Many U.S. national parks and wilderness areas—the Grand Canyon, Yosemite, Shenandoah, and many others—are famous for their beautiful and dramatic scenery. Millions of people visit these areas each year to observe and appreciate nature firsthand. Visibility lies at the heart of this experience—the ability to look out over great vistas to see shapes and colors with crystalline clarity. In parts of the Southwest, the views can be spectacular. But such superb visibility is possible only when the air is extremely clean and particle concentrations are low. Even small increases in particle concentrations can substantially degrade visibility.
Fine particles in the atmosphere absorb and scatter light, thus limiting visual range, shifting colors, and obscuring the details of distant objects. These particles are the main constituent of the pervasive haze that impairs visibility over much of the United States.1 Most of these particles are anthropogenic pollutants, either emitted as particles or formed in the atmosphere by gas-to-particle conversion.
The geographic areas affected by haze have increased with population growth and the spread of industrialization (Clark and Munn, 1986). Haze has become a common landscape feature in many densely populated parts of the United States, especially in the East and in California. But episodes of decreased visibility are also common over large areas
remote from major centers of population and industrial development. The affected areas include many of the nation's most beautiful national parks and wilderness areas.
Studies have shown that varying degrees of visibility impairment occur at many park monitoring stations virtually all the time and that air pollution is responsible for the impairment (NPS, 1988). In recognition of the deteriorated visibility, the 1977 Amendments to the Clean Air Act (Section 169A) establish a national goal of preventing and remedying visibility impairment due to anthropogenic pollution in mandatory Class I areas, which include most large national parks and wilderness areas in the United States.2
This goal will be difficult to achieve because many types of pollutant sources degrade visibility. Many different types of sources produce the same types of chemicals, including those that are most important in visibility degradation. This makes it difficult to assign responsibility unambiguously to a source (for example, a specific coal-burning power plant) or even to a specific class of sources (for example, electric power plants or motor vehicles). Many techniques have been devised to try to resolve these problems. However, the extent to which these techniques can be used in attributing visibility impairment is uncertain, as is their usefulness in estimating the effect that different control strategies might have on visibility.
This report presents working principles for determining the relative importance of anthropogenic contributions to haze in mandatory Class I areas and for assessing various source control measures. It also provides guidelines for alleviating gaps in present knowledge about the sources and formation of haze, air-quality modeling, and emission-control techniques.
This report was prepared by the Committee on Haze in National Parks and Wilderness Areas. The committee was convened by the National Research Council's Board on Environmental Studies and Toxi-
cology in collaboration with the Board on Atmospheric Sciences and Climate. The committee comprised members with 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 (a political subdivision of Arizona), and Chevron Corporation.
The committee also issued an interim report (NRC, 1990) evaluating the Winter Haze Intensive Tracer Experiment (WHITEX), the 1987 National Park Service (NPS) report on the causes of wintertime haze in the region between the Grand Canyon and Canyonlands National park. The WHITEX report 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). The committee concluded that, at some times during the study period, NGS contributed significantly to haze in the GCNP; however, WHITEX did not quantitatively determine the fraction of sulfate (SO42-) particles and resulting haze attributable to NGS emissions.
HAZE FORMATION AND VISIBILITY IMPAIRMENT
Visibility impairment is primarily due to the interaction of light with particles in the atmosphere; gaseous pollutants usually play a small role. Particles interact with light by two important mechanisms: They can absorb light, and they can scatter light in a direction different from that of the incident light. The magnitude of these effects depends on several factors, the most important of which are the size and composition of the particles and the wavelength of the incident light. Despite the complexity of these phenomena, a variety of commonly available techniques makes it possible to characterize the optical properties of the atmosphere and to identify and quantify the pollutants that affect visibility.
Visibility refers to the degree to which the atmosphere is transparent to visible light. Traditional meteorological usage equates visibility with
visual range, the greatest distance at which a large black object can be distinguished against the horizon sky. One of the principal indices of visibility is the extinction coefficient,3 defined as the fraction of light that is attenuated by scattering or absorption as the light beam traverses a unit of the atmosphere. The extinction coefficient is a measure of the rate at which energy is lost or redirected through interactions with gases and suspended particles in the atmosphere. In a uniform atmosphere, light extinction can be shown to be inversely proportional to visual range. Light extinction is directly related to perceptual cues of overall human judgements of visual air quality The 1977 Clean Air Act Amendments and EPA's implementation of these amendments extend the term visibility to cover freedom from discoloration, reduced contrast, and other visible departures from the natural atmosphere.
