precipitation is to
check for a balance of positive and negative ions. In
principle, liquid precipitation is an aqueous solution
bearing no net positive or negative charge; thus cation
(or positive ion) concentrations must equal anion (or
negative ion) concentrations. To the extent that the
cations and anions are out of balance, the measured set
of ions is incomplete (some have been missed), the
measurements are inaccurate (biased and/or imprecise),
or a combination of these factors is at play. The charge
balance was tested by taking the concentrations of the
five largest contributors to the ion strength of individual
samples and calculating a quantity labeled “ion
percent difference.” This quantity was defined by
Stensland and Bowersox (1984) as

Ion Percent Difference = (anion sum - cation sum)/
(anion sum + cation sum) x 100 percent (2)

where (anion sum) is the chemical equivalent, or ionic
charge, concentration of SO, plus NO, and (cation
sum) is the concentration of Ht plus NH,* plus Ca**.
Ion percent differences calculated in this way are
summarized in figure 6. For 87.5 percent of the data in
this figure, ion percent differences are less than 15
percent. This is consistent with the CAL experience for
samples with an ion strength of about 0.2 milliequiva-
lents per liter (meq/L) and pH values < 5.0, characteris-
tics that are typical of Illinois precipitation. Based on
the NADP/NTN Quality Assurance Plan (NADP/NTN,
1990), samples are not even targeted for reanalysis
until ion percent differences are 15 percent, using the
ten-ion set of measurements. A comparison of the
original analysis with reanalysis results at the CAL
verified that the original analyses were almost always
correct, even when ion percent differences were 20
percent (Stensland and Bowersox, 1984). This study
reported that 0.2 percent of the original numbers were
in error. Thus, the magnitude of ion percent differences
with the five-ion set is not markedly higher than for the
more complete ten-ion set of measurements.

166

Finally, if analytical imprecision alone is to blame for
the ion percent differences from the five-ion set, the
number of positive and negative values taken over all
samples would be virtually the same. Figure 6 shows a
breakdown of the frequency of occurrence of positive
and negative differences. Positive differences account
for about 60 percent and negative differences for about
40 percent of the cases. This means that a cation deficit
is 50 percent more likely to occur than an anion deficit
(i.e., 3 cation deficits to every 2 anion deficits). This
imbalance could result from an analytical bias or from
ions not measured or not considered. Indeed, this im-
balance is consistent with the ions not considered in the
five-ion set. Looking at the averages in figure 3, Mg**,
Na‘, and K+ together exceed CI at every site. The
skewness in the ion balance results in figure 6 reflects
the charge imbalance in ions left out of the calculation.

It follows from these results that five ions dominate the
ion strength of Illinois precipitation: SO,* > Ht > NO,
> NH,’ > Ca**. While these ions effectively control the
solution stoichiometry, there is a small net cation defi-
cit from the remaining ions. Illinois precipitation is best
characterized as a dilute solution of mineral sulfuric
(H,SO,) and nitric (HNO,) acids, partially neutralized
by dissolved NH,* and Ca**. The sulfur and nitrogen
oxides result almost entirely from the combustion of
fossil fuels, while the calcium and ammonium are from
terrestrial sources. Since the five ions so dominate the
chemistry of Illinois precipitation and since the other
measured ions have no deleterious effects at the levels
reported, the summary of wet deposition in this chapter
focuses on the spatial and temporal distributions of
these five ions.

Statistical distributions of pollutant concentrations.
Precipitation in Illinois has a range of ion strengths
from ~ 25 pteq/L to > 7,000 peq/L, a factor of ~ 300
from low to high. Ion strength measures the inorganic
pollutant concentration in ion equivalents, so it indi-
cates how polluted the rain or snow is. A typical rain
sample from an Illinois site might have a pH of 4.4;
and if this were entirely due to dilute sulfuric acid, the
ion strength would be ~ 80 jteq/L. The distributions of
total ion strengths of wet deposition samples are shown
for each site in figure 7. All of the median concentra-
tions (located at the center of the notches in the box
plots) are > 80 peq/L, which is another indication that
Illinois precipitation contains more than sulfuric acid
(i.e., more than H* and SO,>). It is evident in this figure
that more than half of the samples have an ion concen-
tration between 100 and 300 peq/L. From the results in
the previous section, 90 percent of this is due to just
five ions.

ATMOSPHERIC DEPOSITION TRENDS IN ILLINOIS

100

Percentile Avg ( Range)

Max 98.7 (97.5-98.7)
90th 96.2 (95.5-96.8)
75th 94.7 (94.1-95.6)
50th 92.4 (91.4-94.1)
25th 88.4 (87.1-91.9)
10th 82.5 (78.9-87.6)
Min 47.3 (47.3-69.2)

90

80

70

Percent of Inorganic lons
(H+SO04+N03+NH4+Ca)

60

50

All Illinois Sites

Figure 5. Distribution of the ratios (in percent) of the sum of concentrations of five ions (H*, SO,, NO;, NH,’,
and Ca**) to the total measured inorganic ion concentrations in individual Illinois samples. The percentage
values at seven reference percentiles in the distribution are listed, along with the range of percentages at the

eight Illinois sites, when ranked separately (see table | for a list of sites).

167

ATMOSPHERIC DEPOSITION TRENDS IN ILLINOIS

100
00 KO aaa

[_] 10-14.9
80 5-9.9

4 Y 0-4.9

a i

30 rise tio, pUdenks of

ION PERCENT DIFFERENCE
a
ro)

TOTAL POSITIVE NEGATIVE

Figure 6. lon percent differences calculated from the five-ion set (SO,, H*, NO;, NH,*, and Ca**). Differences
appear in the five percentage frequency classes shown in the legend. Absolute values of the differences were
used in compiling the frequency distributions labeled “TOTAL”. Frequency distributions for net “POSITIVE”

and “NEGATIVE” differences are also shown.

168

ATMOSPHERIC DEPOSITION TRENDS IN ILLINOIS

fhop
1400
is
=
® 17200 Max=7327
=
So
‘< 1000
=
D
< 800
O
on
> 600
6 400
ee ee ces ee ee
< ea fe ee
Fst Ae)
19 Neca 7 11 ele ag: Aa i NAR =e)

Sle ee

Figure 7. Distributions of total inorganic ion concentrations (Total Ion Strength) in Illinois precipitation at
NADP/NTN sites (see figure 1 for site locations). Boxes enclose the 25th to the 75th percentiles and the notches
mark the median and its 95 percent confidence interval. Box widths are proportional to the square root of the
number of observations. Lines (whiskers) extend to the minimum and maximum except where indicated.
Horizontal ticks on the whiskers indicate the 10th and 90th percentiles. For comparison, the shaded band marks
the location of the notch for IL11.

169

ATMOSPHERIC DEPOSITION TRENDS IN ILLINOIS

A shaded band fills the notch (the median and its 95
percent confidence interval) of the box (25th to the
75th percentile) for Bondville (IL11) data in figure 7.
Located in rural east-central Illinois, Bondville is
roughly halfway between the three northern (IL19,
IL18, and IL78) and three southern sampling locations
(IL47/99, IL35, and IL63). The location of the notch
for a site, either fully above or below this shaded area,
indicates that the median pollutant concentration at that
site is significantly above or below (respectively) the
concentration at Bondville. Only at the Argonne site
(IL19) in the Chicago area is the median ion strength
significantly above (nearly 10 percent) that at Bond-
ville, probably due to the influence of nearby urban
sources. Median pollutant concentrations appear to be
significantly below the Bondville level at the Mon-
mouth site (IL78) in western Illinois and at the Car-
bondale site (IL35) in the forested southwestern part of
the state. At the remaining sites, median pollutant
concentrations, while below the Bondville value, are
within the (shaded) confidence interval and are not
significantly different than this reference. No north to
south gradient is evident in these overall pollutant
concentrations, outside of the possible urban influence
on Argonne data from the Chicago area.

Free acidity, or the concentration of H* in precipitation
is measured and reported in pH units; and in Illinois the
observed pH range is from 3.10 to 8.45 (0.004 to 794
pieq/L). Acid rain has been defined as having a pH
below 5.6, a value that is based on the free acidity of
water in equilibrium with atmospheric carbon dioxide
or CO, (Barrett and Brodin, 1955). Of Illinois wet
deposition samples, 91.3 percent have pH values < 5.6.
This 5.6 reference is based on the assumption that,
except for CO,, natural pollutants scavenged from the
air by precipitation are neither acidic nor basic; i.e.,
they are neutral. Measurements from remote regions of
the world, where natural pollutants dominate anthropo-
genic influences, belie this assumption (e.g., Galloway
et al., 1982). “No single value for pH or any analyte
can be regarded as a worldwide background value;”
however, the average pH in humid remote areas is
closer to 5.0 than 5.6 (Sisterson et al., 1990). Of Illinois
wet deposition samples, 85.6 percent have pH values <
5.0. Taking either of these definitions, 8 to 9 out of
every 10 weekly precipitation samples in Illinois can
be termed “acidic deposition.”

