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Protecting Visibility in National Parks and Wilderness Areas 6 Emission Controls and Visibility The Committee on Haze in National Parks and Wilderness Areas is charged with developing working principles for assessing the relative importance of anthropogenic sources of emissions that contribute to haze in Class I areas and for assessing various alternative measures for source control. For assessing controls, the committee has focused on principles that can be derived from scientific knowledge of visibility impairment. Any effective strategy to accomplish the congressionally established goal of remedying and preventing anthropogenic visibility impairment in Class I areas (Clean Air Act, §169A) requires limiting the emissions of pollutants that reduce visibility. Visibility problems in Class I areas are mostly the result of regional haze, rather than the effect of emissions from one or a few individual sources at specific sites. Therefore, a strategy that relies only on influencing the location of new sources, although perhaps useful in some situations, would not by itself prove effective. Moreover, such a strategy would not, of course, remedy the visibility impairment caused by existing sources. Because most impairment of visibility is regional, an effective program to improve visibility in Class I areas must operate over large geographic areas. Not only would such a program benefit Class I areas, but it would improve visibility outside of these areas. Efforts to reduce haze in Shenandoah National Park or Great Smoky Mountains National Park would improve visibility in large parts of the East; the same is true for Class I areas in the West. Class I areas cannot be regarded as potential islands of clean air in a polluted sea. The first step in designing a visibility strategy is to characterize the
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Protecting Visibility in National Parks and Wilderness Areas particles and gases that impair visibility and, to the extent possible, to apportion the impairment among contributing sources. The source apportionment methods (see Chapter 5 and Appendix C) should be appropriate to the temporal and geographic scale of the visibility problem. A visibility strategy could include controls on several different pollutants and source types. Although, sulfur dioxide, the precursor to sulfates, is often the most important single cause of impairment, control of fine and coarse particulate matter, oxides of nitrogen, ammonia (which is possibly a limiting factor in nitrate formation), and volatile organic compounds (VOCs) also must be considered. In this chapter the committee provides an example, using a speciated rollback model, to apportion anthropogenic light extinction among source types in the eastern, southwestern, and northwestern United States. This exercise is not intended as an operational evaluation of regional haze in those regions. Instead, it is presented to illustrate some of the issues that arise in any apportionment of visibility impairment. The next step in designing a strategy is to determine whether control measures exist or can be developed to reduce the emissions that impair visibility. Appendix D describes control options for the principal source categories identified in the example. (There could, of course, be situations where other sources are also of concern.) Many of these control techniques are effective and commercially available; others are still under development. Next, it is necessary to assess the effectiveness of alternative control measures. The committee used the speciated rollback modeling exercise to estimate the visibility improvements that would result from application of commercially available pollution control methods and technologies to major contributing sources in the three regions modeled. The committee's analysis shows that the application of these controls could noticeably improve visibility in the areas modeled. This exercise is not intended to signify the committee's recommendation for adopting any specific control strategy. Such a decision involves issues outside the bounds of science and the committee's expertise. Rather, the exercise allows an estimate to be made of the extent to which visibility could be improved with commercially available technology. Others must decide whether the public welfare requires the improvement and whether a program to do so would best use a technology-based approach or some alternative, such as an air-quality management
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Protecting Visibility in National Parks and Wilderness Areas approach or market-based mechanisms (see Chapter 3). Similarly, the committee does not recommend the use of a specific methodology to generate alternative control strategies for analysis. Appendix E illustrates the use of linear programming and cost-effectiveness analysis to do so; however, there are a variety of other less formal techniques. The committee is aware of the limitations of its identification of control measures. First, there can be differences of opinion about what control technologies are proven and commercially available. Such judgments are, like choices about how to shape a control strategy, ultimately based on economic and policy considerations as well as on technology. Second, the committee has not assessed the effects of such strategies as land-use planning and increased energy efficiency that rely on changes in behavior rather than on the application of control techniques. This is because of our lack of knowledge about the possible effects of such strategies; it is not the result of any disposition against them. The committee did not estimate the cost of the modeled control program; such an estimate is beyond the committee's expertise. Moreover, a cost estimate would not be very informative in isolation from cost estimates for other