This report discusses visibility primarily in terms of physical parameters such as visual range and extinction. However, the ultimate goal of visibility protection programs is to improve visibility as judged by human observers. Therefore it is important to relate measurable physical parameters to human judgments of visibility also known as visual air quality. These judgments are complex and depend on many factors, including sun angle, air color, and scene composition.
Most visibility impairment can be traced to five particulate substances: sulfates, nitrates, organics, elemental carbon, and soil dust. Water, which can combine with sulfates, nitrates, and some organics, is also a major cause of visibility impairment. Elemental carbon, soil dust, and some organics are emitted directly to the atmosphere; these are called primary particles. Sulfates, nitrates, and other organics are formed in the atmosphere from gaseous precursors; these are known as secondary particles. Improving and protecting visibility thus requires removing both gases and particles from source emissions; this is more complex and costly than removing primary particles alone.
The complex chemical and physical processes that result in the formation of visibility-impairing particles are not fully understood. However, these processes are known to enhance the effect of human activities on visibility. The particles that scatter light most efficiently per unit mass are those of approximately the same size as wavelengths of visible light (0.4–0.7/ µm). As a result of atmospheric processes, a large fraction of anthropogenic airborne particles accumulates in the 0.1 to 1.0 µm diameter range. These particles can survive in the atmosphere for several days and be transported hundreds of kilometers. During transport, the emissions from many different sources can become mixed, making it difficult to assess the effects of individual sources on visibility. This mixing leads to the formation of regional haze, which is now a major threat to visibility in U.S. parklands.
Visibility degradation is closely connected with other effects of air pollution. The human activities that reduce visibility also can impair human health and cause other environmental effects. For example, some of the major effluent gases emitted by fossil-fuel combustion react to form oxidants and acids that are health and environmental hazards as well as visibility-degrading airborne particles. The particles that degrade visibility can also influence climate by interacting with incoming solar radiation and by modifying cloud formation (Trijonis et al., 1990; Charlson et al., 1991). Because of these connections, visibility degradation has become recognized as an indicator of multiple human-health effects and environmental effects resulting from air-pollution all over the world (e.g., Gilpin, 1978; Holgate et al., 1982; Clark and Munn, 1986). Understanding these multiple connections could elucidate the potential environmental benefits of air quality improvement strategies.
DIFFICULTIES IN DEVELOPING EFFECTIVE PROGRAMS
Formulating and implementing effective programs to protect and enhance visibility can be impeded by several political, institutional, and scientific barriers. To control visibility impairment, the air-pollution sources must be identified and, to the extent possible, their relative importance must be determined. This information is difficult to obtain
because of the multiple sources of haze in national parks and wilderness areas, including industrial and utility operations, cars and other mobile sources, as well as burning for management of forest and crop lands.
The controversy over the Navajo Generating Station, affords an example of the difficulties in estimating source contributions to visibility impairment. NGS emits about 148 Mg (163 tons) of SO2 per day, making it one of the largest single SO2 sources in the United States west of the 100th meridian. The plant is located 25 km from the Grand Canyon National Park and 110 km from the Grand Canyon Village tourist area on the Canyon's south rim. NPS, the managing agency for the GCNP, believes that NGS is an important source of SO42- particles that cause wintertime haze in the park. In 1987, a large-scale experiment known as the Winter Haze Intensive Tracer Experiment was carried out to investigate the causes of wintertime haze in the region of GCNP and Canyonlands National Park (NPS, 1989). The experiment called for the injection of a tracer, deuterated methane (CD4), into one NGS stack.
During some haze episodes, significant concentrations of CD4 were detected at a sampling station on the south rim of the Grand Canyon. NPS analyzed data from WHITEX and issued a final report concluding that NGS was responsible for approximately 70% of the anthropogenic particulate sulfate and approximately 40% of the anthropogenic aerosol-related light extinction during selected wintertime periods of haze at the sampling station. This committee evaluated the NPS WHITEX report and concluded that, at some times during the study period, NGS contributed significantly to haze in GCNP. This assessment was based on evaluation of meteorological, photographic, chemical, and other physical evidence. However, the committee also concluded that the data presented in the WHITEX report were insufficient to determine quantitatively the fraction of SO42- aerosol particles and resultant visibility impairment in GCNP that are 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 appear to account for a significant fraction of haze observed in GCNP during the study period. The committee also noted that control of NGS emissions likely would reduce, but not eliminate, wintertime haze at GCNP. Precise quantification of the effect of NGS emissions on surrounding air quality remains to be determined.