Figure 8 plots the distribution of pH measurements at
each Illinois NADP/NTN site. The shaded band, filling
the notch for the Bondville site (IL11), once again
serves as a basis for comparing data among sites.
Bondville pH values at the 10th, 25th, 50th, 75th, and

170

90th percentiles are the lowest among the sites, and the
Bondville median is significantly below the medians at
all but the IL63 and IL99 sites. This is consistent with
the generally higher SO,- and NO, measurements at
Bondville, shown in figures 9 and 10. At the IL78 site
near Monmouth in western Illinois, the median pH is
about 4.6 and the middle half of the distribution, the
25th through the 75th percentiles, is almost completely
above the middle half of the distribution at Bondville.
The free acidity at Monmouth is about half of the
Bondville levels, owing to the combined effects of
lower SO, and NO, and higher Ca** and NH,*. What's
more, the spreads in the pH distributions, indicated by
the differences in the 10th and 90th percentiles (hori-
zontal reference marks on the whiskers in figure 8) are
much larger at Monmouth (IL78), Shabbona (IL18),
and Argonne (IL19) than they are at any of the sites in
the southern half of the state. The more frequent oc-
currence of higher pHs at these sites causes the larger
spread, which exceeds a pH unit (> 10-fold difference
in H*). Higher pHs occur when higher Ca** or NH,* or
both are present to neutralize the SO> and NO, acid-
ity. These conditions occur more often at northern than
southern Illinois sites.

It is the acidity of precipitation, along with the associ-
ated sulfur and nitrogen pollutants, which raises con-
cerns about the potential for damage to agroecosystems
and to unmanaged systems, such as forests. Nearly all
of the reported damage to these ecosystems is the result
of a combination of environmental stresses, including
acidic deposition (Shriner et al., 1990; Barnard et al.,
1990). It is generally not acidic precipitation alone that
is the cause of reported damage. Direct damage, though
rare, has been reported, however. “Phytotoxic concen-
trations of acidic precipitation are generally regarded to
lie below pH 3.6” (Shriner et al., 1990). In Illinois, this
occurs for 0.6 percent of the weekly wet deposition
samples. There are no known reports of direct “acid
rain” damage to Illinois crops or forests in the litera-
ture, due to these rare, very acidic events.

Figures 9 and 10 contain the notched box-and-whisker
plots of SO, and NO, concentrations at Illinois sites.
Median SO,- concentrations range from 2.14 to 3.14
mg/L, with an overall median of 2.64 mg/L; and
median NO, concentrations range from 1.37 to 1.84
mg/L, with an overall median of 1.55 mg/L. Sulfate to
nitrate mass concentration ratios of ~ 1 1/2 to | are
typical of Illinois precipitation. In ion equivalent (or
charge) concentration units, this ratio is ~ 2 to 1,
meaning precipitation would have about two units of
sulfuric acid for every unit of nitric acid, if both SO,-
and NO, were totally unneutralized. Using the shaded

LAB pH

ATMOSPHERIC DEPOSITION TRENDS IN ILLINOIS

19 18 78 Tal pce 47 aD 63
Sl heel

Figure 8. Same as figure 7, except for laboratory (CAL) pH measurements.

171

ATMOSPHERIC DEPOSITION TRENDS IN ILLINOIS

ip ee
SITE. 1B

Figure 9. Same as figure 7, except for sulfate (SO,) mass concentrations in mg/L. Horizontal ticks marking the
10th percentiles on the lower whiskers were too close to the 25th percentiles to show. From left (IL19) to right
(IL63) the 10th percentile values are 1.60, 1.25, 1.04, 1.51, 1.03, 1.39, 1.11, and 1.39, respectively.

NO3 (mg/L)

—_>_ —
cor © 5 |S

16

aN

Oo)

1 ii

poy ae

Figure 10. Same as figure 7, except for nitrate (NO;) mass concentrations in mg/L. Horizontal ticks marking the
10th percentiles on the lower whiskers were too close to the 25th percentiles to show. From left (IL19) to right
(IL63) the 10th percentile values are 0.97, 0.79, 0.64, 0.79, 0.58, 0.57, 0.60, and 0.59, respectively. Also the
lowest five values at IL35 and the minimum at ILI1, IL18, IL47, and IL63 were at the analytical limit of

172

detection (0.93 mg/L).

band in the figures as a reference, the similarity in
behavior of these two pollutants becomes apparent.
While the magnitudes of the respective SO," and NO,
deviations from the reference (i.e., the IL11 median
with its 95 percent confidence interval) are not identi-
cal, IL19 is above IL11 and has the overall highest
concentrations for both ions. In general, SO," and NO,
concentrations for IL18, IL35, IL47, IL63, IL78, and
IL99 are below IL11; and IL99 data are lowest. Choos-
ing reference points other than the median with its
confidence interval gives similar results when the two
ions are compared; thus covariance is evident in the
SO,-and NO, concentration profiles.

To quantify the covariance that appears in figures 9 and
10, correlations for SO,- and NO, concentrations were
calculated. Valid samples from all eight Illinois sites
were used in the calculations. As a basis for compari-
son, correlations among the other three major ions and
SO, and NO, were included in the analysis. Results
are presented in table 2, which lists the correlation
coefficients or R-values for the relationships between
each ion and the other four ion concentrations in
individual precipitation samples. Only linear relation-
ships were tried, which means that the magnitude of
the R-values indicates the degree to which one concen-
tration is proportionate to the other concentration being
tested. High positive R-values indicate that as one
concentration goes up the other goes up in the same
fractional (or proportionate) amount. A value of 1.0
indicates a perfect positive correlation. Negative R-
values mean that as one ion goes up, the other goes down.
It is apparent from the results in table 2 that many of
the R-values are > 0.60 and are similar in value, but the
SO,- and NO, correlation is best; it has the highest
R-value (0.74). Squaring the R-values measures the
fraction of ion concentration variance that is accounted
for by covariance; and for SO,- and NO,,, R? in percent
is 55. In Illinois precipitation, over half of the variation
in SO; concentrations can be explained by the fact that
they track NO, concentrations, and vice versa.

One implication of the correlative behavior of SO, and
NO, in precipitation is that ecosystem exposure to high
SO,; is likely to be accompanied by high NO,. Recall
from figure 4 and the discussion in the previous section
that SO,* and NO, have the highest and third highest
average concentrations, respectively, in Illinois precip-
itation. Free acidity, H*, has the second highest average
concentration. In table 2, the correlations of H* and
SO," (R? = 38 percent) and H* and NO, (R? = 19 per-
cent) are not as high as for several other pollutants,
however. Notwithstanding the lower correlations of H*
with so; or NO,, comparisons of the concentration

ATMOSPHERIC DEPOSITION TRENDS IN ILLINOIS

distributions (i.e., the notched box-and-whisker plots)
of these three pollutants in figures 8-10 show that
where the notch falls above the shaded band in figure
8, it falls below the band in figures 9 and 10, except for
the Argonne (IL19) site. In other words, generally
higher pH values at a site and generally lower SO,* and
NO, concentrations go together, and vice versa. Where
so, is high, so too are H* and NO,; and from the
correlation results, when SO,- is high, NO, is more
likely than any other ion to be high too. The excep-
tional behavior at IL19 is another indication of urban
Chicago influences on Argonne precipitation. Higher
airborne dust levels with higher accompanying Ca**
concentrations may be neutralizing the higher SO,- and
NO, in IL19 precipitation.

Other implications of the correlative behavior of SO,
and NO, in precipitation are that

1. Precipitation scavenges these two pollutants from
the air similarly.

2. The mixing ratio of these pollutants in air varies
over a narrow range.

3. Sulfur and nitrogen oxides are emitted from the
same sources.

4. A combination of these factors is at work in the
atmosphere.

Figure 11 presents the NH,* concentration distribu-
tions in precipitation at Illinois sites. Median concen-
trations range from 0.26 to 0.45 mg/L, with an overall
median of 0.35 mg/L. Unlike the SO, distributions,
in which all values are above the analytical detection
limit, and NO, distributions, with only 0.3 percent
detection limit values (see figure 10), the frequency
of occurrence of detection limits in the NH,* concen-
trations is 3.6 percent. Detection limit values occur
at every Illinois site, from a low of 1.6 percent at
Argonne (IL19) to a high of 6.0 percent at Carbondale
(IL35); and so each of the lower whiskers in figure
11 terminates at 0.02 mg/L, the NH,* detection limit.
That 28 percent of the concentrations are within a
factor of 10 of this detection limit results in greater
analytical uncertainty for NH,* (15-17 percent) than
for the four other major ions (James, 1992). Sites
north (IL19, IL18, and IL78) of Bondville have
higher NH,* concentrations than sites to the south
(IL99, IL47, IL35, and IL63), although only IL35 is
significantly different. This geographical pattern is
markedly different than for pH, SO,", or NO,, but is
similar to the Ca** pattern in figure 12. Of note, too,
is that the NH,* concentrations at the Argonne site
(IL19) are not the highest, as clearly was the case for
SO," and NO,.