control strategies. Approaches that rely on behavioral changes or that provide incentives for increasing efficiency and reducing the cost of controls might be more cost-effective than the approach modeled. Because of data limitations, the modeling exercise does not cover the entire United States; it excludes Alaska, Hawaii, and those portions of the West outside the Pacific Coast, Idaho, Nevada, and the Four Corners states. The excluded area contains important Class I areas such as Yellowstone National Park. APPORTIONMENT OF REGIONAL HAZE USING A SPECIATED ROLLBACK MODEL The committee used a speciated rollback model to apportion anthropogenic light extinction among source types in three large regions of the United States—the East (states east of the Mississippi River), the Southwest (California, Nevada, Arizona, New Mexico, Utah, and Colorado), and the Pacific Northwest (Oregon, Washington, and Idaho). This modeling exercise is presented to illustrate issues that arise in any appor-
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Protecting Visibility in National Parks and Wilderness Areas tionment, and is not intended as an endorsement of speciated rollback modeling in favor of other approaches. Appendix C describes the limitations of speciated rollback modeling. The model presented here treats haze in an aggregated, average sense for the entire modeled region. It does not indicate which source areas within the region contribute the most pollution to a specific receptor area, such as a national park. Moreover, it does not distinguish an episodic source, whose impact is pronounced on a few days and nonexistent on most, from a continuous source whose constant effect is imperceptible. More-advanced models (see Chapter 5 and Appendix C) could provide more detailed information about source-receptor relationships. To address problems with regional haze, speciated rollback models must be extended beyond their typical use as a tool for assigning pollutant concentrations to sources. The extended rollback models developed here pertain to anthropogenic light extinction, not just aerosol concentrations. The apportionment is achieved in two steps: First, visibility impairment is allocated among aerosol types by means of light extinction budget calculations (see Chapter 4), and then each aerosol type is related to emissions through the linear rollback model. This analysis is concerned only with anthropogenic visibility impairment. This means that the results depend on the apportionment of material between natural and anthropogenic sources, which is itself an estimate. The relative contributions of natural and anthropogenic sources to the organic and dust portions of the airborne particles are very poorly characterized; natural sources are arbitrarily assumed here to account for one-third and one-half of the respective totals. The footnotes to Table 6-1 summarize the natural-anthropogenic partitioning of the other aerosol types; the consistent use of fractions is intended to highlight the approximate nature of the values. Based on these assumptions, the fractions of average light extinction that results from anthropogenic sources are about seven-eighths in the East, five-eighths in the Pacific Northwest, and three-eighths in the cleanest areas of the Southwest. Table 6-1 presents extinction budgets that relate the anthropogenic fraction of light extinction to aerosol constituents in the three regions. The numbers are based on the overall extinction budget by aerosol constituents and the portion of each constituent that results from anthropogenic sources. As described in the footnotes to Table 6-1 and the references cited therein, deriving these two sets of information is a non-trivial
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Protecting Visibility in National Parks and Wilderness Areas task involving the assemblage and cross comparison of numerous data sets. In each region, all anthropogenic pollutants are assumed to be emitted within that region. Emissions from major sources across the border in TABLE 6-1 Visibility Model Results: Anthropogenic Light Extinction Budgetsa Eastb Southwestc Northwestd Sulfates 65 39 33 Organics 14 18 28 Elemental carbon 11 14 15 Suspended dust 2 15 7 Nitrates 5 9 13 Nitrogen dioxide 3 5 4 a Percentage contribution by specific pollutant to anthropogenic light extinction in three regions of the United States. b Based on Table 9, Table 18, Figure 45, Appendix A, and Appendix E of NAPAP Visibility SOS/T Report (Trijonis et al., 1990). It is assumed that sulfates (3% natural) account for 60% of non-Rayleigh extinction, organics (33% natural) account for 18%, elemental carbon (3% natural) accounts for 10%, Suspended dust (50% natural) accounts for 4%, nitrates (10% natural) account for 5%, and nitrogen dioxide (10% natural) accounts for 3%. c Based on Table 9, Table 18, Figure 45, Appendix A, and Appendix E of the: NAPAP Visibility SOS/T Report (Trijonis et al., 1990). It is assumed that sulfates (10% natural) account for 33% of non-Rayleigh extinction, organics (33% natural) account for 20%, elemental carbon (10% natural) accounts for 12%, suspended dust (50% natural) accounts for 23%, nitrates (10% natural) account for 8%, and nitrogen dioxide (10% natural) accounts for 4%. d Extinction efficiencies (relative to organics) are chosen as 1.5 for sulfates, 2.5 for elemental carbon, 0.3 for fine crustal materials, and 1.5 for nitrates (Trijonis et al., 1988, 1990). Coarse dust extinction is assumed to be three times fine dust extinction (Trijonis et al., 1988, 1990). Natural aerosol particle fractions are assumed to be one-tenth for sulfates, one-third for organics, one-tenth for elemental carbon, one-half for crustal materials, and one-tenth for nitrates. These assumptions are applied using the fine mass concentrations in Trijonis et al., (1990). The percentage contribution for nitrogen dioxide is assumed to be 4%.
