2
Visibility Conditions in the United States

This chapter briefly characterizes overall visibility conditions in the United States and the emission sources that affect visibility. By comparing current spatial, temporal, and statistical visibility patterns with patterns in airborne particle concentrations, the committee presents information about the causes of visibility degradation. Studies that have related historical visibility trends to historical emission trends are summarized, and major emission source types (natural and anthropogenic), that affect visibility are discussed.

Visibility impairment episodes can range in scale from local plumes to widespread regional haze. The sources of locally visible plumes are easy to identify, for example, the smoke from a power plant stack or from a burning field. However, when plumes are carried by winds, they become more diffuse, and the sources are identified less readily. In regions with many sources, the plumes can merge and become mixed with the emissions from many small sources, such as motor vehicles. The result is a widespread haze in which individual contributions from the various sources are virtually indistinguishable. This latter condition—regional haze—is the main focus of this chapter.

The most intense regional haze in the United States occurs in the East, where haze often is linked to high concentrations of ambient sulfate (SO42-). SO42- concentrations are highest in the summertime during meteorological conditions that are usually associated with the western half of slow-moving high-pressure systems. Under such stagnant conditions, pollutants from many different sources can accumulate, causing severe and widespread visibility degradation.



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Protecting Visibility in National Parks and Wilderness Areas 2 Visibility Conditions in the United States This chapter briefly characterizes overall visibility conditions in the United States and the emission sources that affect visibility. By comparing current spatial, temporal, and statistical visibility patterns with patterns in airborne particle concentrations, the committee presents information about the causes of visibility degradation. Studies that have related historical visibility trends to historical emission trends are summarized, and major emission source types (natural and anthropogenic), that affect visibility are discussed. Visibility impairment episodes can range in scale from local plumes to widespread regional haze. The sources of locally visible plumes are easy to identify, for example, the smoke from a power plant stack or from a burning field. However, when plumes are carried by winds, they become more diffuse, and the sources are identified less readily. In regions with many sources, the plumes can merge and become mixed with the emissions from many small sources, such as motor vehicles. The result is a widespread haze in which individual contributions from the various sources are virtually indistinguishable. This latter condition—regional haze—is the main focus of this chapter. The most intense regional haze in the United States occurs in the East, where haze often is linked to high concentrations of ambient sulfate (SO42-). SO42- concentrations are highest in the summertime during meteorological conditions that are usually associated with the western half of slow-moving high-pressure systems. Under such stagnant conditions, pollutants from many different sources can accumulate, causing severe and widespread visibility degradation.

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Protecting Visibility in National Parks and Wilderness Areas It is important to recognize, however, that regional hazes are not necessarily caused by local emissions, nor do they depend on stagnant meteorological conditions. In the absence of precipitation, airborne particles (and their gaseous precursors) can exist in the atmosphere for many days and can be carried great distances by winds. Studies have shown that regional hazes are often associated with transport from distant sources. Haze episodes at eastern sites are well correlated with previous occurrences (more than 36 hours earlier) of low wind speeds (periods of stagnation) in upwind source regions (Samson, 1978). High SO42- concentrations at rural areas of upstate New York (including the Adirondack Mountains) most often are associated with winds coming from the south and southwest (Galvin et al., 1978). High SO42- concentrations at Shenandoah National Park, Virginia, usually are associated with moderate wind flows from the west to northwest (Wolff et al., 1982). CURRENT VISIBILITY CONDITIONS Geographical Patterns Figure 2-1 shows isopleths of median visual range at rural U.S. sites (Trijonis et al., 1990). The spatial patterns in this map are based on airport observations of visual range which differ from measurements of standard visual range.1 Airport visual range measurements are based on the identification by human observers of targets at known distances from the observation point, whereas standard visual range is calculated from light extinction measurements. However, airport observations have been calibrated against the results of instrumental studies for standard visual range, and the airport data presented in these figures have been adjusted accordingly. Figure 2-1 shows that the mountainous Southwest has the best visibility in the country. Median standard visual range exceeds 150 km in the region comprising Utah, Colorado, Nevada, northern Arizona, north 1   Standard visual range is defined as the greatest distance at which a standard observer can discern a large black object against the horizon sky under uniform lighting conditions.

