International Air Quality Advisory Board SPECIAL REPORT ON TRANSBOUNDARY AIR QUALITY ISSUES November 1998 5. REGIONAL ISSUES 5.1 Artifact Regions For the purposes of this report, the Canada­United States border area has been divided into five regions: Arctic­Far North, Pacific, Mountain­Prairie, Great Lakes­Ontario, and Eastern. For each of these "artifact regions," this chapter describes the status of air quality and atmospheric deposition, identifies sensitive receptors of air pollution, and describes any significant transboundary air management strategies in the states and provinces. It must be noted, however, that these regions do not represent airsheds, which, depending on the pollutant of interest, can extend many kilometres in all directions from the border. The Board notes that the boundaries defined for the purpose of linking sources (both point and area) and receptors in a specific airshed will vary in size and shape, depending on the location of sources and receptors, the extent of transport of the individual pollutant, prevailing meteorology, and contributing source types. 5.2 Arctic­Far North The Alaska­Yukon and Alaska­British Columbia borders areas are sparsely populated, with virtually no industry. It appears that, although there are localized occasions of poor air quality, there are no major transboundary air pollution concerns in the region. However, some transboundary issues along the Alaskan border were noted in the Commission's 1995 Comments Received on the Canada­United States Air Quality Agreement -- A Report to the Governments of Canada and the United States. These concerns dealt with smoke from open burning and wildfires, ice fog, and haze. A number of wide-ranging air pollution issues affects the Arctic as a whole. Arctic haze is air pollution in the form of fine droplets of sulfuric acid, particles of soot, and other contaminants carried north from areas such as Eastern and Western Europe. It is widespread and can be detected in large parts of the Far North during winter and early spring. Potential effects include decreased visibility, acid snow and a possible contribution to climate change. Persistent toxic substances are usually generated and released from anthropogenic activities such as agriculture and industry, almost exclusively in lower latitudes. There is great concern about the long-range air transport of these substances from distant sources and their subsequent deposition in the Arctic environment. Northern native populations are particularly susceptible to the effects of toxic substances such as PCBs, DDT, and mercury because these pollutants tend to bioaccumulate to high levels in the tissues of the fish and wildlife on which native populations rely for food. These pollutants are not generally released locally. Specific pollution incidents such as the Exxon-Valdez oil spill also create transboundary concerns. This region is known for its large tracts of protected wilderness. Wrangell-St. Elias National Park and Preserve in Alaska, contiguous with Kluane National Park in the Yukon, is the largest national park complex in North America. Visitors to these and other wildlands along the border have recently found degraded scenic vistas due to visibility-reducing particles, generated principally from wildfires and prescribed forest burns. There are currently no regional air management strategies in place for this region. Vigilance is needed in the event that resource utilization, such as drilling for oil and gas and mineral extraction in the Arctic National Wildlife Refuge, is contemplated in the future. 5.3 Pacific The transboundary pollution released from a smelter at Trail, British Columbia, into the adjacent U.S. state of Washington approximately 60 years ago was one of the initial references to the IJC. This smelter is now being rebuilt, and emissions of particulate, SO2, and lead have declined over the past ten years, with the expectation of further decline on completion of the present work. Some forest stands of the Cascade Mountains in Washington State are downwind from Vancouver and may be affected by elevated levels of ozone. The transfer of photochemical pollutants in both directions (north and south) is significant. Aerosol nitrates and ozone formed in the Fraser Valley as a result of emissions from urban Vancouver can have impacts, including decreased visibility, on adjacent areas of the United States. Levels of ozone in the Canadian sector of the Fraser Valley have reached 200 ppb on occasion; more commonly, daily peaks are below 100 ppb. However, health studies of farm workers near Abbotsford picking fruit for ten hours a day have shown that even an ozone peak below 80 ppb has a measurable effect on lung function. It is believed that current ozone levels are affecting the productivity of sensitive crops, such as alfalfa grass, in the Fraser Valley; further control programs will be required to address them. Levels of particulate pollution are generally low in coastal regions. In British Columbia as a whole, higher values for particulate matter are recorded in