Efforts to meet the national visibility goal have been hindered by a lack of commitment to meeting the goal. Visibility impairment is at least as well understood as any other effect of air pollution. Yet the nation has not given the same priority to meeting the national visibility goal as it has to addressing other air pollution problems. For instance, Section 169A(f) of the Clean Air Act makes it clear that the EPA is not required to achieve the visibility goal by any particular date. Rather, states are obliged only to make ''reasonable progress'' towards the goal and the federal government has devoted only modest resources to visibility regulation and research. In contrast, the act requires that the health-based primary air-quality standards be attained within a specified time.
This ambiguity in the act reflects the nation's uncertainty about the visibility goal and the costs of achieving it. Such conflict can be seen even within agencies that have visibility protection as part of their missions. For instance, the Forest Service and NPS routinely use prescribed burning as a forest-management practice, yet such burning conflicts with their responsibility to protect visibility in Class I areas. Within the Department of the Interior (DOI), the NPS mission of preserving natural areas conflicts with DOI's traditional interest in resource development. A striking example of this conflict arose during discussions of whether pollution control equipment should be installed on NGS. This issue placed NPS in conflict with the Bureau of Reclamation, which holds part ownership in the power plant.
A fundamental difficulty linked to those described above has been the general approach taken thus far to visibility impairment. EPA's current regulations require retrofitting of controls only on those sources whose contributions to visibility impairment in mandatory Class I areas can be shown through "visual observation or other techniques the State deems appropriate." A source's contribution to regional haze, however, usually cannot be detected through visual observation. At most, observation and similar techniques (such as photography) can detect visible plumes from individual sources. Visible plumes, however, appear to be minor contributors to visibility impairment in Class I areas.
Efforts to decide whether a particular source is contributing to regional haze have thus far encountered grave obstacles. Studies designed to estimate the effect of a particular source on surrounding visibility are
expensive, and the results can be uncertain and controversial. For instance, WHITEX cost more than $5 million, and Congress has appropriated $2.5 million initially for a similar study at the Mohave power station.
SCOPE OF THE REPORT
Recently proposed strategies suggest that visibility regulation should be based on controlling a large number of sources across a broad geographic area, and the Clean Air Act Amendments of 1990 take steps in this direction. In addition to examining methods aimed at assessing the contribution of individual sources to visibility impairment, the committee considered techniques that apportion contributions to regional haze among categories of sources or among geographic regions. The result is a set of working principles that can be used to address either single-source or regional-scale problems.
The committee kept in mind that visibility protection can take many forms and that the choice of form influences the types of analytical tools that can be used to study visibility problems. Estimates of source contributions to visibility impairment should account not only for existing sources, but also for proposed new sources that seek permits under the prevention of significant deterioration program of the Clean Air Act or other visibility-protection programs. The committee stresses that such sources must be examined as part of an overall analysis of the regional haze problem within which the effects of specific sources are embedded. In addition, the committee identified gaps in present understanding of haze formation and set priorities for information-gathering and research in these areas. The committee also established working principles to improve knowledge of these processes and to aid in the development of more effective control strategies.
In a visibility protection program, decisions must be made as to the best strategy to reduce emissions. This report presents working principles for decision makers to use in assessing alternative strategies. These principles are intended to assist in the design of emission-management plans specifically for visibility protection. However, they will also help in the assessment of visibility effects of actions taken primarily for other purposes—for example, the effect on visibility that will result from
attainment of the National Ambient Air Quality Standards for inhalable particles.
Chapter 2 describes visibility degradation in different parts of the country resulting from specific natural and anthropogenic sources of gases and particles. Chapters 3 and 4 provide the legal and technical foundation for the report. Chapter 3 presents the regulatory and institutional framework for visibility protection, including the Clean Air Act. That chapter also sets forth broad regulatory paradigms that were noted by the committee as possible frameworks within which future technical analyses might have to be conducted. Chapter 4 describes 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. Chapter 4 also reviews atmospheric chemical and physical measurement techniques.
Chapter 5 delineates in some detail various source identification and apportionment techniques. That chapter also discusses the relative difficulty of implementing these approaches and suggests detailed evaluation criteria. Technological and administrative feasibility, economic efficiency, flexibility, balance, and error analyses and biases are considered. Chapter 6 describes the emission reduction potential of various source control measures and briefly illustrates the translation of control measures into a rough prediction of effects on visibility. It provides guidance for decision makers as to the best means to develop control strategies.
Chapter 7 sets forth the committee's recommendations for the best approaches to remedy current scientific and technical gaps that limit present understanding of source effects on visibility and ability to evaluate control measures. Chapter 7 also summarizes the policy implications of the current scientific understanding of visibility impairment. The committee urges that the present program be reoriented to address regional haze and the apportionment of haze among groups of sources rather than the present single-source-oriented approach.