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Protecting Visibility in National Parks and Wilderness Areas FIGURE 2-1 Estimated median standard visual range (km) for rural (suburban and nonurban) areas of the United States. Values are based on airport median visual ranges multiplied by 1.3 to account for differences in detection thresholds in estimating standard visual range. Data included for all days (all weather conditions). Data are for 1974–1976, but recent studies indicate that current conditions are approximately the same as shown here. Median values are reported instead of averages because a significant portion of the data are less than the lowest extinction threshold (above the farthest visibility marker). Averages, presented in other figures, are highly correlated with median values. Source: Trijonis et al., 1990.

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Protecting Visibility in National Parks and Wilderness Areas western New Mexico, and southwestern Wyoming. In the adjoining regions to the north and south, median standard visual range is also quite good, exceeding 100 km. However, visual range decreases sharply to the east and west of this area. Median visual range falls to less than 50 km in a narrow band along the northern Pacific coast, less than 30 km in the central valley of California, and to less than 15 km in the Los Angeles basin (Trijonis, 1982a). Although some parts of the East (e.g., New England) have moderately good visibility levels (about 40–60 km), median visual range is generally less than 30 km in the large area east of the Mississippi and south of the Great Lakes. Observations show a distinct relationship between visual range and altitude. On average, visual range is somewhat greater at higher altitudes than in the surrounding areas (Trijonis, 1982a; Air Resources Specialists, 1988). Many national parks are located at higher elevations than the sites from which the data were obtained for Figure 2-1, and the visual range in some national parks could be as much as 50% higher than indicated in the figure (Trijonis et al., 1990). The National Park Service (NPS) routinely measures particle concentrations and composition in many national parks and wilderness areas. Most of those parks and wilderness areas are located in the West; consequently, few data are collected for the eastern portion of the country. The geographical patterns in the annual average data are summarized in Figures 2-2 through 2-7, which show fine-particle (less than 2.5 µm diameter) mass concentration (Figure 2-2), fine particulate sulfur (Figure 2-3), fine soil-derived materials (Figure 2-4), and absorption coefficient (Figure 2-5). (The absorption coefficient is directly related to the concentration of elemental carbon.) Figure 2-6 shows the distribution of the remaining fine-particle mass (the total fine-particle mass minus the concentrations of fine sulfate, elemental carbon, and soil particles). Figure 2-7 presents data for estimated nonsulfate hydrogen (the total hydrogen concentration less the hydrogen that is associated with sulfates). The remaining mass and the nonsulfate hydrogen are believed to be qualitatively related to the spatial distribution of organic aerosols. These figures, as well as rural data sets reported in the National Acid Precipitation Assessment Program (NAPAP) Visibility State of Science and Technology Report (Trijonis et al., 1990), indicate the following differences between the air quality of the rural West (particularly the arid, mountainous Southwest) and that of the rural East (particularly the area south of the Great Lakes and east of the Mississippi):

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Protecting Visibility in National Parks and Wilderness Areas FIGURE 2-2 Average concentrations of fine-particle mass ( µg/m3) from the National Park Service network, 1983–1986. Source: Eldred et al., 1987 (From Trijonis et al., 1990). FIGURE 2-3 Average concentrations of fine-particle sulfur (ng/m3 ) from the National Park Service network, 1983–1986. Source: Eldred et al., 1987 (From Trijonis et al., 1990).

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Protecting Visibility in National Parks and Wilderness Areas FIGURE 2-4 Average concentrations of fine-particle soil materials (ng/m 3) from the National Park Service network, 1983–1986. Source: Eldred et al., 1987 (From Trijonis et al., 1990). FIGURE 2-5 Average fine-particle absorption coefficient (Mm-1) from the National Park Service network, 1983–1986. Fine elemental carbon concentrations in µg/m3 can be estimated by dividing the fine-particle absorption coefficient (in Mm-1) by 10. Source: Eldred et al., 1987 (From Trijonis et al., 1990).