inland cities such as Prince George than on the coast. The geography of the Lower Fraser Valley also allows significant transfers across the boundary from south to north. Quantities of SO2 from refineries in Whatcom County, Washington, and of ammonia from agricultural activities affect pollution levels in the Fraser Valley. There is some indication of low pH precipitation on Saturna Island and Victoria as a result of transport from Puget Sound. Monitoring networks are in place on both sides of the boundary, as far up as 40 km (25 miles) north in the Fraser Valley, and collaborative consultation between the U.S. regulators in Whatcom County and regional and provincial authorities in British Columbia occurs through regular meetings of a joint council established for this purpose. The adequacy of the monitoring, however, remains a concern. The monitoring network in the Fraser Valley is adequate for regulatory purposes, with a dense and comprehensive monitoring program conducted by the Greater Vancouver Regional District (G.V.R.D.). Beyond the borders of the G.V.R.D., however, there are only three monitoring sites on the Canadian side of the border and one on the U.S. side. This is not adequate to satisfactorily track changes in air quality. Proposals to add regional-level monitoring to Whatcom County are now being formulated. The governments noted that they are also working on the Georgia Basin Ecosystem Initiative, which will assist in characterizing and managing these issues in coming years. 5.4 Mountain­Prairie This region extends from the Cascade Mountains east to the Rocky Mountains and into the Great Plains region of Canada and the United States, including much of the provinces of Manitoba, Saskatchewan, Alberta, and British Columbia, and the states of North Dakota, Montana, Idaho, and Washington. This region is characterized by low population density but significant resource utilization, including mining, timber harvesting, and energy development. It contains some of the largest intact wilderness areas in North America and is home to many of the large ungulates (hoofed species) and carnivore species (e.g. wolves) that are subject to preservation efforts. Regional and local sources of particulate and gaseous emissions include wood production operations (e.g. beehive burners, slash burning, pulp and sawmills), wildfires and prescribed burns, resource extraction (e.g. mines, oil and gas drilling and processing, smelters), fertilizer production and use, feedlot operations, agriculture, and fossil fuel combustion (e.g. coal-fired electrical generating facilities). The few large point sources of criteria pollutant emissions include smelters and oil and gas facilities in Alberta, Montana, and North Dakota. Transport of elevated ozone concentrations from the Greater Vancouver and Seattle areas to the mountains has been measured. This transport from urban areas, coupled with possible stratospheric intrusions of ozone, account for elevated ozone concentrations at high elevations. Recent studies of forest fire emissions in Alberta indicate that large burns can contribute to elevated ozone concentrations downwind. U.S. NADP wetfall monitoring in Washington, Idaho, and western Montana and North Dakota showed the annual average pH in the border states to range from 5.1 to 5.6, levels that are close to "background" values. Deposition of sulfate was in the range of 1 to 3 kilograms per hectare (0.89 to 2.7 pounds per acre), with nitrate and ammonium concentrations being elevated in eastern North Dakota as a result of emissions from agricultural operations and feedlots. A U.S. Rocky Mountain study showed snow deposition of sulfur and nitrogen compounds in the border region to be low when compared with loadings measured in the Front Range in Colorado, downwind of some large coal-fired power plants and the population centers of Denver­Boulder­Fort Collins. Thus, acid precipitation is not currently indicated as a principal issue in this portion of the boundary, but certain localized areas do show cause for concern. In the Cascade Mountains in Washington, snow deposition downwind of the Puget Sound area contains elevated concentrations of sulfur, nitrogen, and trace metals. Visibility monitoring in parks and preserves shows the impact of both forest fires (wild fires, prescribed burns, and slash burning) and industrial emissions from sources in the border states and provinces. Monitoring at the contiguous preserves of Waterton Lakes National Park in Alberta and Glacier National Park in Montana indicated that two thirds of the particles that reduce visibility in these two parks originate in Canada, and one half of that contribution comes from sources within Alberta. A special visibility study carried out in Kootney National Park in the Canadian Rockies recorded layered haze due to local sources of wood combustion, and uniform, regional haze due to transport of particles from wildfires burning in the western United States. In this region, there are many major national parks and few population centers on both sides of the border, with