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effective progress. Because visibility impairment is completely reversible 

 and more easily monitored than most pollutant effects, it may be suited to a 

 program incorporating feedback from trial and observation. The need for 

 additional research does not imply that further regulatory action, if 

 otherwise warranted, to improve visibility in Class I areas would be 

 premature . 



The committee felt that the slowness of progress to date has been due largely 

 to a lack of commitment to an adequate government effort to protect and 

 improve visibility and to sponsor the research and monitoring needed to better 

 characterize the nature and origin of haze in various areas. The federal 

 government has accorded the national visibility goal less priority than other 

 clean-air objectives. Even to the extent that Congress has acted, EPA, the 

 Department of Interior, and the Department of Agriculture have been slow to 

 carry out their regulatory responsibilities or to seek resources for research. 



Although the causes of visibility impairment are reasonably well understood, 

 additional research is still necessary in some areas. Visibility research 

 would benefit from increased integration with other air-quality research, and 

 from wider participation by the scientific community. The committee 

 recommended establishing an independent science advisory panel with EPA 

 sponsorship to help guide the research elements of the national visibility 

 program. I think the committee would have been encouraged by the recent 

 efforts of the Committee on Environment and Natural Resources at interagency 

 coordination. 



Visibility impairment can be attributed to emission sources on a regional 

 scale through the use of several kinds of models . 



After identifying which pollutants are impairing visibility in a given region, 

 it is useful to apportion these pollutants among contributing sources to the 

 extent possible so that the relative effectiveness of alternative control 

 measures can be evaluated. No single source -apportionment method is 

 necessarily best for all visibility problems, and the committee's 

 recommendations in this area lean heavily on hybridization of complementary 

 approaches. In general, the best approach for evaluating emission sources is 

 a nested progression from simpler and more direct models to more complex and 

 detailed methods. Simpler methods are most effective in the early stages of 

 source apportionment, with the more complex methods being applied, if 

 necessary, to resolve difficult technical issues. The simpler models are 

 available today and could be used as the basis for designing regional 

 visibility programs; the more complex models could be used to refine those 

 programs over time. As a vehicle to illustrate issues that arise in any 

 apportionment, the committee's final report demonstrated the use of a simple 

 model, speciated rollback, to apportion spatially and temporally averaged 