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Protecting Visibility in National Parks and Wilderness Areas FIGURE 2-6 Average concentrations of remaining fine-particle mass (µg/m3) from the National Park Service network, 1983–1986. The fine particle remaining mass is defined as the total fine-particle mass minus the concentration of fine sulfate, elemental carbon, and soil particles. Organic carbon is a major component of the remaining fine-particle mass. Source: Eldred et al., 1987 (From Trijonis et al., 1990). FIGURE 2-7 Average concentrations of fine-particle nonsulfate hydrogen mass (ng/m3) from the National Park Service network, 1983–1986. The nonsulfate hydrogen is calculated from the total hydrogen concentration by subtracting the hydrogen that is associated with sulfates. Source: Eldred et al., 1987 (From Trijonis et al., 1990).

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Protecting Visibility in National Parks and Wilderness Areas SO42- concentrations are about six times greater in the rural East than in the rural West. Fine soil concentrations are about the same in the rural East and West. Elemental carbon and organic particle concentrations (as reflected by the remaining fine particle mass and the fine-particle nonsulfate hydrogen) are about two times greater in the rural East. (This finding agrees with the data for 19 rural sites reported by Shah et al., 1986.) Fine nitrate (NO3-) concentrations are about the same in both regions, about 0.5 to 1.0 µg/m3. (See denuder nitrate data in Trijonis et al., 1990.) Seasonal Patterns Two data sets show the seasonal visibility patterns for North America. The first data set is quarterly median light extinction data obtained at NPS automated camera sites for 1986–1988 (see Table 2-1 and Appendix B). The second involves analyses of data at U.S. and Canadian airports as shown in Figures 2-8a and 2-8b. Both data sets reveal an extremely strong seasonal feature that occurs on a large geographical scale—visibility is lowest in the summer in the region south of the Great Lakes and east of the Mississippi. The seasonality in light extinction measured at the two NPS sites in this region (Great Smokies and Shenandoah) is clearly evident in Figure 2-9; here, light extinction is more than twice as great in the summer as during the other seasons. The visibility minimum observed during the summer in the East is, in large part, due to maximal SO42- concentrations during the summer. Aerosol data show that SO42- concentrations in the East are nearly twice as high in the summer as during the rest of the year (Trijonis et al., 1990). Figure 2-10 shows seasonal patterns in the East for extinction, fine-particle mass, and SO42- (Trijonis, 1982a); light extinction correlates with total fine-particle mass and fine sulfate, and SO42- constitutes about half of the total fine-particle mass. Neither of the two available data sets are adequate for a definitive analysis of the more subtle seasonal visibility patterns in New England and the western two-thirds of the United States. The NPS data are biased due to treatment of data on days when targets were snow-cov-

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Protecting Visibility in National Parks and Wilderness Areas TABLE 2-1 Quarterly Median Visual Ranges for National Park Service Automated Camera Sites, 1986–1988   Seasonal Median Visual Range (km) Site Dec.–Feb. March–May June–Aug. Sept.–Nov. Acadia Park, ME 85 64 69 64 Arches Park, UT 140 158 189 166 Bandelier Monument, NM 171 154 156 178 Big Bend Park, TX 168 128 145 185 Black Canyon Monument, CO 134 146 159 140 Bryce Canyon Park, UT 243 182 184 189 Bridget Wilderness, WY 52 60 165 84 Buffalo RiVer, AR 61 43 46 64 Capitol Reef Park, UT 102 144 172 158 Capulin Volcano, NM 156 125 136 110 Carlsbad Caverns Park, NM 156 177 110 137 Chaco Culture NHP, NM 155 168 166 162 Chiricahua Monument, AZ 170 162 134 166 Colorado Monument, CO 119 151 171 156 Craig (BLM), CO 75 123 148 107 Crater Lake Park, OR 105 83 148 66 Craters of the Moon, ID 66 129 140 128 Death Valley Monument, CA 203 133 92 117 Dinosaur Monument, CO 131 133 162 178 Glacier Park, MT 35 58 152 82 Glen Canyon Area, AZ 151 152 149 146 Grand Teton Park, WY 18 119 127 94 Great Basin Park, NV 225 174 195 138 Great Sand Dunes, CO 158 123 114 140 Great Smoky Mountains, TN 49 54 20 50 Green River Area, WY 131 67 176 164 Guadalupe Mountains, TX 150 120 106 125 Isle Royale Park, MI 24 49 66 58 Jarbidge Wilderness, NV 64 24 136 92 Joshua Tree Monument, CA 244 140 115 139 Lake Mead Area, NV 234 143 156 149 Lassen Volcanic Park, CA 107 94 156 164 Lava Beds Monument, CA 154 146 158 112