a number of areas on the U.S. side designated as Class I protected areas under the U.S. CAA. Therefore, concern is focused on effects on natural resources, surface water quality, and preservation of scenic vistas in these large protected areas. Resource characteristics that might be affected by anthropogenic air pollution include high-elevation lake and stream water quality, forest productivity, grassland soil fertility, and visibility in scenic areas. Both the Rocky Mountains and the Cascade Range contain large numbers of high-elevation lakes and streams, many of which have low buffering capacity. In general, these fresh waters are not currently affected by deposition of acids or sulfur or nitrogen compounds, with the exception of some ultra-low buffered lakes in the Washington Cascades. Many of the forested lands in this region are suffering from drought, insect infestations, and the consequences of poor management through fire suppression, but it is unlikely that air pollution has significantly affected these coniferous forests. 5.5 Great Lakes­Ontario The Great Lakes constitute the largest body of surface fresh water on earth and one of the planet's most valuable natural resources. The Great Lakes Basin includes the actual Great Lakes and over 760,000 square kilometres (295,000 square miles) of land that drains into them. Home to roughly 36 million people, the Great Lakes Basin is one of North America's major industrial and agricultural regions. The Great Lakes contain significant levels of hazardous air pollutants, including PTSs, due to both local sources of pollution and long-range atmospheric transport (as illustrated for PCBs in Figure 5-1). Over the past several years, annual average ozone levels in the basin have been consistently higher than the 15 ppb Canadian national annual ambient air quality objective. In general, annual average levels of total suspended particulate (TSP) in Ontario have not declined over the last several years. Sulfate levels in the Basin have declined slightly over the same period. Figure 5-1 The Atmospheric Pathway Strachan, W.M.J. and S.J. Eisenreich 1986. "Mass Balancing of Toxic Chemicals in the Great Lakes: The Role of Atmospheric Deposition." Workshop on Estimation of Atmospheric Loading of Toxic Chemicals to the Great Lakes Basin - October 1986. International Joint Commission Studies conducted in the Great Lakes region and elsewhere have provided strong evidence linking air pollutants such as ground-level ozone, airborne particles and acid aerosols to indicators of adverse effects on respiratory health, including decreased lung function in children, increased hospital admission for respiratory diseases, and increased health costs. Figures 5-2 and 5-3 compare ozone levels and sulphate concentrations with hospital admissions. Figure 5-2 Daily Maximum One Hour Ozone Level (ppb) (Recorded on previous day) Burnett R.T., Dales, R.E., Raizenne M.E., Krewski D., Summers P.W., Roberts G.R. Raad-Young M., Dann T. and J.R. Brook, 1993: Effects of low ambient levels of ozone and sulphates on the frequency of respiratory admissions to Ontario hospitals. Envir. Res., 65, 172-194. Figure 5-3 Daily average sulphate level (g/m3) (Recorded on previous day) Burnett R.T., Dales R.E., Raizenne M.E., Krewski D., Summers P.W., Roberts G.R. Raad-Young M., Dann T. and J.R. Brook, 1993: Effects of low ambient levels of ozone and sulphates on the frequency of respiratory admissions to Ontario hospitals. Envir. Res., 65: 172-194. These findings demonstrate that adverse effects on respiratory and cardiac health are associated with relatively low air pollution levels, and that there does not appear to be a threshold level for ozone below which no adverse health effects are observed. Those at higher risk for adverse health effects from exposure to airborne contaminants include the very young, the elderly, smokers, and those with pre-existing cardiac conditions or respiratory diseases. The levels of exposure of residents of the Great Lakes Basin to some PTSs have significantly decreased during the past 20 years. For example, the levels of DDT and its degradation products in the breast milk of Canadian women have decreased thirtyfold since 1967. Since the 1980s, the levels of lead in the blood of Ontario children have decreased over threefold, in parallel with the reduction in use of lead in gasoline. Nevertheless, certain PTSs remain a concern in the Great Lakes Basin, including organochlorine pesticides, PCBs, dioxins, lead, and methyl mercury. Table 5-1 lists the PTSs currently covered under the 1997 Canada-U.S. Strategy for the Virtual Elimination of Persistent Toxic Substances. LEVEL I Aldrin ^ Dieldrin *^ Benzo(a)pyrene {B(a)P} * Chlordane ^ DDT, DDD, DDE *^ Hexachlorobenzene (HCB) *^ Alkylated lead * Mercury * and its compounds Mirex *^ Octachlorostyrene PCBs *^ Dioxins (PCDD; 2,3,7,8-TCDD) *^ Furans (PCDF; 2,3,7,8-TCDF) *^ Toxaphene *^ NOTE: Hexabromobiphenyl and Pentachlorophenol are listed as POPs on the CEC Council Resolution #95-5 but are not included on the Strategy listing of