173

ATMOSPHERIC DEPOSITION TRENDS IN ILLINOIS

Table 2. Linear Pearson Correlation Coefficients (R Values) and R-squared Values, for the Pairwise Correlations among the
Five Major Inorganic Ion Concentrations in Illinois Precipitation (number of observations = 2711; all values significant at
better than the .01 percent level except the H* and Ca™ correlation, which is significant at the 5 percent level)

sO; NO, Ca** NH,* H*
R R* RR RR R R* ee
so; 10 1.0 ja. 15S SOW wis 63 40 62 38
NO, 74-ilt 55 Hora snty 66 43 69 AT 44 19
Ca** Bo) ees 66 43 10 FTO Se a0 -06 .00
NH,* 63 40 69 47 55 a0 1 10 iL) pli
Ht 162. BR 44 19 -06 .00 hk sr i 10 1.0

Note:
*Bonferonni-adjusted probabilities (Wilkinson, 1990).

174

ATMOSPHERIC DEPOSITION TRENDS IN ILLINOIS

MAX=78.2

NH4 (mg/L)

1S crak cae recuse 11. 6 9682 ni titer watidlog on
SITE ID

Figure 11. Same as figure 7, except for ammonium (NH,) mass concentrations in mg/L. Horizontal ticks
marking the 10th percentiles on the lower whiskers were too close to the 25th percentiles and the minimums to
show. From left (IL19) to right (IL63) the 10th percentile values are 0.14, 0.16, 0.12, 0.10, 0.07, 0.07, 0.06, and
0.08, respectively. Also all of the minimum values and from 1.6 to 6.0 percent of the other low values in the
distribution are the analytical limit of detection (0.02 mg/L).

Ca (mg/L)

ayn l= teil Taliep tin
=) A St

Figure 12. Same as figure 7, except for calcium (Ca) mass concentrations in mg/L. Horizontal ticks marking the
10th percentiles on the lower whiskers were too close to the 25th percentiles and the minimums to show. From
left (IL19) to right (IL63) the 10th percentile values are 0.0997, 0.077, 0.060, 0.058, 0.034, 0.050, 0.050, and
0.053, respectively. Also the lowest three values at IL35 and the minimum at IL19 are the analytical limit of

detection (0.009 mg/L).

ATMOSPHERIC DEPOSITION TRENDS IN ILLINOIS

Ammonium is a nutrient and is one of two nitrogen
compounds deposited by precipitation, the other being
NO,,.. In mass concentrations units, NO, and NH,’ rank
third and fourth in importance in Illinois precipitation,
with overall median concentrations of 1.55 and 0.35
mg/L, respectively. In these units, the NO, median is
over four times the NH,* median. To assess the impor-
tance of these two pollutants as nutrients deposited on
Illinois crops, forests, lakes, and streams, however, it is
their nitrogen content that is relevant. As a fraction of
the total mass, nitrogen is 22.6 percent of NO, and
77.6 percent of NH,*. Converting the overall median
concentrations, above, to nitrogen (N) mass units yields
0.35 and 0.27 mg/L for NO, and NH,,*, respectively.
Expressed this way, the median N from NO, is just 30
percent more than the median N from NH,*. What’s
more, NH,* and NO, have the second highest correla-
tion coefficient of all the pollutants reported in table 2.
R? is 0.47, meaning 47 percent of the total variation in
these two N compounds in precipitation is accounted
for by their covariance.

Exposure studies have not shown that crop or forest
damage has occurred from the ambient N levels
found in wet deposition in the United States (Shriner
et al., 1990; Barnard et al., 1990), however, even for
the most extreme events. On crops, the N from NO,
and NH,_ is likely to enhance growth and productiv-
ity. On certain unmanaged high-elevation forests,
there is some evidence that the N addition from wet
deposition over time may stimulate excess growth,
which could predispose trees to winter damage
(Shriner et al., 1990). Illinois forests are not exposed
to the harsh conditions that sometimes occur at high
elevations and are not likely to be affected negatively
by N deposition, but this should be assessed. It is not
known to what extent wet-deposited N accumulates
in Illinois lakes and may contribute to water quality
problems and eutrophication.

One final observation from figure 11 is the lone flier
reported at the Shabbona site (IL18). The NH," in this
sample was 78.2 mg/L, more than ten times the second
highest value at this site or the maximum at any other
site. This same sample appears as the lone flier in
figure 7 and is a flier for potassium and orthophos-
phate, as well. Although this sample passed all of the
screening criteria discussed in the earlier section on
screening procedures, it is a clear anomaly. No visible
contaminants were reported, but the extreme NH,*,
PO,;, and K* strongly suggest contamination by ferti-
lizer. It has a disproportionately large influence on
correlation analyses and so was excluded from the
results presented in table 2.

176

The notched box-and-whisker plots for Ca** appear at
each of the eight Illinois sites in figure 12. Median Ca**
concentrations range from 0.159 to 0.296 mg/L, with
an overall median of 0.238 mg/L. A geographic pattern
of higher values in northern Illinois than in southern
Illinois, described in the discussion on NH,., is indi-
cated in this figure. The highest concentrations occur at
the Argonne site (IL19), where Ca** values are 8 to 26
percent above the second highest site values at every
reference percentile (minimum, 10th, 25th, 50th, 75th,
and 90th) in the distribution except the maximum.
Despite the higher concentrations at IL19, none of the
notches appears either completely above or below the
shaded band, which means that all of the median con-
centrations can be interpreted as being within the 95
percent confidence interval of the Bondville (IL11)
median. This is consistent with the large relative uncer-
tainty or variance in Ca** data. Seventeen percent of all
Ca* concentrations are within a factor of ten of 0.009
mg/L, the analytical detection limit; and precision
estimates are typically 10 to 16 percent (James, 1992).

There are no reports of real or potential ecological
damage from Ca** in precipitation. It is important in
Illinois wet deposition because it is at sufficiently high
concentrations to offset the free acidity effected by
SO, and NO,. It is the only major ion that has a nega-
tive correlation coefficient with H* in table 2, although
almost no variance (R? ~ 0) is explained by this rela-
tionship. A study to explain the sharply lower pH
(higher H*) in eastern U.S. precipitation in the 1970s,
compared with the 1950s, reported that Ca** (and
magnesium) decreases, not So; and NO, increases,
were the cause (Stensland and Semonin, 1982). High
airborne dust levels from widespread droughts in the
central United States were cited as the major source of
the higher Ca*tin the 1950s. Soil aerosols, road dust
(Barnard et al., 1986), and construction and demolition
activities are important sources of Ca** today. Calcium
remains one of five major ions in Illinois precipitation.

Overall, figures 9-12 exhibit some features that are
common to precipitation chemistry data from all
NADP/NTN sites (Knapp et al., 1988). Pollutant
concentrations tend to be skewed by a few large outliers.
This is evident in the asymmetry of the notched box-
and-whisker plots. The upper whiskers (top 25 percent)
are often several times longer than the lower whiskers
(bottom 25 percent); and the distances from the tick
marks at the 90th percentiles and the ends of the whiskers
(the top 10 percent) are often several times longer than
the distances between the 75th and 90th percentiles.
This skewness is typical of many environmental
variables, particularly chemical concentrations, which

ATMOSPHERIC DEPOSITION TRENDS IN ILLINOIS

are bounded by zero but have no upper limit. A log
transformation of the data removes a considerable
amount of this asymmetry. Indeed, log Ca** concen-
trations have a distribution that cannot be distinguished
from a normal distribution, which is the classic bell-
shaped curve. A statistical test of the hypothesis: log
concentration distribution equals normal distribution,
was used to make this observation (STATGRAPHICS,
1992; Kolmogrox-Smirrov “goodness-of-fit” test at the
one percent confidence level). Applying the same test
to log NO,, NH,*, or H* concentrations does not result
in normal distributions, i.e., the probability is < one
percent that log concentrations have a normal distribu-
tion. For H*, this is tantamount to the observation that
pH measurements do not have a normal distribution,
since pH is proportional to log (H*). Log SO,-
concentrations cannot be distinguished from a normal
distribution at the one percent confidence level but can
at the 10 percent level.

