soil dust. Although some of these particles are emitted directly into the atmosphere, others, such as SO42-, are the result of the transformation in the atmosphere of gaseous emissions such as sulfur dioxide (SO2). Because of their physical properties, airborne particles and their gaseous precursors can exist in the atmosphere for several days; during this time, winds can carry these materials great distances (e.g., typically hundreds of miles). This leads to the formation of a widespread regional haze.

Visibility problems in Class I areas are predominantly the result of regional haze from many sources, rather than individual plumes caused by a few 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 be effective in general. Moreover, such a strategy would not remedy the visibility impairment caused by existing sources until these sources are replaced.

Progress towards the national goal of remedying and preventing anthropogenic visibility impairment in Class I areas will require regional programs that operate over large geographic areas.

Because most visibility impairment in Class I areas results from the transport by winds of emissions and secondary airborne particles over great distances, focusing only on sources immediately adjacent to Class I areas—as under the current program—is unlikely to improve visibility effectively.

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.

Nonetheless, the assessment of the contribution of individual sources to haze will remain important 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. The committee has set out working principles in Chapter 5 and Appendix C for attributing visibility impairment to single sources.

The committee doubts, however, that such attributions could be the basis for a workable visibility protection program. It would be extremely time-consuming and expensive to try to determine the percent contri-

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