Protecting Visibility in National Parks and Wilderness Areas Executive Summary
Many visitors to America's national parks and wilderness areas are unable to enjoy some of the beautiful and dramatic views that would prevail in the absence of air pollution. Scenic vistas in most U.S. park-lands are often diminished by haze that reduces contrast, washes out colors, and renders distant landscape features indistinct or invisible.1 The National Park Service (NPS) has reported that visibility impairment caused by air pollution occurs in varying degrees at many park monitoring stations virtually all the time. Today, the average visual range in most of the western United States, including national parks and wilderness areas, is 100–150 km (about 60–100 miles), or about one-half to two-thirds of the natural visual range that would exist in the absence of air pollution.2 In most of the East, including parklands, the average visual range is less than 30 km (about 20 miles), or about one-fifth of the natural visual range.3
Visibility degradation in parklands is a consequence of broader regional-scale visibility impairment. The causes of this impairment are well understood. Most impairment is caused by fine particles that ab
sorb or scatter light. Some of these particles (primary particles) are emitted directly to the atmosphere; others (secondary particles) are formed in the atmosphere from gaseous precursors. Visibility-reducing particles and their precursors can remain in the atmosphere for several days and can be carried tens, hundreds, or thousands of kilometers downwind from their sources to remote locations, such as national parks and wilderness areas. During transport, the emissions from many sources mix together to form a uniform, widespread haze known as regional haze.
Most visibility impairment is caused by five particulate substances (and associated particulate water): sulfates, organic matter, elemental carbon (soot), nitrates, and soil dust. The major cause of reduced visibility in the East is sulfate particles, formed principally from sulfur dioxide (SO2) emitted by coal combustion in electric utility boilers. In the West, the other four particle types play a relatively greater role than in the East. The causes and severity of visibility impairment vary over time and from one place to another, depending on meteorological conditions, sunlight, and the size and proximity of emission sources.
Congress in 1977 established a national goal of correcting and preventing pollution-related visibility impairment affecting large national parks and wilderness areas, termed ''mandatory Class I areas.''4 However, the federal government and the states have been extremely slow in developing an effective visibility protection program. The present program lacks sufficient resources, and it targets few of the major types of sources of visibility impairment in Class I areas. As a result, little progress has been made toward the national visibility goal established by Congress 15 years ago.
The Clean Air Act includes two emissions control programs specifically concerned with visibility in national parks and wilderness areas. One of these, the Prevention of Significant Deterioration (PSD) program, is directed mainly at new sources; the other, a visibility protection program, largely is aimed at existing sources.
The PSD program requires that each new or expanded "major emitting facility" locating in a "clean air area" install the "best available control technology", and it establishes increments (allowable increases) that limit the cumulative increases in pollution levels in clean air areas. Many large national parks and wilderness areas are designated as Class I areas and therefore are subject to the most stringent increments. The PSD program has protected visibility to some extent by reducing the growth of emissions of pollutants that contribute to regional haze. The program's requirement that major new sources locating in clean air areas install the best available control technology has been particularly important.
But the limits on growth in air pollutant concentrations established by the PSD program have been only partially effective. First, the restrictive Class I increments apply only to large parks created before enactment of the Clean Air Act Amendments of 1977; many other scenic areas receive no special protection. Second, it is not even clear that the Class I increments ensure effective protection against new sources that might cause visibility impairment. The increments do not distinguish between particles in the 0.1–1.0 µm range—which have the greatest potential to degrade visibility—and larger particles. Moreover, increments focus on the concentration of pollution at a given time and place; but visibility impairment depends on the total magnitude of fine particulate matter between an object and an observer.
A 1990 report by the U.S. General Accounting Office (GAO) discussed other flaws in the PSD program. The GAO found that federal land managers had not fully met their responsibilities to review PSD permit applications, due to lack of time, staff, and data and also due to the failure of the U.S. Environmental Protection Agency (EPA) to forward permit applications. Moreover, many sources of visibility impairment in national parks and wilderness areas are exempt from PSD requirements, because they are considered minor sources, or because they existed before the PSD program took effect in the 1970s.