PTSs. LEVEL II Cadmium and its compounds 1,4-Dichlorobenzene 3,3'-Dichlorobenzidine Dinitropyrene Endrin ^ Heptachlor and Heptachlor epoxide Hexachlorobutadiene Hexachloro-1,3-butadiene Hexachlorocyclohexane (including alpha, beta, delta, lindane) 4,4'-Methylenebis (2 Chloroaniline) Pentachlorobenzene Pentachlorophenol Tetrachlorobenzene (1,2,3,4- and 1,2,4,5-) Tributyl tin Polycyclic Aromatic Hydrocarbons (PAHs)^ as a group, including but not limited to:     Anthracene     Benzo(a)anthracene     Benzo(g,h,i)perylene     Perylene     Phenanthrene Table 5-1: Level I and Level II Persistent Toxic Substances (PTSs) identified in the Canada-U.S. Strategy for the Virtual Elimination of Persistent Toxic Substances. The Critical Pollutants identified by the IJC Water Quality Board in 1985 are indicated with an asterisk (*) and the POPs from the Commission for Environmental Co-operation (CEC) Council Resolution #95-5 identified with a caret (^). Because these contaminants are persistent and frequently subjected to long-range transport, control measures must be implemented for both local sources and sources distant from the lakes. When the first estimates of deposition of these contaminants were attempted in the late 1970s, concentrations were relatively high and the lakes were seen solely as receptors. This is illustrated by Figure 5-4, which shows the source regions for toxaphene deposited at Egbert, Ontario, in the northern portion of the Great Lakes Basin. Numbers shown in each state are estimated annual uses of toxaphene in tons per year, circa 1980. The use of toxaphene as a pesticide has since been largely discontinued in the United States. Figure 5-4 R.M. Hoff et al., 1993. Measurement of PCCs in air in southern Ontario, ChemospHere 27: 2057-62 More recently, with the reduction in concentrations of some PTSs in incoming air masses, the lakes are now becoming a source. One estimate of loadings of toxic chemicals to Lake Ontario made by Hoff et al. in 1996 (see Figure 5-5) shows volatilization of chemicals from the lakes to be an important factor. While one axis ("y") is described as "% of loading," the portion below the zero point represents a negative loading, that is, a release or volatilization of the contaminants into the atmosphere. If the depositional components of the flux (wet deposition, dry deposition, and gas absorption) are plotted as a fraction of the total loading (represented by the area above the zero per cent line) for most of the organochlorine compounds, gas absorption is the most important process. When the volatilization of the chemical out of the water column is considered, however, it can be several times larger than all the combined atmospheric inputs. For alpha- and gamma-hexachlorocyclohexane, Figure 5-5 shows that the concentration of these compounds in the atmosphere is in near equilibrium with Lake Ontario, with inputs nearly equal to the volatilization output. For the DDT-like species and PCBs, however, the lake is now a source to the atmosphere. For the metals and PAHs, the flux is still largely depositional (except for the lightest molecular weight PAHs, which have higher volatility). Recently these results have been interpreted to mean that the lakes are responding very quickly to the atmosphere's ability to carry off the chemicals above them. This is consistent with current understanding of the grasshopper and cold condensation effects, through which chemicals continue to migrate through the environment from warmer to colder climates. The Great Lakes are now a source of many chemicals to other regions of the globe. Figure 5-5 Hoff, R.M., W.M.J. Strachan, C.W. Sweet, C.H. Chan, M. Shackleton, T.F. Bidleman, K.A. Brice, D.A. Burniston, S. Cussion, D.F. Gatz, K. Harlin, W.H. Schroeder, 1996. Atmospheric Deposition of Toxic Chemicals to the Great Lakes: A Review of Data through 1994, Atmos. Environ. 30, 3505-3527. As noted in Section 4, an air monitoring network around the Great Lakes, the Integrated Atmospheric Deposition Network, has one regional background station on each of the Great Lakes. The network provides good regional estimates of concentrations of several organic and inorganic chemicals. Recent information, developed as part of the ongoing Lake Michigan Mass Balance Study, has confirmed that Chicago is the source of large quantities of PCBs. A major portion of PCB transport from Chicago has been undetected by IADN monitoring, which has focused on measuring deposition in remote areas. This finding confirms the need for an additional emphasis on measuring sources and ambient concentrations in and around urban areas to obtain a better understanding of transport and deposition of air pollutants. In its current work under the Great Lakes Water Quality Agreement, the Board will continue to consider the extent to which routine monitoring capabilities need to be extended to include urban areas. 