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.


These are national wilderness areas and national memorial parks larger than 5,000 acres, national parks over 6,000 acres, and international parks. Any such area must have been in existence on August 7, 1977, the date the Clean Air Act Amendments of 1977 were signed into law, to be considered a mandatory Class I area.

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