Supplemental evidence should accompany all model attainment demonstrations.

Generally, those modeling analyses that show that attainment will be reached in the future with

some margin of safety (e.g., estimated concentrations below 82 ppb for ozone, 14.5 ug/m3 for

annual PM2.5, and 62 ug/m3 for 24-hour PM2.5) will need more limited supporting material. For

other attainment cases, in which the projected future design value is closer to the NAAQS, more

rigorous supporting analyses should be completed. As noted earlier (see section 2.2), there may

be some areas that can expect to achieve timely attainment despite failing the model attainment

test, and vice versa. This section of the guidance will discuss some specific additional analyses

that can be used to supplement the model projections. Of particular interest are analyses that

help determine whether the model likely overpredicts, underpredicts, or accurately predicts the

air quality improvement projected to occur by the attainment date. States should review these

supplemental analyses, in combination with the modeling analysis, in a “weight of evidence”

assessment of whether each area is likely to achieve timely attainment. Additional examples of

possible weight of evidence determinations are provided in existing EPA guidance (U.S. EPA,

1996a). Again, it should be noted that no single supplemental analysis can serve as an adequate

substitute for the air quality model; however, in aggregate, supplemental analyses may provide

information which may indicate a different outcome than the modeled test.

In Section 7.1, we identify several broad types of analyses which can be used to

corroborate one another as part of a set of supplemental analyses. States should utilize each of

these types of analysis to ensure that conclusions regarding adequacy of a proposed strategy are

based on a variety of independent analyses. In Section 7.2, we more specifically address the

submittal of a weight of evidence demonstration and recommend a framework for compiling and

submitting a weight of evidence demonstration.



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