5.6 Eastern The Eastern region of the Canada­United States border geographically covers the provinces of New Brunswick, Nova Scotia and Quebec, and the states of Maine, New Hampshire, and Vermont. The border in this region contains many shared resources, including the coastal water of the Gulf of Maine, the Saint Croix boundary river, and the watersheds of the St. Lawrence River and Lake Champlain. The region is best characterized as diverse and vulnerable. The shared resources of the region include a diverse mixture of geological, hydrological, topographical, climatological, and biological features. This is matched by a diverse pattern of development, from the virtually undeveloped mountainous regions of the Laurentian Highlands and the Northern Appalachian Mountains to major urban centers like Montreal and New York City. The region is quite vulnerable to air pollution because of its location downwind of the entire continent. Distant upwind decisions affect the environmental health of this region and its inhabitants. Efforts to control local sources of air pollution have been largely successful. Monitoring of pollutants such as carbon monoxide (CO), SO2, and NO2 has shown levels to be within the limits established by the respective federal governments. Monitors on both sides of the border, however, are recording levels of ozone that exceed the human health standards in both countries. Ozone and its precursors are known to freely flow across the border, especially along the Atlantic coast. Attempts to quantify the levels of fine particulate matter along the border are under way, with monitoring plans for this pollutant under development in the United States. Previous studies documenting the levels of sulfate, a significant component of fine particulate matter, found evidence that high sulfate levels are causing visibility impairments and contributing to acidification of the region. Extrapolation of data from these studies suggests that areas within the region may experience levels of PM2.5 above the standard recently adopted by the United States to protect human health. Many unique resources exist within the Eastern region, and a full cataloguing of them is beyond the scope of this report. To further characterize the threat of air pollution, however, it is useful to examine air pollution and deposition in five smaller areas, or receptors, within the region -- Kejimkujik in Nova Scotia, Acadia National Park in Maine, Casco Bay (one of the Great Waters in Maine), Hubbard Brook in New Hampshire, and Lake Champlain in Vermont, New York and Quebec (a transboundary lake that is also one of the Great Waters). Kejimkujik Kejimkujik, Nova Scotia, is the eastern-most acid deposition monitoring site in North America. Since the early 1980s, surface water chemistry has been monitored in this area to verify the ecological benefit from emissions control. Although concentrations of sulfate in rain have decreased in most of the Eastern region since the early 1990s, some locations such as Kejimkujik have statistically significant increasing trends in sulfate levels. An extension of an earlier analysis of water quality trends in Nova Scotia, Newfoundland, Quebec, and Ontario yielded the following results: 51 per cent of 202 monitored sites showed decreasing sulfate; 1 per cent showed increasing sulfate; and 48 per cent showed no significant trend. A more recent regional analysis showed that 11 per cent of monitoring sites in these provinces continue to acidify; 33 per cent are recovering; and 56 per cent exhibit no statistical trend. The greatest difference between these results and those from the earlier analysis is that a substantial number of lakes in Nova Scotia and Newfoundland have shifted from the improving class to the class without an acidity trend. Acadia National Park Acadia National Park is an area of more than 19,000 hectares (47,000 acres) along the mid-coast of Maine that includes rich and diverse flora, birds, and mammal species. Downwind from large urban and industrial areas, it periodically experiences high concentrations of air pollutants. Ozone concentrations, for example, have violated federal standards and routinely exceeded standards set by the State of Maine. Sulfates are the largest single contributor to visibility impairment. National Park Service studies have focused on the sulfur content of aerosols in extreme fog events, approximating the relative contributions of different source areas to aerosol concentrations at receptor sites. As shown in Figure 5-6, nickel smelting facilities in the area around Sudbury, Ontario, contribute the most sulfur to Acadia National Park (29 per cent). It is estimated that coal-fired power plants in the New York­Philadelphia area contribute approximately 15 per cent, while plants in northern New York contribute about 24 per cent. Midwestern SO2 sources -- primarily in Michigan -- contribute about 9 per cent. Figure 5-6 Acadia National Park Fraction of sulfate arriving at Acadia from various source regions. "Characteristics and Origins of Haze in the Continental United States." W.C. Malm, Earth-Science Reviews, 33(1992) 1-36, Elseview Science Pub. Casco Bay Concern with atmospheric deposition to Casco Bay, Maine, focuses on