The other visibility protection program under the Clean Air Act requires states to establish measures to achieve "reasonable progress" towards the national visibility goal and to require the installation of the "best available retrofit technology" on large sources contributing to visibility impairment in mandatory Class I areas. In 1980, EPA issued rules aimed primarily at controlling "plume blight" (impairment due to visible plumes from nearby individual sources). At that time, EPA also
expressed its intention to regulate regional haze at some future date "when improvement in monitoring techniques provides more data on source-specific levels of visibility impairment, regional-scale models become more refined, and scientific knowledge about the relationships between air pollutants and visibility impairment improves." More than a decade later, despite major advances in monitoring techniques, regional-scale models, and scientific knowledge of visibility impairment, EPA has yet to issue rules for regulating regional haze. Instead, EPA's rules require only the regulation of impairment that is attributable to individual sources through the use of simple techniques. This has greatly weakened the visibility program's effectiveness. Fourteen years passed until the first pollution source was required to control its emissions under this program.
Emission-control measures already adopted or planned will not solve the nation's visibility problems. The acid rain control program established by the 1990 Clean Air Act Amendments has been predicted to reduce SO2 emissions in the East by about 36% by 2010. That reduction probably will improve visibility in much of the East but will eliminate only a fraction of the anthropogenic visibility impairment. In the West, where most Class I areas are located, projections done for EPA indicate that the acid rain control program will halve, but not entirely prevent, expected growth in SO2 emissions between now and 2010.
THE CHARGE TO THE COMMITTEE
This report was prepared by the National Research Council's Committee on Haze in National Parks and Wilderness Areas. The committee was convened by the Council's Board on Environmental Studies and Toxicology in collaboration with the Board on Atmospheric Sciences and Climate of the Commission on Geosciences, Environment, and Resources. The committee's members have expertise in meteorology, atmospheric chemistry, air-pollution monitoring and modeling, statistics, control technology, and environmental law and public policy. The committee's work was sponsored by the U.S. Department of the Interior (National Park Service, Bureau of Reclamation, and Office of Environmental Quality), U.S. Department of Energy, U.S. Environmental Protection Agency, U.S. Department of Agriculture (Forest Service), the Arizona Salt River Project, and Chevron Corporation.
The committee was charged to develop working principles for assessing the relative importance of anthropogenic emission sources that contribute to haze in Class I areas and for considering various alternative source control measures. It also was charged to recommend strategies for filling critical scientific and technical gaps in the information and data bases on (1) methods for determining individual source contributions, (2) regional and seasonal factors that affect haze, (3) strategies for improving air-quality models, (4) the interactive role of photochemical oxidants, and (5) scientific and technological considerations in choosing emission control measures.
In 1990, the committee published an interim report, Haze in the Grand Canyon, which evaluated the National Park Service's Winter Haze Intensive Tracer Experiment (WHITEX) report on the causes of wintertime haze in the region between the Grand Canyon and Canyon-lands National Park. The WHITEX report by NPS had asserted that the Navajo Generating Station (NGS), a large coal-fired power plant in Page, Arizona, is a principal contributor to visibility impairment in Grand Canyon National Park (GCNP). Our committee's interim report concluded that, at some times during the study period, NGS contributed significantly to haze in GCNP, but that WHITEX failed to quantitatively determine the fraction of sulfate particles and resulting haze attributable to NGS emissions. The committee identified flaws in the models used to estimate NGS's contribution, in the interpretation of those models, and in the data base. The committee found that sources other than NGS appeared to account for a significant fraction of haze observed in GCNP during the study period. Thus, if NGS emissions were to be controlled, visibility impairment in GCNP would be reduced but not eliminated.