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Protecting Visibility in National Parks and Wilderness Areas northern Mexico and southern Canada are neglected, as is the anthropogenic background that is found in the most remote regions of the globe. Because the analysis neglects the anthropogenic portion of the extra-regional background, it will tend to overestimate the effects of changing regional emissions. This problem would become more severe if the analysis were conducted for smaller study regions. Table 6-2 lists, for each region, the percentage contribution from each major source type to emissions of pollutants that contribute to haze formation. Primary particles and gaseous precursors of secondary particles are included. Tables 6-3 through 6-5 apportion light extinction using the speciated rollback model. In keeping with the overall level of the analysis, the following simplistic precursor relationships are assumed: Sulfate concentration is proportional to emissions of sulfur oxides (SOx). Half of the organic airborne particle concentration is proportional to emissions of primary organic particulate matter (PM); the other half is proportional to emissions of VOC. The elemental carbon concentration is proportional to emissions of elemental carbon. The suspended dust concentration is proportional to emissions of suspended dust. Half of the nitrate concentration is proportional to emissions of ammonia (NH3); the other half is proportional to emissions of oxides of nitrogen (NOx). The nitrogen dioxide (NO2) concentration is proportional to emissions of NOx. The relationship for nitrate reflects an arbitrary assumption about the distribution of circumstances in which NOx or NH3 is the limiting reactant in forming ammonium nitrate particles, and the relationship for organics reflects an assumed partitioning between primary and secondary material. These two relationships are clearly more uncertain than the others. Tables 6-3 through 6-5 show that in all three regions, electric utilities, gasoline-fueled vehicles, and diesel-fueled mobile sources are either the top three anthropogenic sources of light extinction or are three of the top four sources. According to this modeling approach, in the East, electric
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Protecting Visibility in National Parks and Wilderness Areas utility SOx alone are responsible for slightly more than one-half of anthropogenic light extinction. This is because sulfates are the predominant component of anthropogenic haze in the East and electric utilities are the predominant emitter of SO2 in the East. For the Southwest and Northwest, however, no single source category is dominant. To illustrate the use of the tables, we can estimate the effect on visibility of the SO2 emission reductions in the East called for by the 1990 Clean Air Act amendments. According to NAPAP (NAPAP, 1991a,b), the amendments require about a 36% reduction in SO2 emissions in the East (equivalent to a 46% reduction in electric utility SO 2 emissions; see Table 6-2). From Table 6-3, we deduce that a 46% reduction in SO2 from electric utilities will produce a 46% × 51% = 23% reduction in anthropogenic light extinction in the East. Because about seven-eighths of total light extinction in the East is anthropogenic, the total improvement in light extinction would be 7/8 × 23% = 20%. This result is in agreement with the NAPAP visibility assessment that used light extinction budgets similar to those used here but that was based on a deterministic model for sulfate transport and chemical reaction in the atmosphere, the Regional Acid Deposition Model (RADM). POTENTIAL VISIBILITY IMPROVEMENTS FROM EMISSION CONTROLS This section assesses the potential for visibility improvements from application of commercially available technology to control emissions. The assessment is for illustration and should not be construed as advocating a specific control strategy. The method could, however, be applied to estimate the effectiveness of emissions reductions being considered by policy makers, especially to compare the effects of alternative control strategies. Appendix D describes control methods for major sources of visibility impairment in the regions modeled: electric utilities, industrial coal combustion, the petroleum and chemical industries and industrial oil combustion, nonferrous smelters, diesel-fueled and gasoline-fueled motor vehicles, fugitive dust, feedlots and livestock waste management, residential wood burning, forest management burning, and organic solvent evaporation. These methods are summarized in Table 6-6.
