The purpose of this document is to provide guidance to EPA Regional, State, and Tribal

air quality management authorities and the general public with respect to 40 CFR 51.904(a)(2)

and (3). The guidance may not apply to a particular situation, depending upon the

circumstances. Any decisions by EPA regarding a particular overwhelming transport

demonstration will only be made following notice and opportunity for public review and

comment. Therefore, interested parties are free to raise questions and objections about the

appropriateness of the application of this guidance to a particular situation; EPA will consider

whether or not the guidelines set forth in this document are appropriate in that situation. This

guidance is a living document and may be revised periodically without public notice. The EPA

welcomes public comments on this document at any time and will consider those comments in

any future revisions of this guidance document. Readers of this document are cautioned not to

regard statements recommending the use of certain procedures or defaults as either precluding

other procedures or information or providing guarantees that using these procedures or defaults


DRAFT 6-29-05

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,



will result in actions that are fully approvable.

2.0 Criteria for assessing overwhelming transport

Assuming a subpart 1 ozone area meets the requirement of section 51.904(a)(1)

demonstrating that it is rural, there are two additional criteria that must be met under

51.904(a)(2) and (3) for the area to be classified as an OTA:

51.904(a)(2): transport of ozone and/or precursors into the area is so overwhelming that

the contribution of local emissions to observed 8-hour ozone concentrations above the level of

the NAAQS in the area is relatively minor; and

51.904(a)(3): emissions within the area must not make a significant contribution to the

ozone concentrations in other downwind areas.


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