It should be noted that EPA is aware that transport of ozone and ozone precursors has
contributed, and is contributing, to ozone nonattainment in many areas. Recent assessments of
regional ozone control approaches1, 2 have concluded that a nitrogen oxides (NOx) control
strategy would be most effective for reducing regional scale ozone and ozone transport. To that
end, EPA has promulgated several rules (e.g., NOx SIP Call3, Tier-2/Low Sulfur Gasoline
Vehicle4, Heavy Duty Diesel Engine5, the Clean Air NonRoad Diesel rule6, and the Clean Air
1OTAG Final Report; Ozone Transport Assessment Group, 1997.
2An Assessment of Tropospheric Ozone Pollution - A North American Perspective; NARSTO, July 2000.
3Finding of Significant Contribution and Rulemaking for Certain States in the Ozone Transport
Assessment Group Region for Purposes of Reducing Regional Transport of Ozone; Rule; USEPA, October 1998.
4Control of Air Pollution From New Motor Vehicles: Tier 2 Motor Vehicle Emissions Standards and
Gasoline Sulfur Control Requirements; Final Rule; USEPA, February 2000.
5Control of Emissions of Air Pollution from 2004 and Later Model Year Heavy-duty Highway Engines and
Vehicles; Final Rule; USEPA, October 2000.
6Control of Emissions of Air Pollution From Nonroad Diesel Engines and Fuel; Final Rule, USEPA, May
7Clean Air Interstate Rule, Final Rule March 10, 2005, USEPA.
8The Ozone Report: Measuring Progress through 2003; USEPA, April 2004.
9Paragraph (a)(1) indicates an OTA must meet the criteria as specified for rural transport areas under
section 182(h) of the CAA; however, EPA is proposing to revise this to allow for an area that meets the section
182(h) criteria--except it is adjacent to a county or counties in an adjacent C/MSA, where the adjacent
county/counties is/are designated attainment.
Interstate Rule7) over the past decade, that are designed, in part, to lower regional emissions of
NOx. Over time, these rules are expected to reduce the magnitude and the geographic extent of
the nation’s 8-hour ozone problem. There is already some evidence8 that improvements in ozone
air quality over the eastern United States since the mid-1990s have coincided with continued
decreases of regional NOx emissions, in conjunction with local VOC control programs.
It is expected that there will be subpart 1 ozone areas affected by transport that will not
be eligible for the overwhelming transport area (OTA) classification because they do not meet
the definition of a rural area in 40 CFR 51.904(a)(1).9 However, EPA restricted the OTA
classification to rural areas because these areas will generally not have significant sources of
emissions to control and therefore are not likely to contribute much to their own nonattainment
problem. Areas that are not rural, even if they are affected to a significant degree by transport,
can generally be shown to contribute to their own and to other areas’ nonattainment problems.
Upon request by the State, EPA would conduct notice-and-comment rulemaking to
classify an area as an OTA. The EPA will propose that an area will be classified as an OTA if it
is rural, demonstrates that the nonattainment problem in the area is due to overwhelming
transport, and shows the area does not significantly contribute to ozone in other areas. This
guidance document outlines EPA's recommended approach for demonstrating overwhelming
transport and lack of significant contribution.
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