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Protecting Visibility in National Parks and Wilderness Areas TABLE 6-2 Percentage Contribution of Source Categories to Emissions in the East, Southwest, and Nortwesta,b,c SOx Organic Particles VOCs Elemental Carbonc Suspended Dust NH3 NOx East Electric utilities (nearly all from coal-fired power plants) 78 Neg Neg Neg Neg Neg 39 Diesel-fueled mobile sources (trucks, buses, train, jets, ships, heavy equipment) 1 1/2 1 Neg 34 Neg 31 47 29 Negd Negd Neg Neg 16 26 Gasoline vehicles Petroleum and chemical industries and industrial oil combustion 4 1/2 7 Neg Neg 11 Neg Neg Neg Neg Neg Neg Neg Neg Neg Industrial coal combustion Neg 20 13 15 Neg Neg Neg Residential wood burning Fugitive dust (presumably predominantly from on-road and off-road traffic) Neg Neg Neg Neg Neg Neg Neg Neg 100 Neg Neg 66 Neg Neg Feedlots and livestock waste management 8 46 45 9 Neg 34 19 Miscellaneous
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Protecting Visibility in National Parks and Wilderness Areas SOx Organic Particles VOCs Elemental Carbonc Suspended Dust NH3 NOx Southwest Electric utilities (nearly all from coal-fired power plants) 33 Neg Neg Neg Neg Neg 19 Diesel-fueled mobile, sources (trucks, buses, trains, jets, ships, heavy equipment) 12 5 Neg 52 Negd Neg 23 Gasoline vehicles 5 38 42 31 Negd Neg 32 Petroleum and chemical industries and industrial oil combustion 22 Neg 12 Neg Neg Neg Neg Copper smelters 19 Neg Neg Neg Neg Neg Neg Fugitive dust (presumably predominantly from on-road and off-road traffic) Neg Neg Neg Neg 100 Neg Neg Residential wood burning Neg 8 5 6 Neg Neg Neg Feedlots and livestock waste management Neg Neg Neg Neg Neg 75 Neg Miscellaneous 9 49 41 11 Neg 25 26
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Protecting Visibility in National Parks and Wilderness Areas SOx Organic Particles VOCs Elemental Carbonc Suspended Dust NH3 NOx Northwest Electric utilities (nearly all from coal-fired power plants) 30 Neg Neg Neg Neg Neg 8 Diesel-fueled mobile sources (trucks, buses, trains, jets, ships, heavy equipment) 12 Neg Neg 37 Neg4 Neg 29 Gasoline vehicles 4 15 31 16 Neg4 Neg 36 Petroleum and chemical industries and industrial oil combustion 19 Neg 10 Neg Neg Neg Neg Residential wood burning Neg 22 25 22 Neg Neg Neg Forest management burning Neg 45 13 20 Neg Neg Neg Fugitive dust (presumably predominantly from on-road and off-road traffic) Neg Neg Neg Neg 100 Neg Neg Feedlots and livestock waste management Neg Neg Neg Neg Neg 81 Neg Primary metallurgical process 8 Neg 15 Neg Neg Neg Neg Organic solvent evaporation Neg Neg 15 Neg Neg Neg Neg Miscellaneous 27 18 6 5 Neg 19 27
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Protecting Visibility in National Parks and Wilderness Areas SOx, oxides of sulfur; VOCs, volatile organic compounds; NH3, ammonia; NOx, oxides of nitrogen; Neg, negligible. a Based on the 1985 NAPAP Inventory (Zimmerman et al, 1988a; Battye, pets. comm., E.H. Pechan and Associates, Durham; N.C., 1990), with the exception that copper smelter emissions have been updated to 1988 (E. Trexler, pers. comm., DOE, Washington, D.C., 1990) because of the large recent changes in emissions from that source category. b Source categories are included for a region only if they are estimated to contribute to at least 2% of total anthropogenic haze. Individual emissions are noted as Negligible (Neg) if the anthropogenic haze contribution from that source emission is less than 0.5%. c The elemental carbon/organic carbon fractions of PM2.5 emissions are assumed to be 70/20 for diesel-fueled, mobile sources, 10/40 for gasoline vehicles, 10/45 for residential wood burning, and 6/60 for forest management burning (Gray, 1986; Trijonis et al., 1988; Core, pers. comm., Oregon Department of Environmental Quality, Portland, Oregon, 1990). It is assumed that all other urban sources account for the same amount of organic PM emissions as the total of gasoline and diesel mobile sources but only for 10% as much elemental carbon (Gray, 1986; Trijonis et al., 1988). d Suspended dust from vehicular traffic is treated as a separate category.
