We recommend several qualitative means for recognizing model limitations and resulting

uncertainties when preparing an attainment demonstration. First, we recommend using models

in a relative sense in concert with observed air quality data (i.e., taking the ratio of future to

present predicted air quality and multiplying it times an “ambient” design value)7. As described

later, we believe this approach should reduce some of the uncertainty attendant with using

absolute model predictions alone. Second, we recommend that a modeling analysis be preceded

by analyses of available air quality, meteorological, and emissions data to gain a qualitative

understanding of an area’s nonattainment problem. Such a description should be used to help

guide a model application and may provide a reality check on the model’s predictions. Third, we

recommend that States/Tribes use several model outputs, as well as other supporting analyses, to

provide corroborative evidence concerning the adequacy of a proposed strategy for meeting the

NAAQS. Modeling results and other supporting analyses can be weighed to determine whether

or not the resulting evidence suggests a proposed control strategy is adequate to meet the

NAAQS. Finally, we identify several activities/analyses which States/Tribes could undertake, if

they so choose, to apply models and corroborative approaches in subsequent reviews and

analyses of a control strategy, such as mid-course reviews. These subsequent reviews are useful

for determining whether a SIP is achieving progress as expected.

Premise 2. For many areas, nested regional/urban model applications will be needed to

support the attainment demonstration. Available air quality data suggest ozone and PM2.5

concentrations approach levels specified in the NAAQS throughout much of the eastern U.S. and

in large parts of California (U.S. EPA 2004a, U.S. EPA 2004b). A number of analyses

(U.S.EPA, 1998b and U.S. EPA, 2005a and 2005b) show that regional ozone and PM transport

can impact areas several hundred miles or more downwind. The regional extent of ozone and PM

transport patterns and distances in some areas will likely necessitate nested regional model



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