Observing that pollutant concentrations in precipitation
are skewed (figures 9-12) and knowing how to describe
the concentration distributions with mathematical
expressions helps effects researched to study and
understand the eccosystem response to pollutant
experience. What’s important is that pollutant concen-
trations in Illinois precipitation often range over two or
three orders of magnitude, beginning near zero. This
means that ecosystems are exposed to many low and
moderate concentrations and to a few concentrations
that may be 100 to 1000 times higher. In addition, there
is a substantial amount of covariance among the ions,
which means that these high concentrations tend to
occur at the same time. Recall that the frequency of
occurrence of pH < 3.6, at which direct damage to
some plants has been reported (Shriner et al., 1990), is
0.6 percent in Illinois. For these same samples, SO is
in the top 1.3 percent, NO, is in the top 9.6 percent,
and NH," is in the top 16 percent; Ca** values are low
and high. This correlative behavior of precipitation
concentrations is quantified in table 2: for SO," and
NO,,, R’ is 55 percent; for NH,* and NO,, Ca** and
NO,,, and NH,’ and SO,;, R? is 40 to 47 percent.
Among the five major ions, the H* correlations are
lowest; however, when Ht is correlated with the other
ions in a charge balance relationship, R? improves to 80
percent. This is yet another way to show that Illinois
precipitation is a dilute solution of sulfuric and nitric
acids, partially neutralized by ammonium and calcium.

Dry Deposition. The complex phenomena of pollutant
mass transfer by nonprecipitation processes from the
atmosphere to receptors at the earth’s surface, i.e., dry
deposition, are often explained and examined by way

of analogy to an electrical circuit. Indeed, such a
conceptual model forms the basis for coupling together
the computational methods used to arrive at the
NOAA/ATDD results for V,, (Hicks et al., 1991). The
series resistance model shown in figure 13 (after
Davidson, 1988) considers the airborne concentration
of a particular chemical to be analogous to an electrical
potential or voltage. The receptor surface is then taken
to be a sink, at zero potential, or at the ground state.
Pollutant transfer from the atmosphere to the surface
actually occurs when particles or gas molecules pass
through various layers in the atmosphere and there is a
physical or chemical reaction at the surface. Different
processes control the rate of transfer in each layer. In
the series resistance analogy, this transfer is an electri-
cal current that flows through a series of resistors,
which control the amount of current that passes.

In their simplest form then, the three “‘resistors’’ in the
series resistance model represent atmospheric pro-
cesses. R, or aerodynamic resistance represents the
turbulent processes in the lower levels of the atmo-
sphere that mix and deliver airborne materials to
thenear surface atmosphere. R, or boundary-layer
resistance represents the effect of the receptor surface
on small-scale air turbulence and the rate at which
particles and gas molecules can be brought into close
enough proximity to the receptor surface to actually
react chemically or “stick” physically. R, or surface
resistance represents the ability of the surface to retain
the materials brought into reactive proximity. A fourth
resistor (R, or gravitational settling resistance) repre-
sents the rate at which particles literally “fall” if they
are large or massive enough that gravitational effects
dominate the process.

As in the analogous electrical circuit, the higher the
resistance, the lower the current. The resistance with
the highest value becomes the overall rate limiter for
the entire process. Mathematical tools that apply to the
analyses of electrical circuits apply exactly to this
conceptual framework. Additional resistances in series
or parallel to the simple three-resistor model can be
used to differentiate and examine the effects of addi-
tional processes. Large to moderate scale atmospheric
processes, and thus daily weather patterns, affect pri-
marily R,. The surface type and configuration affect the
way a receptor is coupled to the atmosphere, and thus
R,. Finally, the type of material at the surface, its
biological state, composition, physical state, chemical
reactivity, and especially wetness, impact R.. Overlaying
the resistance analogy, and adding additional com-
plexity to the deposition rate estimation, is the fact that
different pollutants have different resistances. The

177

ATMOSPHERIC DEPOSITION TRENDS IN ILLINOIS

Atmosphere

Ra

Re

Ground

Notes:

R, = aerodynamic resistance

R, = boundary-layer resistance

R, = surface resistance

R, = gravitational settling resistance

Figure 13. Schematic representation of dry deposition processes in the so-called series resistance analogy.

178

aerodynamic size and solubility of aerosols, and the
reactivity of gases with various types of surfaces greatly
impact the R values. In a simple series resistance
circuit, the overall resistance (R,,,,,,) is just the sum of
the individual elements:

Row = R,+ R, +R,

This is the basic form of the model for gases and fine
aerosol. For these physical forms of pollutants in the
atmosphere, gravity has little or no effect on the depo-
sition rate, and R, has a very large value relative to the
other three terms. Where parallel resistances exist in an
electrical circuit, R, ca is found by inverse summation,
so that including all four resistances in the model
shown in figure 13 produces the equation:

1/Ryy = (AR, +R, +R) + (R,)

For very large particles, such as aerosol with aerodynamic
diameters of several jim, or raindrops whose size is
measured in millimeters (mm), R, becomes a relatively
very small number. In this case, the (/R,) term becomes
very large, and the effect of gravity dominates the process.
V,, is a conductance, or inverse resistance term, and is
calculated simply as

Therefore, many atmospheric and surface parameters
are considered to produce a single V, value, which is
specific for the pollutant type and average surface type
modeled. The two NOAA/ATDD sites (Bondville and
Argonne) were originally selected to represent region-
ally typical conditions. The Argonne site is at the
southwestern fringe of the Chicago metropolitan area.
and is characterized by mixed forest and suburban
residential vegetation, with numerous relatively local
pollution sources. In contrast, the Bondville site is
farmland, rotated annually between soybean and corn
production, with very few nearby pollution sources.
The V, estimation model takes into account these
general site characteristics, as well as day-to-day
changes in vegetation status and land use reported by
the site operators. In the NOAA/ATDD model, large
particles, small particles, ozone, nitric acid, and sulfur
dioxide are each treated as a separate case for which V,
values are computed weekly. Finally, this conductivity
term is multiplied by the potential term, concentration,
and the sampling interval, in units of time (here, one
week) to arrive at the dry deposition amount:

Deposition = V,, * concentration * time

ATMOSPHERIC DEPOSITION TRENDS IN ILLINOIS

In the data reported here, this equation was used with
appropriate conversion factors to express all dry depo-
sition amounts as kilograms of material depositing per
year to one hectare of area (kg ha" yr'). To convert to
pounds per year to one acre (Ibs ac yr'), multiply the
deposition value shown by 1.12.

The first step in examining the spatial variation in dry
deposition, involves a comparison of the V,, results for
the two NOAA/ATDD sites. Figure 14 shows the distri-
bution of weekly mean V, values observed at the two
sites during the 1985-1992 period for which data are
available. For each of the three pollutants shown
(HNO,, SO,, and aerosol), the median V gat Argonne is
somewhat higher than at Bondville due to the site char-
acteristics and long-term climatic differences in the two
sites. Most important among these factors is the greater
roughness scale at Argonne, due to land use and vege-
tation type. These factors are expressed primarily in the
R, and R, components of the V, computation. Larger
differences are seen in figure 14 between the three
pollutant species than between sites.This is due to the
R, component, and the differences are caused largely
by the varying affinities of the pollutants for surfaces.
Aerosol particles may “stick” or “bounce off” surfaces,
and R, is influenced by the physical nature of the
surface. SO, gas is very reactive with surfaces when
aqueous films are present to facilitate the chemical
reactions that convert SO, to SO, at the surface, so the
R, component for SO, is strongly controlled by surface
wetness. HNO, is also a gas, but it has a very small R,
value for all surface types, whether wet or dry, and thus
HNO, has the largest V, values overall.

Figure 15 is similar to figure 14, but shows the corre-
sponding Argonne and Bondville weekly mean air
concentration distributions for HNO,, SO,, and SO,
(SO,; is strongly associated with small aerosol, i.e.,
diameter < 1 jm). The differences here are attributable
to the suburban versus rural nature of the two sites.
The SO, concentration at Argonne is somewhat higher,
simply due to the higher density of SO,-emitting
sources in the Chicago-Joliet region, as compared to
the agricultural region around Bondville. SO,> in fine
aerosol, by contrast, is largely a secondary pollutant,
formed by the reaction of gas phase SO, over time.
Thus, SO,* concentrations tend to vary regionally,
rather than locally; concentration distribution and the
local sources may have somewhat less of a direct
impact. HNO, concentrations appear to be quite similar
at both sites; the high rate of dry deposition, the
reactivity of the gas with aerosol constituents, and the
volatile nature of some of the reaction products,
preclude generalizations about HNO, distributions.