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Protecting Visibility in National Parks and Wilderness Areas Source category Emissions Control methods Comments Fugitive dust Dust particles Improved agricultural land management; roadway surface improvements; road sweeping and flushing; improved material storage and transfer Technologies are widely available and applied; control of natural dust emissions is often impracticable. Feedlots and livestock waste management NH3 Application of chemical additives to animal waste Can reduce NH3 emissions from feedlots by about 50% Residential wood burning Organic particles, elemental carbon Improved woodstoves Emissions from best existing stove technology stoves are about 80% lower than from conventional woodstoves Forest management burning Organic particles, elemental carbon Reduction of number of acres burned; increased use of residues; burning under increased fuel moisture; helitorch ignition; rapid mop-up Together, these practices can reduce emissions by about 50% Organic solvent evaporation VOCs Reformulation of solvents; use of nonsolvent-based alternatives Some alternatives are available; others are under development a See Appendix D for additional information.
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Protecting Visibility in National Parks and Wilderness Areas Table 6-7 shows the emission reductions assumed for this assessment. The reductions are based on application of commercially available controls alone—that is, controls of proven effectiveness that can be applied or purchased today. For example, SOx reductions from electric utilities are based on application of wet and spray-dry flue-gas desulfurization, but not on the sorbent injection and clean coal technologies still being demonstrated. Reductions in motor vehicle emissions are based on engine improvements, better electronic control, and enhanced inspection and maintenance, but not on the use of alternative fuels or of unproven technologies such as trap oxidizers. Reductions greater than those assumed in Table 6-7 would of course be possible as advanced technologies become available. Emission reductions are specified as 1/4, 1/2, or 3/4 as a reflection of their approximate nature. For a sensitivity analysis, these reductions are replaced by 20%, 40%, and 60% for a low estimate and by 30%, 60%, and 90% for a high estimate. All emission reductions are relative to the 1985 NAPAP inventory used as the basis for the speciated rollback modeling analysis. The approximate haze improvements associated with these reductions are shown in Tables 6-8, 6-9, and 6-10. The reductions of Table 6-7 have been multiplied by the source allocations of Tables 6-3 through 6-5 to yield the anthropogenic light extinction improvements of Tables 6-8 through 6-10. For example, assuming electric utility SOx accounts for 52% of anthropogenic light extinction in the East (Table 6-3), then a 75% reduction in electric utility SOx (Table 6-7) would yield a 39% reduction in eastern anthropogenic light extinction (Table 6-8). The 1990 Clean Air Act amendments will require a reduction of approximately 50% in electric utility SO2 emissions nationwide by the year 2000 (most of the reduction is to take place in the East). The notes to Tables 6-8 through 6-10 state the total percentage decreases in anthropogenic light extinction for the entire control strategy, i.e., the sum of all the entries in each table. Table 6-11 summarizes the visibility improvements that would result from the example emission reductions. For the East, anthropogenic light extinction would be reduced by an estimated 59%. Because natural visibility impairment is small compared to anthropogenic impairment in the East, there would be nearly an equivalent reduction in total light extinction, and median visual range would more than double. As indi-