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Protecting Visibility in National Parks and Wilderness Areas   Seasonal Median Visual Range (km) Site Dec.–Feb. March–May June–Aug. Sept.–Nov. Mesa Verde Park, CO 166 152 156 164 Mount Rainier Park, WA 81 82 102 98 Olympic Park, WA 10 59 94 66 Pinnacles Monument, CA 162 114 132 114 Point Reyes, CA 65 50 36 28 Redwood Park, CA 91 69 42 50 Rocky Mountain Park, CO 136 110 144 132 San Gorgonio Wilderness, CA 266 101 127 140 Shenandoah Park, VA 70 68 24 54 Superstition Mountains, AZ 234 185 153 166 Theodore Roosevelt Park, ND 198 93 133 120 Voyageurs Park, MN 75 99 172 106 Weminuche Wilderness, CO 98 112 150 137 Wind Cave Park, SD 209 119 152 147 Yellowstone Park, WY 55 24 96 63 Yosemite Park, CA 56 75 69 68 Zion Park, UT 184 156 172 164 All data are included except for observations of snow-covered targets. (Winter and spring estimates are problematic at many western sites due to treatment of data obtained during conditions with snow-covered targets.) Seasonal values represent averages of all quarterly medians available. Quarterly medians are based on regressions fit to cumulative frequency plots. Source: Trijonis et al., 1990. ered. In the West, the airport results reported by Husar are problematic because of the lack of sufficiently distant targets at some sites and also because of the limited statistical resolution in the reported results; interpretation also is difficult because the results are based on data from a mixture of urban and rural locations (Trijonis et al., 1990). Definitive results must await a revised analysis of the NPS camera data and airport visibility data or, better yet, an analysis of the transmissometer data from the new NPS monitoring system (see Chapter 4).

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Protecting Visibility in National Parks and Wilderness Areas FIGURE 2-8a Median airport visual range for January, averaged over 1979–1983. Larger circles correspond to lower visual range (greater light extinction). Diameter of circles is proportioned to light extinction. Source; R.B. Husar, pers. comm., Washington University, St. Louis, Mo., 1989 (From Trijonis et al., 1990). Despite these limitations, a few patterns emerge. For example, it is well established that in Arizona and southern California, visibility is lowest during the summer (Trijonis, 1982a; Air Resources Specialists, 1988; Trijonis et al., 1988; and Husar, 1989, pers. comm., Washington University, St. Louis, Mo.). In contrast, for the northern two-thirds of California, the Pacific Northwest, and the northern mountain states, minimal visibility tends to occur during the fall and winter (Trijonis, 1982b; Air Resources Specialists, 1988; and Husar, 1989, pers. comm., Washington University, St. Louis, Mo.). The winter and fall visibility minimum is especially obvious in the central valley of California (Trijonis, 1982b; Husar, 1989, pers. comm., Washington University, St. Louis, Mo.).

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Protecting Visibility in National Parks and Wilderness Areas FIGURE 2-12. Annual trends for 75th percentile extinction coefficient for Northeast and Southeast United States and annual trend of average extinction coefficient for entire eastern United States. Source: Trijonis et al., 1990.

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Protecting Visibility in National Parks and Wilderness Areas FIGURE 2-13. Trends for 75th percentile extinction coefficient for Northeast and Southeast United States, winter and summer. Source: Trijonis et al., 1990.