179

ATMOSPHERIC DEPOSITION TRENDS IN ILLINOIS

Argonne HNO,

Bondville HNO,

Argonne Aerosol

Bondville Aerosol

Argonne SO,

Bondville SO,

4.0

Figure 14. Comparison of the distributions of weekly mean dry deposition velocities (V,) determined at two
Illinois sites (Argonne, IL 19, and Bondville, ILI11) over the period 1985-1992.

180

ATMOSPHERIC DEPOSITION TRENDS IN ILLINOIS

Argonne HNO,

Bondville HNO,

Argonne SO,

Bondville SO,

fssie isa von Argonne SO,
————_{ [| |—— ek kK kK HHO O_O
Bondville SO,

6 12 18 24
Concentration, ug m*

Figure 15. Comparison of the distributions of weekly mean airborne concentrations of three pollutants ug m™) at
two Illinois sites (Argonne, IL19, and Bondville, IL11) over the period 1985-1992.

181

ATMOSPHERIC DEPOSITION TRENDS IN ILLINOIS

Finally, the V, and concentration data are combined in
figure 16. This figure shows the computed dry deposi-
tion amounts for the two NOAA/ATDD sites.

Dry deposition loading estimates for the period 1984-
1990 were computed using annual median pollutant
concentrations from IEPA monitoring data (see Air
Quality Trends in Illinois chapter). Separate medians
were derived for the Chicago and the “Remainder of
State” areas. These concentration values were then
combined with V statistics based on the model results
from the NOAA/ATDD network data set, to produce
the loading estimates shown in figures 17-28. Each
figure presents the annual loading values in kg ha" yr!
for a particular pollutant, with plot (a) showing the
Chicago-area data and plot (b) showing the “Remain-
der of State” data. Note that while data were not
available for all years of the overall period, the

time scale of all plots is constant. For each year, the
distribution of NOAA/ATDD weekly mean V, data
was analyzed. The product of the median pollutant
concentration (from IEPA measurements) and the
annual mean V , value is shown as a horizontal line.
The vertical line represents the loadings at + one
standard deviation in V,,. Finally, the triangles pointing
up and down indicate the result if the maximum and
minimum V F respectively, are used in the loading
calculation.

The Argonne NOAA/ATDD site V,, values were used
to compute the Chicago-area loadings; Bondville V,
values were used for the “Remainder of State” results.
This introduces some additional imprecision in the
results, since there exists much variation in land use,
topography, and vegetation type in the “Remainder of
State” area, while the Bondville V , Values are com-
puted for farmland. While modeling tools (Matt and
Meyers, 1993) and land-use and vegetation type data
(see chapter with GIS data) exist for much finer resolu-
tion estimations of V y development of the calculations
was beyond the scope of the current project.

From this series of plots (figures 17-28), it is obvious
that dry deposition in Illinois tends to be somewhat
higher in the Chicago area. This is due to both higher
airborne concentrations for most pollutants and higher
deposition velocities. For arsenic, manganese, nitrate,
ozone, sulfur dioxide, and sulfate (figures 17, 21-24,
and 26-27), the differences are on the order of 10 to 30
percent. For cadmium, chromium, iron, nickel, lead,
and TSP (figures 18-20, 22, 25, and 28), dry deposition
in the Chicago area exceeds the rest of the state by 200
to 400 percent. This is primarily caused by the differ-
ences in air quality (see Air Quality Trends in Illinois
chapter). Temporal trends in dry deposition generally

182

follow air quality trends, although additional variability
is introduced into the time-series data by interannual
variation in deposition velocities.

Spatial variation in dryfall was examined using the
NADP/NTN “dry bucket’ data from five Illinois sites
for which a long-term record was available (figure 1
and table 1). Figures 29-31 show the distributions of
dryfall loading amounts observed in the 8-week
integrated samples at the five sites for calcium,
ammonium, and sulfate. Dryfall data for other ions (the
dryfall samples were analyzed for all of the same ions
as the wet deposition samples) is not considered to be a
reliable indicator of actual dry deposition rate. This is
because the polyethylene bucket, used in the NADP/
NTN wet/dry collector, cannot account for the aerody-
namic and chemical reactivity characteristics of natural
environmental surfaces. For calcium and the large-
particle fraction of ammonium and sulfate, however,
there appear to be less significant interfering chemical
reactions, and there is a larger R, component due to the
larger particle sizes associated with these chemicals.
Thus, gravitational processes are relatively more
important for particles containing calcium and the
large-particle fraction of ammonium and sulfate, and
the dryfall collector is believed to provide useful data
for these species. In figures 29-31, Argonne (IL19) is
the leftmost box plot distribution. The other five sites
are plotted in roughly north-south order from left to
right. Figure 29 shows a slight tendency for calcium to
be higher in the southern part of the state. Figure 30
shows that the highest and most variable values for
ammonium dryfall occur at Argonne and in northern
Illinois. Argonne shows a slightly higher dryfall
loading (figure 31) for sulfate than the rest of the state,
where a slight trend to higher values in southern Illinois
is also noticeable.

Temporal Trends

Wet Deposition. Trends in the chemical composition
of Illinois precipitation were assessed by first deter-
mining the annual concentration distribution of each
ion at each site. Next, the year-to-year changes in
concentrations were evaluated by applying a Spear-
man rank correlation test to percentile points in the
distributions. Three points were chosen, the 25th,
50th, and 75th percentile values. A summary of the
test results is shown in table 3. None of the trend
estimates were identically zero. A zero trend means
that there is no overall direction in the year-to-year
concentrations, either up or down. As a result, the
changes were labeled as either “up” or “down” in
this table.

ATMOSPHERIC DEPOSITION TRENDS IN ILLINOIS

|e eee a Argonne HNO,
ht
Bondville HNO,

Argonne SO,

eo a Fe +O -
Bondville SO,

ila lili mal mame Eig
eames 2] Bondville SO,

a
Oo 6 12 24 30

Deposition, kg :

18
ha’ yr

Figure 16. Comparison of the distributions of weekly estimates of annualized dry deposition (kg ha" yr’) at two
Illinois sites (Argonne, IL19, and Bondville, ILI1) over the period 1985-1992.

183

ATMOSPHERIC DEPOSITION TRENDS IN ILLINOIS

As Loading, kg ha" yr"

Fi

Cd Loading, kg ha™ yr"

Fig

184

0.005
a. Chicago Loadings b. Rest of IL Loadings
0.004
0.003
0.002
a
a - i
0.001 4 4 - 4 ‘
v v of v
0.000
1983 1984 1985 1986 1987 1988 1989 1990 1991 1983 1984 1985 1986 1987 1988 1989 1990 1991
YEAR YEAR
gure 17. Dry deposition loadings of arsenic (kg ha’ yr') for the Chicago area (a) and the remainder of the

State of Illinois (b), from ILEPA annual median concentration and NOAA/ATDD V,, statistics.

0.005
a. Chicago Loadings b. Rest of IL Loadings

0.004 4
0.003
0.002
0.001
0.000

1983 1984 1985 1986 1987 1988 1989 1990 1991 1983 1984 1985 1986 1987 1988 1989 1990 1991

YEAR YEAR

ure 18. Dry deposition loadings of cadmium (kg ha" yr') for the Chicago area (a) and the remainder of the

State of Illinois (b), from ILEPA annual median concentration and NOAA/ATDD V, statistics.

ATMOSPHERIC DEPOSITION TRENDS IN ILLINOIS

0.010

a. Chicago
Loadings

b. Rest of IL Loadings

0.008

0.006

0.004

Cr Loading, kg ha" yr"

0.002

0.000
1983 1984 1985 1986 1987 1988 1989 1990 1991 1983 1984 1985 1986 1987 1988 1989 1990 1991

YEAR YEAR

Figure 19. Dry deposition loadings of chromium (kg ha’ yr’) for the Chicago area (a) and the remainder of the
State of Illinois (b), from ILEPA annual median concentration and NOAA/ATDD V, statistics.

2.0

a. Chicago Loadings b. Rest of IL Loadings

A ase | eee
3 ate ee pital ta

0.0
1983 1984 1985 1986 1987 1988 1989 1990 1991 1983 1984 1985°1986 1987 1988 1989 1990 1991
YEAR YEAR

Figure 20. Dry deposition loadings of iron (kg ha’ yr’) for the Chicago area (a) and the remainder of the State
of Illinois (b), from ILEPA annual median concentration and NOAA/ATDD V, statistics.