THE COMMITTEE'S APPROACH TO ITS CHARGE
In this final report, the committee examines patterns of visibility degradation and haze-forming pollutant concentrations in various parts of the United States resulting from natural and anthropogenic sources of gases and particles (Chapter 2). It considers the regulatory and institutional frameworks for efforts to improve and protect visibility, including the Clean Air Act (Chapter 3). This report also reviews the scientific understanding of haze formation and visibility impairment, including the meteorological and chemical processes responsible for the transport and
transformation of gases and particles in the atmosphere, as well as chemical and physical measurement techniques (Chapter 4). The approach of first relating source emissions to aerosol composition, and then relating aerosol composition to visibility, is fundamental to most of the committee's analyses.
In evaluating methods for source identification and apportionment, this report considers the technical adequacy (including degree of uncertainty), flexibility, and difficulty of implementation of the various approaches (Chapter 5). In discussing control techniques, the report describes the emissions reduction potential of various control measures and illustrates the translation of control measures into a rough prediction of effects on visibility (Chapter 6). The report also considers policy implications of scientific knowledge about visibility and recommends approaches to remedy scientific and technical gaps that limit present understanding of source effects on visibility and the ability to evaluate control measures (Chapter 7).
GENERAL CONCLUSIONS AND RECOMMENDATIONS
The complete design of a program for protecting and improving visibility in Class I areas must involve many policy issues outside the bounds of science and the committee's expertise. However, present scientific knowledge about visibility impairment in Class I areas has several important implications for policy makers.
Progress toward the national goal of remedying and preventing man-made visibility impairment in Class I areas (Clean Air Act, Section 169A(a)) will require regional programs that operate over large geographic areas and limit emissions of pollutants that can cause regional haze.
Most visibility impairment in national parks and wilderness areas (Class I areas) results from the transport by winds of emissions and secondary airborne particles over great distances (typically hundreds of miles). Consequently, visibility impairment is usually a regional problem, not a local one. Regional haze is caused by the combined effects of emissions from many sources distributed over a large area, rather
than of individual plumes caused by a few sources at specific sites. As a result, a strategy that relies only on influencing the location of new sources, although perhaps useful in some situations, would not be effective in general. And of course, such a strategy would not remedy the visibility impairment caused by existing sources until those sources are replaced.
A program that focuses solely on determining the contribution of individual emission sources to visibility impairment is doomed to failure. Instead, strategies should be adopted that consider many sources simultaneously on a regional basis, although assessment of the effect of individual sources will remain important in some situations.
Because haze is caused by the combined effects of the emissions of many sources, it would be an extremely time-consuming and expensive undertaking to try to determine, one source at a time, the percent contribution of each source to haze. For instance, the efforts to trace the contribution of the Navajo Generating Station to haze in the Grand Canyon National Park took several years and cost millions of dollars without leading to quantitatively definitive answers. Moreover, there are (and will probably continue to be) considerable uncertainties in ascertaining a precise relationship between individual sources and the spatial pattern of regional haze.
Assessment of the contribution of individual sources to haze will remain useful in some situations. For instance, a regional emissions management approach to haze could be combined with a strategy to assess whether locating a new source at a particular location would have especially deleterious effects on visibility. In Chapter 5, the committee has set out working principles for attributing visibility impairment to individual sources.
Visibility impairment can be attributed to emission sources on a regional scale through the use of several kinds of models. In general, the best approach for evaluating emission sources is a nested progression from simpler and more direct models to more complex and detailed methods. The simpler models are available today and could be used as the basis for designing
regional visibility programs; the more complex models could be used to refine those programs over time.
After identifying which pollutants are impairing visibility for a given region, it is useful to apportion visibility impairment among contributing sources to the extent possible so that the relative effectiveness of alternative control measures can be evaluated. Source apportionment models of varying degrees of accuracy and complexity can be used to analyze regional haze problems, although no single source-apportionment method is necessarily best for all visibility problems. Simpler methods are most effective in the early stages of source apportionment, with the more complex methods being applied, if necessary, to resolve difficult technical issues.
For regional haze problems, the committee recommends use of the following models, in order of increasing sophistication:
Speciated rollback models. These are simple, spatially averaged models that assume changes in pollutant concentrations to be directly proportional to changes in regional emissions of these pollutants or their precursors.
