context of providing illustrative control scenarios for selected areas, and understanding the

contribution of different source categories, source regions and pollutant emissions to air quality

across the U.S. The RSM can be used in a variety of ways: (1) strategy design and assessment

(e.g. comparison of urban vs. regional controls; comparison across sectors; comparison across

pollutants); (2) optimization (develop optimal combinations of controls to attain standards at

minimum cost); (3) model sensitivity (systematically evaluate the relative sensitivity of modeled

ozone and PM levels to changes in emissions inputs.

18.6 How Should the Results of the Model Evaluation be Assessed (Interpreted)?

In EPA guidance for the 1-hour ozone attainment demonstrations (U.S. EPA, 1991a),

several statistical goals were identified for operational model performance. These goals were

identified by assessing past modeling applications of ozone models and determining common

ranges of bias, error, and accuracy (Tesche et al., 1990). The 1-hour guidance noted that because

of differences in the quality of the applications considered, it was inappropriate to establish "rigid

criterion for model acceptance or rejection" (i.e., no pass/fail test). It was recommended that

these ranges should be used in conjunction with the additional qualitative procedures to assess

overall model performance.51

With the additional experience of more than a decade of photochemical modeling, it is

clear that there is no single definitive test for evaluating model performance. All of the tests

identified in Sections 18.2 and 18.3 have strengths and weaknesses. Further, even within a single

performance test, it is not appropriate to assign “bright line” criteria that distinguish between

adequate and inadequate model performance. In this regard, EPA recommends that a “weight of

evidence” approach (like that described in Section 7) be used to determine whether a particular

modeling application is valid for assessing the future attainment status of an area. EPA

recommends that States/Tribes undertake a variety of performance tests and weigh them

qualitatively to assess model performance. Provided suitable data bases are available, greater

weight should be given to those tests which assess the model capabilities most closely related to

how the model is used in the modeled attainment test. Generally, additional confidence should be

attributed to model base case applications in which a variety of the tests described above are

applied and the results indicate that the model is performing well. From an operational

standpoint, EPA recommends that States/Tribes compare their evaluation results against similar

modeling exercises to ensure that the model performance approximates the quality of other

applications. To aid in this comparison, we have summarized performance metrics (including

information on models and notes on inputs) of recent ozone, PM2.5, and regional haze applications

in Appendix B.


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