PAHs, PCBs, nitrogen, phosphorus, sulfates, pesticides, and mercury and other trace metals. Recent sediment studies show elevated concentrations of cadmium, lead, mercury, PAHs, PCBs, silver, and zinc near population centers, in waste discharges, and in rural eastern Casco Bay, which is remote from known sources (Wade et al., 1995). A circulation model study of Casco Bay did not clearly indicate possible localized sources of these pollutants, suggesting atmospheric deposition as a significant source (Pearce et al., 1994). Hubbard Brook, New Hampshire Long-term data from the Hubbard Brook Experimental Forest in New Hampshire indicate that, although only small changes in stream acidity have been observed, large quantities of base cations have been lost from soils. These losses are apparently due to declines in base cation deposition, as well as leaching by acidic deposition. As a result, the response of soil and stream water chemistry to increases or decreases in acidic deposition might be substantially delayed. Because many factors beyond atmospheric deposition affect acidity trends, however, a more extensive study is required to more closely determine the role of deposition. Lake Champlain Lake Champlain is the sixth largest inland water body in the United States. Because the lake passes through the border and drains north into the St. Lawrence River, it is a receptor of transboundary importance. Air pollution threats to Lake Champlain manifest themselves as fish consumption advisories due to mercury and PCBs; increased levels of arsenic, PAHs, PCBs and lead in lake sediments; impairments of visibility; increases in regional ozone and fine particulate matter; and acid precipitation. Monitoring/Modeling Issues There is good coordination of ozone measurements in the immediate vicinity of the Maine­New Brunswick border. An international co-ordination effort exists to ensure that data from each network are of comparable quality and that an "airshed" approach is taken to data analysis. The region hosts a number of regional, national, and international sites that provide the basis for studies to document long-range transport of ozone, particulate matter, acidifying air pollution, and PTSs. Regional atmospheric transport models have confirmed the long-range transport of ozone and its precursors, as well as particulate matter, acidifying air pollution and PTSs. There is an immediate need to develop mass balance models at specific receptor locations to begin to understand the contribution of PTSs from various pathways and sources to these locations. These mass balance models should allow the development of more effective management strategies. Air Quality Strategies in the Eastern Region The need for regional strategies to address air pollution in the Eastern region is well recognized and has led to the formation of several regional cooperative studies, regional air quality organizations, and regional air quality management strategies. For example, the Northeastern States for Co-ordinated Air Use Management (NESCAUM) is a thirty-year-old organization established by the governors of the six New England states, New York, and New Jersey. NESCAUM provides a forum to coordinate air quality management issues within the northeastern United States. NESCAUM has recently been working to interface with its counterparts in Eastern Canada. The New England Governors and Eastern Premiers Association (NEG/EP) is a binational organization that provided leadership in developing a regional Acid Rain Control Plan well in advance of efforts by the federal governments. The NEG/EP recently passed two resolutions recognizing the threat of i) mercury pollution, and ii) acid deposition and calling for further controls. The U.S. CAA also recognized the need for a regional solution to the ozone problems by designating a Northeast Ozone Transport Region (OTR) and establishing an OTR Commission (composed of official representatives from each of the 12 states in the region and the District of Columbia, plus the Administrators from the three northeastern EPA Regions). The Commission is charged with recommending to the federal government control measures for implementation throughout the region to reduce ozone levels below the federal standard. A second U.S. effort to identify and coordinate abatement plans for ozone is the Ozone Transport Assessment Group (OTAG), which was established by the Environmental Council of States (ECOS) in 1995. OTAG comprises a 37-state geographic region in the eastern half of the country (plus the District of Columbia), and its work has been summarized by the Board in previous reports. All of these efforts make it clear that the key to effective control and significantly improved air quality rests with continued and improved monitoring and the further development of binational approaches and strategies for emission reductions. Recommendation The Board recommends that any regional control strategies to limit transboundary air pollution be based on source transport and receptor regions as defined by the pollutant, meteorology and contributing sources.