Hybrid combinations of chemical mass balance receptor models with a different source-oriented secondary particulate mass formation model, and used with empirical data for pollutant scattering and absorption efficiencies. Receptor models are models that infer source contributions by characterizing atmospheric aerosol samples, often using chemical elements or compounds in those samples to identify emissions from particular source types. Hybrid models are formed by combining two or more separate modeling techniques.
Hybrid combinations of mechanistic models for transport and secondary particulate mass formation with measured particle size-distribution data to facilitate light scattering calculations. Mechanistic models are 3-dimensional, computer-based models that simulate the atmospheric transport, dispersion, chemical conversion, and deposition of pollutants as faithfully as possible.
Speciated rollback models are available now; in Chapter 6 the committee uses such a model to illustrate apportionment of regional haze. The recommended hybrid combinations could be assembled from available components.
To assess the contribution of an existing single source to visibility impairment, photographic and other source identification methods could be used in simple cases. More complex situations require the use of hybrid combinations of chemical mass-balance or tracer techniques with secondary particle models that include explicit transport calculations and an adequate treatment of background pollutants. For complex applications that require the greatest sophistication, the most advanced reactive plume models available should be used with measured data on particle properties in such plumes and should be accompanied by an adequate treatment of background pollutants.
To assess new single sources, the most advanced reactive plume models available should be used with measured data on particle properties in the plumes of similar sources and accompanied by an adequate treatment of background pollutants.
To analyze a single source at the regional scale, a description of the source in question should be inserted into an appropriately chosen multiple-source description of the regional haze problem.
The next step in designing a visibility protection strategy is to determine whether methods for controlling visibility-impairing emissions exist or can be developed and to assess the effects of alternative sets of controls. The committee's analysis of one control scenario indicates that application of commercially available emission controls would reduce but not eliminate anthropogenic visibility impairment; the greatest improvement would be in the East. (This analysis should not be construed as endorsing a technology-based or any other specific control strategy.)
Visibility policy and control strategies might need to be different in the West than in the East.
Haze in the East and in the West differ in important ways. Haze in the East is six times more intense than in the West because of the much higher levels of pollution in the East. Were all anthropogenic pollution to disappear, visibility would still be greater (by about 50 percent) in the West. In relatively clean areas, small increases in pollutant concentrations can markedly degrade visibility; increases of the same magnitude are less noticeable in more polluted areas. Hence, visibility in Class I areas in the West is particularly vulnerable to increased levels of pollution. Moreover, the West contains most of the nation's large national parks and wilderness areas, which can be fully appreciated only when
visibility is excellent. The East, however, contains a large population to enjoy the benefits of any improvement in visibility in that region.
In the East, sulfates derived from SO2 emissions from coal-fired power plants account for about one-half of all anthropogenic light extinction. Reductions in these emissions are expected to occur in the next two decades as a result of the 1990 Clean Air Act Amendments' acid rain control program. In the West, no single source category dominates; therefore, an effective control strategy would have to cover many source types, such as electric utilities, gasoline- and diesel-fueled vehicles, petroleum and chemical industrial sources, forest-management burning, and fugitive dust.
Efforts to improve visibility in Class I areas also would benefit visibility outside these areas.
Because most visibility impairment is regional in scale, the same haze that degrades visibility within or looking out from a national park also degrades visibility outside it. Class I areas cannot be regarded as potential islands of clean air in a polluted sea.
Reducing emissions for visibility improvement could help alleviate other air-quality problems, just as other types of air-quality improvements could help visibility.
The substances that contribute to regional haze also contribute to a variety of other undesirable effects on human health and the environment. For example, SO2 is a precursor of sulfuric acid in acid rain, oxides of nitrogen (NOx) and volatile organic compounds (VOCs) are precursors of lower-atmosphere ozone, and fine atmospheric particles are a respiratory hazard. Such particles can influence climate by interacting with incoming solar radiation and by modifying cloud formation. Policy makers should consider the linkages between visibility and other air-quality problems when designing and assessing control strategies.
