The first of these criteria concerns whether an area is being affected by overwhelming

transport and the second whether the area is significantly contributing to another nonattainment

area. Analyses for both of these criteria will involve assembling emissions, air quality,

meteorological, and/or photochemical grid modeling data; and making an informed decision

regarding contribution based on the results of the composite set of analyses. This aggregation of

data is generally referred to as “weight of evidence”10 and is discussed in detail in EPA modeling

guidance on 8-hour ozone attainment demonstrations.11 The end product of this “weight of

evidence” determination is a document which describes analyses performed, data bases used, key

assumptions and outcomes of each analysis, and why a State believes that the evidence, viewed

as a whole, supports a conclusion that the area is overwhelmingly affected by transport and does

not significantly contribute to downwind problems.

The following types of analyses are examples of elements of possible weight of evidence

determinations supporting whether an area should receive an OTA classification. It is expected

10 The term “weight of evidence” has a specific meaning in the context of an attainment demonstration. In

terms of an attainment demonstration, a weight of evidence demonstration is a set of analyses that show an area can

be expected to attain the standard in the future, despite a modeling projection that may not demonstrate that all

monitors in an area will be monitoring attainment. In the context of the overwhelming transport guidance we are

using the term “weight of evidence” in its more generic sense, that is, a series of technical analyses that when

aggregated tend to support a particular air quality conclusion. Because in many cases, the same types of ambient

analyses can support both goals – attainment demonstrations and overwhelming transport determinations – we crossreference

our modeling guidance for more information on performing a weight of evidence analysis.

11 Guidance on the Use of Models and Other Analyses in Attainment Demonstrations for the 8-Hour Ozone

NAAQS (Draft Final); USEPA, February 2004,


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