185

ATMOSPHERIC DEPOSITION TRENDS IN ILLINOIS

0.20

b. Rest of IL Loadings

a. Chicago Loadings

0.15

0.10

0.05

Mn Loading, kg ha™ yr"

4 4
a 4 a f
watts Sa
0.00
1983 1984 1985 1986 1987 1988 1989 1990 1991 1983 1984 1985 1986 1987 1988 1989 1990 1991
YEAR YEAR

Figure 21. Dry deposition loadings of manganese (kg ha™ yr’) for the Chicago area (a) and the remainder of the
State of Illinois (b), from ILEPA annual median concentration and NOAA/ATDD V,, statistics.

0.020

a. Chicago Loadings

b. Rest of IL Loadings

°
°o
=
a

0.010 a

Lig

0.005

NI Loading, kg ha” yr™

N,

0.000
1983 1984 1985 1986 1987 1988 1989 1990 1991 1983 1984 1985 1986 1987 1988 1989 1990 1991

YEAR YEAR

Figure 22. Dry deposition loadings of nickel (kg ha yr’) for the Chicago area (a) and the remainder of the
State of Illinois (b), from ILEPA annual median concentration and NOAA/ATDD V,, statistics.

186

ATMOSPHERIC DEPOSITION TRENDS IN ILLINOIS

20

a. Chicago Loadings

b. Rest of IL Loadings

-
a

_
°

i a

Sy ey t oP td

1983 1984 1985 1986 1987 1988 1989 1990 1991 1983 1984 1985 1986 1987 1988 1989 1990 1991

NO, Loading, kg ha™ yr“ -
a

YEAR YEAR

Figure 23. Dry deposition loadings of nitrate (kg ha’ yr') for the Chicago area (a) and the remainder of the
State of Illinois (b), from ILEPA annual median concentration and NOAA/ATDD V, statistics.

a. Chicago Loadings b. Rest of IL Loadings

Ozone Loading, kg ha” yr"

0.0
1983 1984 1985 1986 1987 1988 1989 1990 1991 1983 1984 1985 1986 1987 1988 1989 1990 1991

YEAR YEAR

Figure 24. Dry deposition loadings of ozone (kg ha” yr’) for the Chicago area (a) and the remainder of the
State of Illinois (b), from ILEPA annual median concentration and NOAA/ATDD V, statistics.

187

ATMOSPHERIC DEPOSITION TRENDS IN ILLINOIS

0.20

a. Chicago Loadings b. Rest of IL Loadings

0.15 <

0.05 thbeid

wv,

Pb Loading, kg ha™ yr"'

a 4 a
bt. 4
0.00
1983 1984 1985 1986 1987 1988 1989 1990 1991 1983 1984 1985 1986 1987 1988 1989 1990 1991
YEAR YEAR

Figure 25. Dry deposition loadings of lead (kg ha’ yr’) for the Chicago area (a) and the remainder of the State
of Illinois (b), from ILEPA annual median concentration and NOAA/ATDD V, statistics.

30
a. Chicago b. Rest of IL Loadings ,
Loadings

_

N
°o

SO, Loading, kg ha™ yr"
3°

to]
1983 1984 1985 1986 1987 1988 1989 1990 1991 1983 1984 1985 1986 1987 1988 1989 1990 1991
YEAR YEAR

Figure 26. Dry deposition loadings of sulfur dioxide (kg ha”' yr') for the Chicago area (a) and the remainder of
the State of Illinois (b), from ILEPA annual median concentration and NOAA/ATDD V, statistics.

188

ATMOSPHERIC DEPOSITION TRENDS IN ILLINOIS

20

a. Chicago Loadings b. Rest of IL Loadings

ere my
° a

SO, Loading, kg ha” yr”
a

°
1983 1984 1985 1986 1987 1988 1989 1990 1991 1983 1984 1985 1986 1987 1988 1989 1990 1991
YEAR YEAR

Figure 27. Dry deposition loadings of sulfate (kg ha’ yr’) for the Chicago area (a) and the remainder of the
State of Illinois (b), from ILEPA annual median concentration and NOAA/ATDD JV, statistics.

100
a. Chicago Loadings b. Rest of IL Loadings

a

> a @
°o °o °

TSP Loading, kg ha™ yr™

nN
°

°
1983 1984 1985 1986 1987 1988 1989 1990 1991 1983 1984 1985 1986 1987 1988 1989 1990 1991
YEAR YEAR

Figure 28. Dry deposition loadings of TSP or mass of aerosol particles (kg ha’ yr’)
for the Chicago area (a) and the remainder of the State of Illinois (b),
from ILEPA annual median concentration and NOAA/ATDD V, statistics.

189

ATMOSPHERIC DEPOSITION TRENDS IN ILLINOIS

15

_
°o

Calcium Dryfall, kg ha’ yr‘
a

IL19 IL18 IL11 IL47 IL35 IL63
Site ID

Figure 29. Box plots of the dryfall loadings for calcium (kg ha’ yr') measured at six NADP/NTN sites in Illinois
(see table 1 for precise sampling intervals included, which vary slightly for each site).

30

i)
o

Ammonium Dryfall, kg ha’ yr"
}

IL19 IL18 IL11 IL47 IL35 IL63

Site ID
Figure 30. Box plots of the dryfall loadings for large-particle ammonium (kg ha” yr")
measured at six NADP/NTN sites in Illinois (see table 1 for precise sampling intervals included,

which vary slightly for each site).

190

30

nN
°o

Sulfate Dryfall, kg ha’ yr"
°

ATMOSPHERIC DEPOSITION TRENDS IN ILLINOIS

3

IL19 IL18 IL11 IL47 IL35 IL63

Site ID

Figure 31. Box plots of the dryfall loadings for large-particle sulfate (kg ha’ yr')
measured at six NADP/NTN sites in Illinois (see table 1 for precise sampling intervals included,

which vary slightly for each site).

Table 3. Changes in Concentrations of Major Inorganic Ions in Illinois Precipitation

Ions

Ca*

NH,*

NO,

So,

pH

Note:

Percentile
25
50
75

25
50
75

25
50
75

25
50
75
25
50
75

Number with significant

Number of sites change*

Up Down Up Down
1 6 0 3
0 7 0 2
1 6 0 2
3 4 0 1
3 4 0 1
3 4 0 0
2 ) 0 0
Z 5 0 1
1 6 0 0
0 7 0
0 7 0 0
1 6 0 1
7 0 0 0
6 1 0
5 2 0 0

*Using a Spearman rank correlation test, reject H, (5 percent level): no change in concentration.

191

ATMOSPHERIC DEPOSITION TRENDS IN ILLINOIS

For Ca**, results show that the concentrations de-
creased at nearly all sites and percentiles. Of the 21
tests (3 percentiles at 7 locations), only 7 were statisti-
cally significant at the 5 percent level for NO, and
NH,,*: the evidence is weak for a net change in either
direction, however. These two pollutants were rela-
tively unchanged in precipitation during the last decade.
For SO,;, the weight of evidence once again shows a
downward trend in concentration, although only 4 of 21
tests were significant (at the 5 percent level). And
finally, for pH the test results indicate an increase over
time. Although none of these tests was significant, the
conclusion that pH has risen is consistent, chemically,
with the drop in SO,>. Stoichiometrically, the decrease
in sulfate was larger than the decrease in calcium,
accounting for a net decrease in acidity or a rise in pH.

Seasonal variation sin V * concentration, and deposition
at Bondville are shown in figures 32-34. These figures
show a LOWESS-fit curve (Cleveland, 1979) through
all weekly mean data from the NOAA/ATDD site
collected in the period 1985-1992. The results illustrate
the impact of the cyclic variation in the environment
that affects the dry deposition process. In figure 32, the
enhancement of the V,, of ozone and SO, in the summer
months is obvious, and is largely due to a reduction in
R, (see figure 9) as biologically active surfaces, i.e.,
vegetation, and liquid water films proliferate during the
growing season. R, increases, and V . is thus reduced
for these surface-reactive pollutants during the winter
season. Fine-particle V, also shows a tendency for
higher warm-season values, but there is a much smaller
increase. V,, values for fine particles and HNO,
respond to 1) a decrease in R,, which occurs with the
increase in reactive area and roughness of the surface
as the vegetation canopy expands, and 2) a decrease in
R,, which occurs due to and produces increased
atmospheric turbulence and instability produced by
summer convection. Figure 33 shows the concentration
data for fine particulate NO, and SO,”, and gaseous
SO, and HNO,, measured at Bondville, on a corre-
sponding annual cycle. A strong seasonal cycle appears
only for SO,>. Multiplying the curves of figures 32 and
33 together produces the deposition loading curves of
figure 34. When both concentration and V, have
seasonal cycles that are in phase, or peak simulta-
neously, then the deposition curve has similar but
intensified cycle. This is the case for SO,-, where the
fine-particle V, and the SO, concentration tend to
increase slightly in the warm season; the SO,- deposi-
tion curve then peaks relatively steeply at the same
time period. For SO,, only the V, term has a definite
cyclic seasonal trend, but it is nonetheless reflected in
the deposition cycle because of its magnitude. Finally,

192

in the case of HNO,, the V,, and concentration curves
are not distinct, but appear to be out of phase, with V,
higher in the summer, and concentration higher in the
winter. The resulting deposition curve reveals much
variation, but no distinct seasonality. It is important to
note from figures 32-34 that simple monitoring of
airborne concentration does not necessarily reveal a
true picture of the delivery of pollutants to receptors at
the surface.

