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regulations was because the agency expected that the 

 implementation of title IV of the 1990 amendments--acid rain 

 control measures—would significantly reduce visibility 

 impairment in class I areas caused by sulfur dioxide and nitrogen 

 oxides emissions. However, EPA has since concluded that while 

 reductions of these pollutants could improve visibility in some 

 class I areas in eastern states, the reductions would not solve 

 all of the visibility problems. Although the reductions are 

 estimated to improve visibility by approximately 20 percent on an 

 average day in many eastern parks, Park Service officials told us 

 that this level of improvement will not be apparent to many park 

 visitors . 



In western class I areas, EPA concluded that any reductions 

 in emissions of sulfur dioxide and nitrogen oxides achieved by 

 the implementation of title IV would be offset by increases in 

 pollution caused by population growth and the construction of new 

 emission sources. Therefore, EPA projects little or no change in 

 visibility for national parks and wilderness areas in western 

 states, which account for 126 of the 158 class I areas. 



Lending support to the need for regional haze regulations is 

 a 1993 National Research Council report which concluded that 

 neither existing nor planned emission control programs, including 

 title IV, will solve the nation's visibility impairment problem. 

 The report argued that real progress in reducing visibility 

 impairment will require regional programs that control pollution 

 from sources in large geographic areas. According to the 

 Council's report, visibility impairment is probably as well 

 understood as any other air pollution problem. Consequently, the 

 report concluded that while additional research is worthwhile, 

 current scientific knowledge is adequate and control technologies 

 are available for taking regulatory action to improve visibility. 

 In commenting on the Council's report, the Park Service stated 

 that the report confirmed what it has believed for years, that . 



