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-


Class I areas are areas subject under the Clean Air Act to relatively stringent restrictions on increases in air pollutants over baseline concentrations. Most of the United States is classified as Class II and is subject to less stringent restrictions. Mandatory Class I areas are national parks and wilderness areas not subject to reclassification.

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