From The Changing Illinois Environment: Critical Trends,Volume 2: Air Resources, Technical Report of the Critical Trends Assessment Project

Report Overview

Although considerable progress has been made in improving 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 possible, Illinois environmental officials must set priorities so that the most serious problems receive the most attention. The priorities should be based on an assessment 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 information 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 considerably 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 temperatures in Illinois, which increased by 4 to 5F 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.

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 geographic 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 Coefficient. Statistical tests were also carried out to identify significant differences in concentrations between geographic 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 detected. For the state as a whole, seven of twelve pollutant/averaging time data sets tested for time trend showed significant trends (5 percent or better) toward decreasing concentrations over the 1978-1990 period. After accounting for temperature effects, O3 showed a significant decreasing trend (2 percent level), rather than no trend.

In the Chicago area, eight of the twelve data sets 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, O3 (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 concentrations. After accounting for temperature, O3 showed a decreasing trend at the 6 percent level, but not the 5 percent level. Three of nine data sets 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--SO42 (sulfate 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, SO42, As, Cd, and Mn showed significant (5 percent or better) decreases.

Comparison of median and maximum pollutant concentrations 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 NO3 (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 SO2 than the rest of the state.

The MetroEast area generally experienced higher concentrations than the rest of the state for annual mean Pb, annual mean TSP (total suspended particulate matter), 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 O3 and annual mean SO2.

The analyses of spatial distribution of pollutant concentrations in the Chicago area showed that 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 annual mean or 24-hour concentrations, or both, of SO42, 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 environmental 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 characteristics 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 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 loadings, which together with the concentration 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 pollutants, such as sulfur dioxide (SO2) and nitrogen oxides (NOx), 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 atmosphere 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 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. 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 concentrations of dissolved calcium (Ca2+), magnesium (Mg2+), sodium (Na+), potassium (K+), ammonium (NH4+), sulfate (SO42-), nitrate (NO3-), chloride (Cl-), orthophosphate (PO43-), 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.

Dry deposition includes the mass transfer of pollutants to the surface by a variety of physicochemical processes: 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 difference, 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 different than the ones affecting nitric acid, a highly reactive, soluble gas. The dry deposition of gases and submicron 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 parameter known as the "deposition velocity" Vd. The dry deposition flux is the product of Vd 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 Atmospheric 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 Cl-, SO4-, particulate NO3-, nitric acid vapor (HNO3), and SO2. 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 submicron aerosols.

The dry deposition of large particles, which have an aerodynamic diameter greater than 1 micrometer (æm), 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 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 (SO42-) > hydrogen ion (H+) > nitrate (NO3-) > ammonium (NH4+) > calcium (Ca2+). 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 NOx 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 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 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 measurements 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 consideration of the magnitude of deposition for many chemicals would indicate that significant impacts are possible. Finally, the impacts of SO42-, SO2, acids, and NO3- deposition, both in precipitation and dry deposition, 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.

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