The time series of annual distributions of dry deposi-
tion loading for several pollutant species, calculated for
the Argonne and Bondville NOAA/ATDD sites, is
plotted in figures 35 and 36. The box plot for each year
indicated shows the distribution of weekly loading
estimates, expressed as an annual total in kg ha™ yr’.
There are no apparent or statistically significant long-
term trends for SO,, SO;°, HNO,, or NO, loadings at
either site during the 1984-1992 period. However,
significant interannual variation exists for dry deposi-
tion of each pollutant. These variations tend to follow
air quality variations, but not exactly, due to the
independent variations in V,. While slight trends
toward decreases in wet deposition have been noted
above, and may be attributed to lower emissions of SO,
and SO,; over the 1980s, such a trend is not apparent in
the dry deposition data. Figures 12-23 represent
additional time-series plots of dry deposition loadings,
based on IEPA air quality data. Annual median
concentrations for the Chicago area and the “Remain-
der of State” area were used in combination with the
NOAA/ATDD V, results to produce the annual
deposition estimates. Trends over time in the IEPA air
quality data are thus reflected in these figures and are
described in detail elsewhere in this report.

Total Deposition

The wet and dry deposition data compiled throughout
this section on atmospheric deposition were combined,
where possible, to arrive at estimates of “total atmo-
spheric deposition,” (figures 37-39). The resultant total
deposition estimates should at best be considered very
tentative. The uncertainties in each of the values com-
bined are relatively large, on the order of tens of per-
cent. These uncertainties are compounded when
summed into the total deposition estimate. However,
these results are a valid statewide estimate, based on
real monitoring data, of the magnitude of inputs of
several chemicals to the earth’s surface. The following
data were used to plot the results of total atmospheric
deposition for calcium (NADP/NTN wet Ca** +
NADP/NTN dryfall Ca**), total nitrogen (NADP/
NTN dryfall NH,* + NOAA/ATDD HNO, + IEPA

ATMOSPHERIC DEPOSITION TRENDS IN ILLINOIS

Bondville, Illinois: Dry Deposition Velocity

Vv, 0,, cm s*
°
bh

=
fo)

V, fine particles, cm s"

N

V, HNO,, cm 8°
—

Month

Figure 32. Seasonal variations of weekly mean dry deposition velocity, V, (cm s') for several airborne pollutants
calculated from observations at Bondville, Illinois, for the period 1985-1992.

193

ATMOSPHERIC DEPOSITION TRENDS IN ILLINOIS

Bondville, Illinois: Concentrations

ig m®

NO

Month

Figure 33. Seasonal variations of weekly mean airborne concentration (ug m™) for several airborne pollutants
calculated from observations at Bondville, Illinois, for the period 1985-1992.

194

ATMOSPHERIC DEPOSITION TRENDS IN ILLINOIS

Bondville, Illinois: Dry Deposition

NO, Flux, kg ha" yr"
9°
a4
°

sd
°
ro)

SO,, kg ha" yr“
oN Sao © ¢

° 1 2 3 4 5 6 7 8 9 10 11 12
30
a
>
re 20
=
io)
x
> 10
°
a
a
Oo
fe) 1 a 3 4 5 6 7 8 9 10 11 12
30
tre
>
i 20
o
oo
2
. 10
°
”
Oo
fs) 1 2 3 4 5 6 7 8 9 10 11 12
Month

Figure 34. Seasonal variations in annualized weekly mean dry deposition loading (kg ha" yr’) for several
airborne pollutants at Bondville, Illinois, for the period 1985-1992.

195

ATMOSPHERIC DEPOSITION TRENDS IN ILLINOIS

Argonne, Illinois: Dry Loadings

NO,, kg ha™ yr“

1984. 1985. 1986. 1987. 1988. 1989. 1990. 1991. 1992.

SO,, kg ha™ yr"

1984. 1985. 1986. 1987. 1988. 1989. 1990. 1991. 1992.

HNO,, kg ha™ yr“

1984. 1985. 1986. 1987. 1988. 1989. 1990. 1991. 1992.

SO,, kg ha" yr™
re)
o

1984. 1985. 1986. 1987. 1988. 1989. 1990. 1991. 1992.
Date

Figure 35. Annual distributions of dry deposition loading (kg ha” yr') for several pollutant species, calculated
for the Argonne, Illinois NOAA/ATDD site.

196

ATMOSPHERIC DEPOSITION TRENDS IN ILLINOIS

Bondville, Illinois: Dry Loadings

NO,, kg ha" yr"

1985. 1986. 1987. 1988. 1989. 1990. 1991. 1992.

SO,,, kg ha” yr™

1985. 1986. 1987. 1988. 1989. 1990. 1991. 1992.

HNO,, kg ha™ yr"

1985. 1986. 1987. 1988. 1989. 1990. 1991. 1992.

SO,, kg ha" yr"

1985. 1986. 1987. 1988. 1989. 1990. 1991. 1992.

Date

Figure 36. Annual distributions of dry deposition loading (kg ha’ yr') for several pollutant species, calculated
for the Bondville, Illinois NOAA/ATDD site.

197

ATMOSPHERIC DEPOSITION TRENDS IN ILLINOIS

Chicago Area

RSSsssg

G Precipitation
@ Dryfall

Calcium Deposition, kg ha’ yr”

85.86.87.88. 89.90.

ss Rest of State
F
= 8
x
4g g
s j g
= Zageg
o 4 Z g Z Z
°
a
2
E D Precipitation
ic) 0 @ Dryfall
6 85.86.87.88.89.90.

Total Atmospheric Deposition - Calcium

Figure 37. Annual estimates of total atmospheric deposition (kg ha” yr') for calcium for the Chicago area and
the remainder of the State of Illinois.

198

ATMOSPHERIC DEPOSITION TRENDS IN ILLINOIS

Chicago Area

ES
‘a
. Z
215
= Y
Y
¢ Z
s r]
sor BY D) Y 7 @ Precipitation-NH4
2 AA BAG Prectpitation-Nos
a 5 anZee Z 0 Gaseous-HNO3
c cae A |
o a a Small Particle-NO3
ae = Dryfall-NH4
: 85.86.87. 88.89.90.

Sin Rest of State

‘s

<£

2 15

e

°

: 12 Z Z y © Precipitatipn-NH4
& A y CO Precipitatibn-NO3
Qa § a /B\e 0 Gaseous-HNO3
S Small Patticle-NO3
° [o} @ Dryfall-

z

85.86.87.88.89.90.

Total Atmospheric Deposition - Nitrogen

Figure 38. Annual estimates of total atmospheric deposition (kg ha™' yr'') for nitrogen for the Chicago area and
the remainder of the State of Illinois.

199

ATMOSPHERIC DEPOSITION TRENDS IN ILLINOIS

Chicago Area

5 30

‘3

=

2 20

ec

2

=

e

& 10 B Precipitation
5 O Gaseous

5 Small Particle
5 ° @ Dryfall

” 85.86.87.88.89.90.

Rest of State

‘5 30

‘e

<£

2 20

2

=

a]

g 10 © Precipitatibn
a O Gaseous

5 Small Particle
S oO @ Dryfall

a

Total Atmospheric Deposition - Sulfur

Figure 39. Annual estimates of total atmospheric deposition (kg ha yr') for sulfur for the Chicago area and the
remainder of the State of Illinois.

200

ATMOSPHERIC DEPOSITION TRENDS IN ILLINOIS

NO, + NADP/NTN wet NO, + NADP/NTN wet
NH,,*), and total sulfur (NADP/NTN dryfall SO,- +
IEPA SO, + IEPA SO, + NADP wet SO,>) in figures
37-39, respectively.

