Additional air quality modeling. Sensitivity tests can be performed to see if

conclusions about trends in “worst” and “best” visibility are robust. One example of such an

analysis is applying a model with and without a more finely resolved nested grid near one or

more Class I areas. A purpose of this would be to see whether conclusions are affected by the

degree of detail in which nearby sources are considered A second example of an analysis would

be to consider alternative future emissions and/or differing growth rate assumptions. This may

be a particular concern for regional haze analyses because the emissions projection period is

generally longer than for most ozone and PM2.5 attainment demonstrations. Uncertainty in

emissions and growth rates become more important as the projection period is lengthened.

If trends in visibility for “worst” and/or “best” days are similar using sensitivity tests,

alternative models and/or alternative modeling approaches, this finding supports conclusions

reached in the uniform rate of progress analysis.

Review of trends. A review of trends generally involves a comparison, sometimes

qualitative, between past trends in reconstructed visibility and estimated changes in emissions

(e.g., early ‘90's to mid ‘00's). The IMPROVE dataset has a longer record than the

corresponding urban FRM and speciation sites. Some IMPROVE sites have data going back to

1989. This information could be used to confirm that more control measures on previously

reduced emissions of a component or its precursors is likely to be useful. It may also be used to

see whether certain PM components are becoming increasingly important sources of light



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