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Protecting Visibility in National Parks and Wilderness Areas TABLE 6-7 Estimated Commercially Available Percentage Reductions of Visibility-Impairing Emissions Source category Emission Reductiona Electric utilities SOx NOx 3/4 1/2 Industrial coal combustion SOx NOx 3/4 1/2 Diesel-fueled motor vehicles SOx NOx Elemental carbon 3/4 1/2 3/4 Gasoline vehicles SOx Organic particles +VOC NOx 3/4 1/2 1/2 Petroleum and chemical industries and industrial oil combustion SOx VOC 3/4 1/2 Fugitive dust Dust particles 1/4 Feedlots and livestock waste management NH3 1/2 Residential wood burning Organic particles, VOCs Elemental carbon 1/4 1/4 Forest management burning Organic particles, VOCs Elemental carbon 1/2 1/2 Organic solvent evaporation VOC 1/2 Copper smelters b SOx a These are illustrative reductions, in most cases from individual units, that are technically feasible using commercially available technologies and methods. They provide the bases for assigning 25%, 50% or 75 % reduction in the base scenario of the illustrative assessments shown in Tables 6-8 to 6-11 (and for selecting 20%/40%/60% and 30%/60%/90% for the low and high reduction scenario of the sensitivity analyses). b Copper smelters, which remain a significant source of SOx in the southwest, were not analyzed for potential SOx reductions because they are already controlled, and the prospects for further reductions are uncertain.
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Protecting Visibility in National Parks and Wilderness Areas TABLE 6-8 Speciated Rollback Model Percentage Reductionsa in Eastern U.S. Anthropogenic Light Extinction for Emission Reductions of Table 6-7. Total Reduction in Eastern U.S. Anthropogenic Light Extinction: 59% ± 12%b Source Sulfates α SOx Organics 1/2 α org PM 1/2 α VOC Elemental Carbon α ELC PM Suspended Dust α Dust PM Nitrates 1/2 a NOx 1/2 α NH3 NO2 α NOx Electric utilities 39 1/2 1 Gasoline-fueled motor vehicles 1 2 1/2 1/2 1/2 Diesel-fueled motor vehicles 1 4 Industrial coal combustion 4 Petroleum and chemical industries and industrial oil combustion 2 1/2 Residential wood burning 1/2 1/2 Fugitive dust 1/2 Feedlots and livestock waste management 1 a The numbers in the table indicate percentage reductions in anthropogenic light extinction achieved by controls applied to each source category and emissions type. Emission sources are presented in the order of their relative contribution to anthropogenic light extinction. b The spread of the results is obtained by replacement of 25%/50%/75% emission reductions with 20%/40%/60% and 30%/60%/90% reductions.
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Protecting Visibility in National Parks and Wilderness Areas TABLE 6-9 Model Percentage Reductionsa in Southwest U.S. Anthropogenic Light Extinction for Emission Reductions of Table 6-7. Total Reduction in Southwest U.S. Anthropogenic Light Extinction: 40% ± 8%b Source Sulfates α SOx Organics 1/2 α org PM 1/2 α VOC Elemental Carbon α ELC PM Suspended Dust α Dust PM Nitrates 1/2 a NOx 1/2 α NH3 NO2 α NOx Gasoline-fuel motor 1 1/2 3 1/2 1/2 1 Electric Utilities 9 1/2 1/2 1/2 Fugitive dust 4 Diesel-fueled motor vehicles 4 5 1/2 1/2 Petroleum and chemical industries and industrial oil combustion 7 1/2 Feedlots and livestock waste management 1 1/2 a The numbers in the table indicate percentage reductions in anthropogenic light extinction achieved by emission reductions applied to each source category and emissions type. Sources are presented in order of their relative contribution to anthropogenic light extinction. b The spread of the results is obtained by replacement of 25%/50%/75% emission reductions with 20%/40%/60% and 30%/60%/90% reductions.