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Protecting Visibility in National Parks and Wilderness Areas FIGURE 2-14a Comparison of SO2 emission trends () and extinction coefficient (+) for the northeast United States during winter months. Source: R.B. Husar, pers. comm., Washington University, St. Louis, Mo., 1989 (From Trijonis et al., 1990; Husar and Wilson, 1993). SOURCES OF VISIBILITY-IMPAIRING MATERIALS Natural Sources Trijonis (1982a, b) and Trijonis et al. (1990) have evaluated natural background visibility (i.e., visibility in the absence of anthropogenic pollution) and the types of materials present in the unpolluted atmosphere for the eastern and western United States. The evaluation incorporates data on the concentrations of the major components of airborne particles: SO42-, organic matter, elemental carbon, nitrates, soil dust, and water. The estimates of natural aerosol concentrations are developed from three types of information: Compilations of data on emissions from natural and human sources;

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Protecting Visibility in National Parks and Wilderness Areas FIGURE 2-14b Comparison of SO2 emission trends () and extinction coefficient (+) for the northeast United States during summer months. Source: R.B. Husar, pers. comm., Washington University, St. Louis, Mo., 1989 (From Trijonis et al., 1990; Husar and Wilson, 1993). Ambient measurements of aerosol species in remote areas (especially in the southern hemisphere); Regression studies of visibility against the concentrations of trace elements associated with anthropogenic and natural emissions. The estimates exclude factors such as precipitation, fog, blowing snow, and sea spray. Because sea spray and fog are excluded, the results might not be representative of natural visibility in coastal areas, rather the natural background estimates are intended to be annual and spatial averages that exclude those areas. For the East, the analysis yields an average natural background visual range of 150 ± 45 km. This is equivalent to a light extinction level of 26 ± 7 Mm-1, or a little more than twice the level caused by Rayleigh (clear-air, ''blue sky'') scattering of light by air molecules. For the arid West, the average background visual range is estimated as 230 ± 35 km, a value equivalent to a light extinction level of 17 ± 2 1/2 Mm-1, which is about 1.5 times the level caused by Rayleigh scattering alone. (The error bounds are estimates of uncertainties in the spatial and annual averages.)

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Protecting Visibility in National Parks and Wilderness Areas FIGURE 2-15a Comparison of SO2 emission trends () and extinction coefficient (+) for the southeast United States during winter months. Source: R.B. Husar, pets. comm., Washington University, St. Louis, Mo., 1989 (From Trijonis et al., 1990). Trijonis et al. (1990) estimated that the major contributors to natural extinction levels in the East are Rayleigh scattering (46%), organics (22%), water (19%), and suspended dust, including coarse particles (9%). The major contributors in the West are Rayleigh scattering (64%), suspended dust (14%), organics (11%), and water (7%). The contribution of water is due to the hygroscopic components in airborne particles. Water is the most uncertain contributor, and it might be overestimated, given the very small amounts of SO42- and nitrates in the natural background particles (Trijonis et al., 1990). A significant fraction of the water in the natural background particles might be associated with organics because natural concentrations of organics are greater by a factor of 10 than those of SO42- or nitrate. Based on the above assessment, the most important sources of natural visibility-reducing particles are sources that emit organic materials and dust particles. Natural organic particles are produced as primary emissions (e.g., wildfire smoke, plant waxes, and pollen) and as a result of conversion from volatile organic compound (VOC) emissions (e.g., terpenes and other hydrocarbons). Natural mineral dust comes from the action of wind on soils.

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Protecting Visibility in National Parks and Wilderness Areas FIGURE 2-15b Comparison of SO2 emission trends () and extinction coefficient (+) for the southeast United States during summer months. Source: R.B. Husar, pers. comm., Washington University, St. Louis, Mo., 1989 (From Trijonis et al., 1990). On an annual average basis, the concentrations of natural particles are generally small compared with concentrations of anthropogenic particles. However, two natural particle sources—wildfires and windblown dust—are extremely episodic, and they can be the dominant cause of visibility reduction at certain times. As illustrated in Figure 2-16, most of the wildfire activity (in terms of acres) occurs in the Rocky Mountain states, the Pacific Coast, and the Southeast. The area most affected by intense dust storms centers around the Texas panhandle (see Figure 2-17). It should be noted that, for both of the above sources, the distinction between natural and anthropogenic influences is somewhat blurred. Anthropogenic Sources Fine particles are the primary cause of anthropogenic haze. Coarse particles (predominantly soil dust) and gaseous nitrogen dioxide (NO2) can also play a significant role. Anthropogenic dry fine aerosol consists almost entirely of just five pollutants: sulfates, organics, elemental

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Protecting Visibility in National Parks and Wilderness Areas FIGURE 2-16 Frequency and extent of wildfires in the United States in 1988. Source: USDA, 1989.