Achieving the national visibility goal will require a substantial, long-term program.
The national visibility goal is unlikely to be achieved in a short time.
Policy makers might develop a comprehensive national visibility improvement strategy as the basis for further regulatory action, and establish milestones against which progress toward the visibility goal could be measured.
Current scientific knowledge is adequate and control technologies are available for taking regulatory action to improve and protect visibility. However, continued national progress toward this goal will require a greater commitment toward atmospheric research, monitoring, and emissions control research and development.
The slowness of progress to date is due largely to a lack of commitment to an adequate government effort to protect and improve visibility and to sponsor the research and monitoring needed to better characterize the nature and origin of haze in various areas. The federal government has accorded the national visibility goal less priority than other clean-air objectives. Even to the extent that Congress has acted, EPA, the Department of Interior, and the Department of Agriculture have been slow to carry out their regulatory responsibilities or to seek resources for research.
The committee addressed the need to alleviate scientific and technical gaps in the areas of visibility and aerosol monitoring and measurement, source apportionment, and emissions control technology. The committee considered what measures might be taken to understand better the sources of haze, possible means of reducing emissions from those sources, and alternative ways of preventing future visibility impairment in Class I areas.
The committee emphasizes that the need for additional research does not imply that further regulatory action, if otherwise warranted, to improve visibility in Class I areas would be premature. The authority of regulatory agencies to act without complete scientific knowledge is clearly implied in the Clean Air Act. Moreover, visibility impairment is probably better understood and more easily measured than any other
air-pollution effect. The remaining gaps in knowledge of visibility are primarily a symptom of the lack of a strong national commitment to enforcing the visibility protection provisions of the Clean Air Act.
Resources for research are limited; therefore, precautions should be taken to ensure that the visibility protection activities of the federal land management agencies, EPA, the Department of Energy, and state and local air agencies are of the highest possible quality. In addition, a greater effort is needed for formal publication of scientific work in independent, peer-reviewed literature.
The committee recommends establishing an independent science advisory panel with EPA sponsorship to help guide the research elements of the national visibility program. This panel could address the need for wider participation by the scientific community in addressing visibility problems.
EPA should build upon and expand its efforts to track the success of the PSD program. In particular, information is needed about the potential of new sources to reduce visibility in Class I areas and about the effects on such areas of the new emissions trading programs of the 1990 Clean Air Act Amendments. EPA's current visibility-screening model needs to be revised to consider the contribution of an individual source to regional haze.
Research on relating human judgments of visibility to objective measures, such as light extinction, should continue. The results should be used to inform decision makers and the public about the perceptibility of predicted visibility changes.
Areas in which research is needed include atmospheric transport and transformations of visibility-impairing pollutants, the development of models that can better apportion haze among sources, and improved instrumentation for routine monitoring and for obtaining data that can be used to evaluate models. Monitoring and research must be closely coordinated. Better models, however, are not enough. Any model, even the simplest or most refined, depends on good empirical data on the airborne particles that cause haze and on their sources. Greater resources are needed to develop these data.
If national visibility monitoring networks are to achieve their goals,
a long-term commitment to establishing and financially supporting these networks is essential. Monitoring programs should be able to relate visibility impairment to its sources on a scale commensurate with regional haze events and the distribution of major emissions sources. Monitoring networks in the East need to be expanded to track visibility improvements associated with reductions in SO2 emissions. Wind observations should be evaluated to ensure that atmospheric transport is represented accurately.
A consensus should be developed on the specific instrumentation to be used for monitoring light extinction. Standards should be established for the performance characteristics of the instrumentation. Future measurement programs should devote increased attention to quality assurance and control. Strengthening the quality assurance and control program of the Interagency Monitoring of Protected Visual Environments (IMPROVE) network should be a high priority.