SUMMARY AND CONCLUSIONS
Deposition Trends and Spatial Variation

Wet deposition in Illinois has been monitored for ten
years or more at eight NADP/NTN sites. The NADP/
NTN reports the concentrations of ten separate chemi-
cal pollutants in precipitation, of which just five ac-
count for about 90 percent of the chemical composition
that causes Illinois precipitation to be acidic. The
pollutants in order of importance are: sulfate (SO,>) >
hydrogen ion (H*) > nitrate (NO,) > ammonium (NH,")
> calcium (Ca**). Illinois precipitation is most simply
described as a dilute solution of mineral sulfuric and
nitric acids, partly neutralized by ammonium and calcium.

Based on statistical tests of time- series data alone,
there is no unambiguous trend that applies to all of the
important pollutants causing acid rain in Illinois. Based
on a “weight of the evidence” analysis, however, sev-
eral points can be made about Illinois precipitation
chemistry changes during the 1980s:

1. Sulfate decreased 2 to 4 percent per year in the
southern third of the state, with smaller decreases
elsewhere.

2. Calcium decreased by 3 to 7 percent per year, ex-
cept at Argonne (suburban Chicago), where it re-
mained steady.

3. Nitrogen species, ammonium and nitrate, remained
unchanged.

4. pH increased slightly, but the increase is too small
and too variable to be quantified.

5. Sulfur dioxide and NO, emissions decreased
slightly.

Dry deposition in Illinois tends to be somewhat higher
in the Chicago area, due to both higher airborne con-
centrations of most pollutants and higher deposition
velocities. For sulfate, nitrate, sulfur dioxide, ozone,
arsenic, and manganese, the differences are on the
order of 10 to 30 percent. For cadmium, chromium,
iron, nickel, lead, and TSP, dry deposition in the
Chicago area exceeds the remainder of the state by 200
to 400 percent; this is caused primarily by the differ-
ences in air quality (see Air Quality Trends in Illinois
chapter). Temporal trends in dry deposition generally
follow air quality trends, although additional variability

is introduced into the time-series data by interannual
variation in deposition velocities. More important for
ecological impacts is the seasonal nature of dry deposi-
tion loadings, with higher deposition velocities for
many pollutants occurring during the warm season,
when biological impacts may also be the greatest.

The total deposition of sulfur in the Chicago area is
about 15 percent higher than in the rest of the state. For
sulfur (sulfate plus sulfur dioxide), the ratio of wet to
dry deposition is about | part wet to 1.5 parts dry. The
deposition of nitrogen in the Chicago area is about 30
percent higher than in the rest of the state. For nitrogen
(nitrate plus ammonium plus nitric acid vapor), the
ratio of wet to dry deposition is about one part wet to
three parts dry.

Deposition to Specific Receptor Systems

The spatial and temporal variation information is most
useful in describing the coupling of the atmosphere to
receptors that are also distributed in space and whose
sensitivity varies temporally with the depositing
pollutants. Acid deposition to forests for example, is
most likely to have an effect during the growing
season, and is much less likely to be harmful in the
dormant season. Toxic deposition to forests, however,
may act through a cumulative effect, in which the
temporal variation is less important to understanding
the impact on the receptor system.

Acid deposition to Lake Michigan presents a special
difficulty in this analysis, since neither wet nor dry
deposition is measured over the lake. The refinement of
estimates based on shoreline measurements is an ongo-
ing research topic, however.

Other Aspects of Atmospheric Deposition

Agricultural systems have been shown to be relatively
insensitive to current levels of wet acid deposition,

but the impact of toxic deposition and dry deposition
of many pollutants is unknown. Ozone has been shown
to have negative impacts on yield and quality of cash
crops in several areas of the United States. The role

of wet and dry toxics deposition as a contributor to
nonpoint source pollution in surface and ground-water
supplies for human consumption is also unknown at
this time. The impact of atmospheric deposition (acid
rain, toxic pollutants, and biological nutrients) to lakes
and streams in Illinois (i.e., non-Lake Michigan waters)
has not been documented, however, consideration of
the magnitude of deposition for many chemicals

201

ATMOSPHERIC DEPOSITION TRENDS IN ILLINOIS

would indicate that significant impacts are possible.
Finally, the impacts of SO,-, SO,, acids, and NO,
deposition on exterior building materials, paints, etc.,
both in precipitation and dry deposition, have been
demonstrated in recent federally sponsored research in
many areas of the United States. Materials impacts in
Illinois are as yet unquantified, but are potentially large.

Enhancement of Atmospheric Deposition
Knowledge

Further analyses of the data presented here and addi-
tional existing data, which were not accessible in the
context of this project, could refine the temporal and
spatial resolution of a combined atmospheric deposi-
tion database. Recommendations for further work
include acquisition of additional extant air quality and
precipitation chemistry data, and the development of
the NOAA/ATDD model to a high-resolution model
specifically for Illinois.

ACKNOWLEDGMENTS

This work was supported by the Illinois Critical Trends
Assessment Project, under the Department of Energy
and Natural Resources contract (SENR CTA-2/CRIT
TREND) with the Illinois State Water Survey and by
the State of Illinois. Data from the National Atmo-
spheric Deposition Program/National Trends Network,
which supports the ISWS’s Central Analytical Labor-
atory through a cooperative agreement with Colorado
State University’s Natural Resource Ecology Labora-
tory, were extensively used. The assistance of Rayford
P. Hosker, Jr., and Lynn Satterfield in obtaining and
formatting the NOAA/ATDD dry deposition data was
much appreciated. The authors acknowledge the diligent
efforts of Joyce Fringer in preparing the manuscript
and Greg Dzurisin in analyzing the data.

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204

EPORT DOCUMENTATION ; 2. REPORT NO.

— 3. Reciplent’s Accession No.
PAGE | ILENR/RE-EA-94/05(1) _
4, Title and Subtitle ¥ & Report Oste
The Changing Illinois Environment: Critical Trends Technical June 1994

Report of the Critical Trends Assessment Project é
Volume I: Air Resources
7. Author(s)

Illinois State Water Survey Division

9. Perfermning Organization Neme and Address
Illinois Department of Energy and Natural Resources

Illinois State Water Survey Division
2204 Griffith Drive
Champaign, IL 61820

8. Performing Organization Rept. No.

10. Project/Task/Work Unit No.

_—_-

11. Contract(C) or Grant(G) No.
(Cc)

(G)

12. Sponsoring Orgunization Nzme and Address 13. Tyce of Report & Period Covered
Illinois Department of Energy and Natural Resources
325 West Adams Street

Springfield, IL 62704-1892

15. Supplementary Notes

16. Abstract (Limit: 200 words)
Air quality measurements made by the Illinois Environmental Protection Agency for 1978-1990

were analyzed for time trends and spatial variations over Illinois. The Spearman Rank
Correlation Coefficient was used to evaluate the statistical significance of trends in
median concentrations. Spatial distributions were plotted for six criteria pollutants, and
sulfate, nitrate, and several metals in the Chicago area for 1980, 1985, and 1990. Down-
ward trends significant at 1% were noted in annual mean lead concentrations in all areas off
Illinois. Particulate matter showed no significant trends statewide or in any geographica
regions. Ozone concentrations were decreasing after removal of temperature effects.
Annual mean sulfate had a downward trend statewide (significant at 5%), in the Chicago
area, and in areas away from major urban areas (2%). No significant trends for nitrate
were detected regionally or statewide. Downward trends were significant at 1% for arsenic
the only other metal with a significant trend statewide or in the Chicago area. No
significant trends were detected for arsenic or cadmium in the Metro East area where both
manganese and iron concentrations were increasing. Spatial concentration patterns of most
pollutants were highly variable in the Chicago area for 1980, 1985, and 1990. Contouring
and comparisons were hampered by recent sharp declines in sampling site number and density

17, Ovcument Ansiysis a. Descripters
Air quality, air pollution criteria pollutants, metals, time trends, spatial variations,
Illinois, Chicago, statistical analysis

b, Idemtifiers/Opet-Ended Terms

s- COSAT! Field/Group
Avallobility Statement No restriction on distribution. 19. Security Class (This Report) 22. Ne, of Pages
Available at IL Depository Libraries or from Unclassified 206
National Technical Information Services, 22, Price
Springfield, VA 22616 Unclassified

Sea ANSI-239.14) See {nsteuctions on Reverse OPTIONAL FORM 272 (4-77)

(Formerly NTIS-3$)
Ospartment of Commerce

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UNIVERSITY OF ILLINOIS-URBANA
70097730362 C002 VO01
THE CHANGING ILLINOIS ENVIRONMENT SPRIN

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