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Protecting Visibility in National Parks and Wilderness Areas FIGURE 2-17 Annual percentage frequency of dusty hours based on hourly observations from 343 weather observation stations that recorded dust, blowing dust, and sand when prevailing visibility was less than 7 miles (11 km). Shaded areas represent no observations of dust. Period covered is approximately 1940 to 1970. Source: Orgill and Sehmel, 1976.

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Protecting Visibility in National Parks and Wilderness Areas carbon, soil dust, and nitrates. Consequently, from the standpoint of visibility, the most important anthropogenic emissions are SO2 (precursor of SO42- particles), primary organic particles, gaseous VOCs (precursor of secondary organic particles), primary elemental carbon particles, soil-derived material, ammonia (NH3) (a precursor of ammonium nitrate), and nitrogen oxides (NOx, which are precursors of nitrate aerosols and NO2). Figure 2-18 illustrates the important anthropogenic sources of each emission in the United States. For some emissions, one source category stands out: SOx: Electric utilities contribute about 70% of the total. NH3: Livestock waste management operations are the dominant source. Elemental carbon: Diesel-fueled mobile sources account for about half of the emissions. Suspended soil dust: Vehicular traffic is presumably the predominant anthropogenic source. For NOx, three categories stand out: electric utilities, gasoline-fueled vehicles, and diesel-fueled mobile sources. VOCs and primary organic particles are emitted in significant quantities from a wide variety of sources; the most important are gasoline-fueled vehicles, residential wood burning, petroleum and chemical industrial sources, solvent evaporation, and burning for forest management. SUMMARY Visibility impairment episodes range in scale from local plumes to widespread regional haze. The most intense regional haze in the United States occurs in the East, where the median standard visual range (calculated from airport data) is generally less than 30 km. The best visibility in the country is found in the arid, mountainous Southwest, where median standard visual range exceeds 150 km. In the adjoining regions to the north and south, median standard visual range is also good, exceeding 100 km. Median visual range falls to less than 50 km along the northern Pacific coast, less than 30 km in the central valley of California, and less than 15 km in the Los Angeles basin. Visibility generally improves with altitude.

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Protecting Visibility in National Parks and Wilderness Areas FIGURE 2-18 Anthropogenic inventory of visibility-related emissions for the United States. Based on the 1985 NAPAP inventory (Zimmerman et al., 1988a; Placet et al., 1990), except copper smelter emissions, which have been updated for 1988 (Trexler, pers. comm., DOE, Washington, D.C., 1990). A pie chart is not shown for suspended dust; vehicular traffic on paved and unpaved roads is assumed to be the predominate source.

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Protecting Visibility in National Parks and Wilderness Areas National Park Service data show that sulfate particle concentrations are about six times greater in the rural East than in the rural West; elemental carbon and organic particle concentrations are about twice as great in the rural East as in the rural West; and concentrations of fine soil dust and nitrates are about the same in both regions. In the East, airport data and NPS camera data reveal a strong summertime visibility minimum that is strongly linked to a summertime maximum in sulfate concentrations. Airport data from the late 1940s through the early 1980s show coherent visibility trends over large regions. In the Northeast, visibility has moderately improved during the winter and moderately decreased during the summer. Visibility in the Southeast has worsened moderately during the winter and substantially during the summer. The greatest increases occurred during the 1950s and 1960s. It is likely that changes in SO2 emissions are largely responsible for these changes in visibility. The average natural background visual range varies from about 150 km in the eastern United States to 230 km in the arid West. The major contributors to natural extinction are Rayleigh scattering, organics, water, and suspended dust. The main constituents of anthropogenic haze are sulfates, organics, elemental carbon, soil dust, nitrates, and water. The principal anthropogenic haze-causing emissions are sulfur dioxide (SO2, a precursor of sulfate particles), primary organic particles, gaseous volatile organic compounds (VOCs, precursors of secondary organic particles), primary elemental carbon, primary crustal material, ammonia (NH3, a precursor of ammonium nitrate), and nitrogen oxides (NOx, precursors of nitrate particles and NO2). Except for VOCs and primary organics, the anthropogenic emission inventories for these species are dominated by relatively few source categories.