Greater attention should be given to the implications that planned changes in airport visibility monitoring hold for research on visibility impairment. Airports should be equipped with integrating nephelometers sensitive enough to measure the range of haze levels encountered in the atmosphere.
Current measurement methods permit reasonable estimates of the average contributions of major aerosol constituents to atmospheric visibility impairment. However, several aerosol measurement methods need to be developed or improved for the following:
Accurate measurement of organic and elemental carbon particles, especially at low concentrations;
Routine measurement of the water content of airborne particles;
Measuring particle size distributions;
Continuous measurement of sulfates, organics, elemental carbon, nitrates, and elemental composition;
Solar-and battery-powered measurement for use in remote areas.
The committee recommends using high-sensitivity integrating nephelometry for routine visibility monitoring. This technique, which mea-
sures the scattering of light from an air sample drawn through an enclosed cell, can provide accurate data at reasonable cost. Nephelometer data can be compared with measured particle concentrations at the same point to determine the contributions of different pollutants to visibility impairment. A readily available, easily serviced, and electronically up-to-date instrument with adequate sensitivity for good and poor visibility is needed. Nephelometer measurements of light scattering should be supplemented with independent measurements of light absorption. Instrumentation for continuous measurements of particle absorption coefficients should be developed.
Source-apportionment models require better input data on source emissions, along with unified procedures for testing individual sources. Emissions data need to be integrated more accurately into overall emissions inventories. The inventories requiring the most improvement are those for primary organic and elemental carbon particles and gaseous VOCs.
Models should be validated using existing data sets from comprehensive field studies. Mechanistic models and hybrid receptor models should be included in validation studies.
Receptor models require substantial source testing and ambient emissions measurements to improve emissions profiles for sources of haze. Standard protocols for the release and sampling of tracers should be developed, along with field studies to verify these protocols. Inexpensive and relatively short-lived tracers are needed to distinguish the emissions of similar sources.
Research should also continue toward the development of advanced mechanistic models. Two kinds of mechanistic models are especially needed: (1) an advanced reactive model for analysis of visibility-impairing plumes from single sources; and (2) a grid-based, multiple-source regional model for analysis of regional haze problems. The development of such models will require significant refinement in the understanding of processes that affect particle size distributions. Critical processes include atmospheric emissions of particles and gases that play a role in the production of secondary particles, and gas-to-particle con-
version. Measurement programs that are intended to acquire such information should be designed in collaboration with modelers to ensure that the results are suitable for model development and validation.
Continued research and development support by government and industry is needed to improve the cost-effectiveness of existing emissions control technologies and to develop new technologies. Desirable areas for further research and development in the near term include
Combined NOx/SO2 control technologies for power plants and industrial boilers;
Low-cost, low-temperature selective catalytic reduction for NOx control;
Better particulate control for diesel vehicle engines;
More efficient batteries for electric-powered vehicles, which could provide greater range before recharging.
Long-term emissions reduction research efforts should focus on low-emission, high-efficiency technologies for replacing or repowering fossil-based electricity generation, more efficient energy use technologies, industrial process modification to minimize emissions, and development of low-emission transportation systems.
Better economic modeling techniques are needed to identify the most cost-effective control strategies. Although the costs and effectiveness of many control techniques are known, it is difficult to translate costs for a specific technology into costs for overall emission reductions in an urban area or region.
FUTURE DIRECTIONS FOR PROTECTING AND IMPROVING VISIBILITY
Present scientific knowledge has important implications for the design of programs to protect and improve visibility. What is needed, overall, is the recognition that any effective visibility protection program must be
aimed at preventing and reducing regional haze. An effective program must, therefore, control a broad array of sources over a large geographic area. Such a program would mark a considerable break from the present approach of focusing on visible plumes from nearby sources and of attempting to determine the effects of individual sources on visibility impairment.
Although visibility impairment is as well understood as any other air pollution effect, gaps in knowledge remain. Filling these gaps will require an increased national commitment to visibility protection research. We believe that the time has come for Congress, EPA, and the states to decide whether to make that commitment.