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Protecting Visibility in National Parks and Wilderness Areas TABLE 6-10 Model Percentage Reductionsa in Northwest U.S. Anthropogenic Light Extinction for Emission Reductions of Table 6-7. Total Reduction in Northwest U.S. Anthropogenic Light Extinction: 41% ± 8%b Source Sulfates α SOx Organics 1/2 α org PM 1/2 α VOC Elemental Carbon α ELC PM Suspended Dust α Dust PM Nitrates 1/2 a NOx 1/2 α NH3 NO2 α NOx Diesel-fueled motor vehicles 3 4 1/2 1 1/2 Gasoline-fueled motor vehicles 1 3 1 1/2 Electric Utilities 7 1/2 1/2 Forest management burning 4 1 1/2 Residential Wood Burning 2 1 Petroleum and chemical industries and industrial oil combustion 4 1/2 1/2 2 Fugitive Dust Feedlots and livestock waste management 2 1/2 Organic solvent evaporation 1 a The numbers in the table indicate percentage reductions in anthropogenic light extinction achieved by emission reductions applied to each source category and emissions type. Sources are presented in order of their relative contribution to anthropogenic light extinction. b The spread of the results is obtained by replacement of 25%/50%/75% emission reductions with 20%/40%/60% and 30%/60%/90% reductions.
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Protecting Visibility in National Parks and Wilderness Areas TABLE 6-11 Summary of Visibility Improvements from the Example Control Program East Southwest Northwest Percentage reduction in anthropogenic light extinctiona 59 40 41 Percentage reduction in total average light extinctionb 52 15 26 Percentage increase in median visual range 108 18 35 (~2550 km)c (~150180km) (~7095 km) a From Tables 6-8 through 6-10 b Assuming natural contributions to total light extinction are 1/8 in the East, 5/8 in the Southwest, and 3/8 in the Northwest. Current median visual rangers are from Figure 2-1 c Median visual range before and after control. cated in Table 6-8, the majority of the improvement would come from controls on electric utility SOX. In the Southwest and Northwest, anthropogenic light extinction would be reduced by about 40%. Accounting for the larger relative contributions from natural sources in those less polluted regions, the reductions in total average light extinction are estimated to be only 15% in the Southwest and 26% in the Northwest. On worst-case pollution days, when natural contributions are relatively less important, light extinction would be reduced correspondingly more. As shown in Tables 6-9 and 6-10, the major source categories that contribute to the improvements would be motor vehicles (gasoline and diesel), electric utilities, industrial petroleum and chemical sources, and (in the Northwest) forest management burning. It is of interest to compare the visibility improvements from the example emissions reductions to the improvements necessary to accomplish the national goal of no anthropogenic impairment of visibility from widespread haze. Achieving the latter goal under average conditions
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Protecting Visibility in National Parks and Wilderness Areas (not for worst-case pollution days) would require anthropogenic light extinction reductions of approximately 98.5% in the East, 83% in the Southwest, and 94% in the Northwest. As noted above, anthropogenic light extinction constitutes about seven-eighths of the average total light extinction in the East, five-eighths in the Northwest, and three-eighths in the cleanest areas of the Southwest. It is assumed that ''no anthropogenic impairment'' on the average means that average anthropogenic light extinction is less than 10% of average natural light extinction. Choosing 15% rather than 10% for this threshold would change the required anthropogenic light extinction reductions from 98.5% to 98% in the East, 83% to 75% in the Southwest, and 94% to 91% in the Northwest. The potential reductions in light extinction from the example emission reductions—59 ± 12% in the East, 40% ± 8% in the Southwest, and 41% ± 8% in the Northwest—fall far short of this. The above analysis supports the following conclusions regarding the visibility improvement that would result from scenarios based upon the application of commercially available controls: In the East, anthropogenic and total light extinction would be reduced by more than one-half. In the Southwest and Northwest, reductions in anthropogenic light extinction would be less than one-half, with total (anthropogenic plus natural) reductions of about one-fourth. The key to improving visibility substantially in the East is to control SOx emissions from electric utilities. Improving visibility substantially in the Southwest and Northwest will require the control of many source categories—especially electric utilities, diesel-fueled and gasoline-fueled motor vehicles, petroleum and chemical industrial sources, forest management burning, and fugitive dust. The visibility improvements in all regions would fall far short of the national goal of no anthropogenic impairment from regional haze. The above analysis is only an example calculation—not an operational formulation and evaluation of a national haze control program. Nevertheless, the qualitative aspects of these conclusions do not appear sensitive to the details of the calculation. This insensitivity to specific numerical assumptions is supported by the compartmentalized nature of errors in the speciated rollback model (see Appendix C) and by the rudimentary sensitivity analysis in Tables 6-8 through 6-10.
