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Progress toward the national visibility goal will require limits on the 

 emissions of pollutants that can cause regional haze. 



Incontrovertible scientific evidence links emissions of air pollutants to the 

 formation of haze that limits visibility and degrades the visual environment. 

 Almost all the effects of air pollution on visibility are caused by airborne 

 particles. In most cases, visibility degradation is caused by five kinds of 

 particulate substances and associated particulate water: sulfates, nitrates, 

 organic matter, soot, and soil dust. Although some of these particles 

 ("primary") are emitted directly into the atmosphere, others ("secondary") are 

 formed downwind, from emitted reactive gases. Airborne particles and their 

 gaseous precursors typically remain in the atmosphere for several days, during 

 which time shifting winds can carry them hundreds of miles. As a result, a 

 strategy that relies only on influencing the location of sources, although 

 perhaps useful in some situations, would not be effective in general. The 

 aggregate emissions of the region will have to be limited. 



Progress toward the national visibility goal will require regional programs 

 that operate over large geographic areas. 



Most visibility impairment in national parks and wilderness areas results from 

 the accumulation of primary emissions and secondary products in air 

 transported over great distances. As a result, visibility impairment is 

 usually a widespread problem, not a local one. That is, the problem is one of 

 regional haze, 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. Focusing only on sources immediately adjacent 

 to Class I areas --as under the current program -- is unlikely to improve 

 visibility effectively. Class I areas cannot be regarded as potential islands 

 of clean air in a polluted sea. Efforts to improve visibility in Class I 

 areas also would benefit visibility outside these areas. 



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 results from the combined emissions of many sources, it is 

 extremely time-consuming and expensive to try to determine, one source at a 

 time, the percentage contributed by each one. Moreover, the attainable 

 uncertainties in the relationship of haze to source emissions increase as 

 increasing resolution among sources is demanded. 



The story of the Navajo Generating Station (NGS) , a large coal-fired power 



