As part of their recommendations to transform the SIP process into one that is more

performance-oriented, the Clean Air Act Advisory Committee (CAAAC) recommended

increased use of weight of evidence within State/Local attainment demonstrations (AQMWG,

2005). One of the workgroup’s recommendations to EPA was that “EPA, in conjunction with

affected stakeholders, should modify its guidance to promote weight-of-evidence (WOE)

demonstrations for both planning and implementation efforts. In particular, these demonstrations

should reduce reliance on modeling data as the centerpiece for SIP/TIP planning, and should

increase use of monitoring data and analyses of monitoring data, especially for tracking progress.

Enhanced tracking and ambient monitoring data is a better use of available resources than

intensive local modeling.”

A weight of evidence (WOE) determination examines results from a diverse set of

analyses, including the outcome of the primary attainment test, and attempts to summarize the

results into an aggregate conclusion with respect to whether a chosen set of control strategies

will result in an area attaining the NAAQS by the appropriate year. The supplemental analyses

discussed above are intended to be part of a WOE determination, although the level of detail

89Few studies have been done to examine similar uncertainties for PM2.5. Based on recent

modeling analyses, a similar range of +- 2-4% of the NAAQS seems appropriate for PM2.5. That

translates to roughly 0.3-0.6 ug/m3 for the annual PM2.5 standard. Consequently, the

recommended weight of evidence range for PM2.5 is nominally +- 0.5 ug/m3.


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