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Protecting Visibility in National Parks and Wilderness Areas RELATIONSHIP BETWEEN VISIBILITY AND OTHER AIR-QUALITY PROBLEMS Visibility is just one of many air-quality problems. The pollutants that impair visibility contribute to other environmental problems, some of which have been or are being considered as objects of federal, state, or local legislation or regulation. For example, controls aimed at reducing acid rain or lowering ambient concentrations of ozone and PM10 could improve visibility in Class I areas; conversely, controls aimed at improving visibility could alleviate other air-quality problems. Policy makers should weigh these linkages in the design and assessment of possible control strategies. The source attribution models discussed in Chapter 5 and Appendix C can provide the technical basis for examining the effects of proposed controls on multiple air-quality problems. Complex mechanistic models that explicitly consider a broad range of chemical interactions are best suited for this analysis. Simpler approaches could be applied to analyses of limited scope. For example, a rollback approach can be used to estimate the effect of SO2 reductions that stem from the 1990 Clean Air Act amendments on visibility in eastern Class I areas (Trijonis et al., 1990). Such an analysis can provide useful information, because sulfates are a dominant component of eastern regional haze, and sulfuric acid is a major component of eastern acid rain. If the effects of NOX or VOC controls were examined, the more complex models would be necessary because the chemical interactions of these pollutants are more complex. The committee recognizes that these approaches must be balanced against other considerations. Although it could be theoretically attractive to calculate beforehand all relevant advantages and disadvantages of proposed controls on emissions, this might not be possible or even desirable in the process of formulating public policy. Moreover, many social and economic factors must be taken into account in deciding how to protect and improve visibility in Class I areas. CONCLUSIONS An effective program to improve visibility in Class I areas must reduce emissions of pollutants that impair visibility. Because most visi-
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Protecting Visibility in National Parks and Wilderness Areas bility impairment in Class I areas is caused by regional haze, an effective program must operate over large geographic areas. The committee used a speciated rollback model to apportion anthropogenic light extinction in the East, Southwest, and Northwest. Its analysis indicates that in the East, SO2 emissions from electric utilities cause more than one-half of all anthropogenic light extinction. In the Southwest and Northwest, no single source category is dominant; the major sources include gasoline-and diesel-fueled motor vehicles, electric utilities, industrial petroleum and chemical sources, and (in the Northwest) forest management burning. Control technologies are available to reduce emissions from all major sources of haze. However, some of these technologies are expensive or of limited effectiveness. Continued support for research and development by government and industry is needed for efforts to improve the cost-effectiveness of existing emissions control technologies and to develop new technologies, especially low-emission technologies for fossil-fuel-based electricity generation (such as coal gasification and fuel cells), more efficient energy use technologies, and renewable sources of energy. The committee estimated the visibility improvements that would result from the application of all commercially available controls in the three regions in the model. This analysis should not be construed as prescribing a specific control strategy. Estimated reductions in anthropogenic light extinction were about 60% in the East, and 40% in the Northwest and Southwest. The corresponding reductions in total light extinction were about 50% in the East, 25% in the Northwest, and 15% in the Southwest. The results fall far short of the national goal of no manmade impairment of visibility. The analysis indicates that the key to improving visibility in the East is to control SO2 emissions from power plants. In the West, substantial improvements in visibility require that many source categories be controlled. In designing and assessing strategies to improve visibility in Class I areas, it is important to consider linkages with other air-quality problems, such as acid rain, PM10, and lower-atmosphere ozone. Many social and economic considerations must also be taken into account in designing control strategies.
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Representative terms from entire chapter: