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1

 OTAG Final Report; Ozone Transport Assessment Group, 1997.

2 An Assessment of Tropospheric Ozone Pollution - A North American Perspective; NARSTO, July 2000.

3 Finding 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.

4 Control of Air Pollution From New Motor Vehicles: Tier 2 Motor Vehicle Emissions Standards and

Gasoline Sulfur Control Requirements; Final Rule; USEPA, February 2000.

5

 Control of Emissions of Air Pollution from 2004 and Later Model Year Heavy-duty Highway Engines and

Vehicles; Final Rule; USEPA, October 2000.

6 Control of Emissions of Air Pollution From Nonroad Diesel Engines and Fuel; Final Rule, USEPA, May

2004.

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Criteria For Assessing Whether an Ozone Nonattainment Area is Affected by

Overwhelming Transport

1.0 Introduction

Section 172(a)(1) of the Clean Air Act provides that EPA has the discretion to classify

ozone nonattainment areas that are not subject to other classification provisions of the Act (e.g.,

section 182(a)(1)). In the Final Rule to Implement the 8-hour Ozone National Ambient Air

Quality Standard (NAAQS) - Phase 1 (Phase 1 Rule), EPA provided that it would not classify

subpart 1 areas, with one exception. EPA created an overwhelming transport classification for

rural nonattainment areas that are not subject to classification under subpart 2 (i.e., subpart 1

ozone areas) and whose ozone problem is the result of transport of ozone and ozone precursors. 

We limited the availability of the classification to certain rural areas that demonstrate that

transport of ozone and/or precursors into the area is so overwhelming that the contribution of

local emissions to observed 8-hour ozone concentration above the level of the NAAQS is

relatively minor and that emissions within the area do not significantly contribute to ozone

concentrations measured in other areas (40 CFR 51.904).

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
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7

 Clean Air Interstate Rule, Final Rule March 10, 2005, USEPA.

8 The Ozone Report: Measuring Progress through 2003; USEPA, April 2004.

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Paragraph (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.

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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.

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
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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, http://www.epa.gov/scram001/guidance/guide/draft-final-o3.pdf 

.

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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.

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
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12 Draxler, R.R. and Rolph, G.D., 2003. HYSPLIT (HYbrid Single-Particle Lagrangian Integrated

Trajectory) Model access via NOAA ARL READY Website (http://www.arl.noaa.gov/ready/hysplit4.html). NOAA

Air Resources Laboratory, Silver Spring, MD.

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that an area petitioning for an OTA classification would complete, and submit to EPA, a full

analysis consisting of evidence from multiple forms of weight of evidence analyses as described

within this guidance. For an area to be classified as an OTA, the large majority of the tests

would have to demonstrate the area meets the criteria in 51.904(a)(2) and (3).

2.1 Ambient Data / Meteorological Analyses

There are numerous types of ambient data analyses that can be completed that show

whether an area is being overwhelmingly influenced by ozone generated outside of the local area

and whether local emissions make a significant contribution to high downwind ozone

concentrations. The following are a subset of possible analyses that States should use in

analyzing whether an overwhelming transport classification is appropriate for an area.

2.1.1 Trajectory analyses and wind roses: One useful analysis to assess the extent of

transport into a given nonattainment region is compiling back trajectories for those days upon

which the ambient NAAQS is exceeded in an area and determining the origin and transport path

of the air masses that result in violations. If a large percentage of days with ozone violations is

associated with air parcel trajectories from areas outside of the area in question, then it may be

concluded that transport is a significant contributor to a large percentage of the violations. 

Shorter-path trajectories (i.e., those that originate in or near the area in question) may indicate a

greater role from local/nearby sources, however long-path trajectories (i.e., those that originate

long distances away from the potential OTA) by themselves are not sufficient evidence for an

overwhelming transport classification. One possible tool for use in calculating back trajectories

for days on which exceedance-level ozone was observed locally is the HYSPLIT12 (HYbrid

Single-Particle Lagrangian Integrated Trajectory) model. It is recommended that 24-, 48-, and

72-hour backward trajectories be calculated for any days within the past three years in which

observed 8-hour ozone concentrations were greater than 85 ppb. Another way to look for

overwhelming transport would involve the use of pollution wind roses. These combine wind

speed and wind direction with observed ozone concentrations. The ambient data can be

stratified by concentration levels and used with regional emissions analyses, discussed in the

next section, to assess transport effects.

To assess the 51.904(a)(3) criteria, States should identify potential downwind areas that

may be affected by the local area in question and perform similar analyses from the perspective

of the downwind area.

It should be noted that there are some limitations associated with the use of trajectories

and wind roses. In particular, it is not possible to apportion contribution to an area’s ozone
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13 Backward trajectories estimate the route that an air parcel took to get to the location in which high ozone

was observed. These trajectories use gridded meteorological data to determine the parcel's trajectory. Because of

the difficulty in representing atmospheric variables, which are continuous in space and time, by discrete gridded data

points, these trajectories should be viewed as uncertain. This uncertainty is difficult to quantify, but 24-72 hour

backward trajectories have potential errors in the hundreds of kilometers. Further, just because an trajectory passed

over a populated area, does not necessarily mean that emissions from that area were loaded into the eventual highozone parcel. The height of the trajectory along the path as it relates to the mixed layer can help indicate whether

upwind emissions loading takes place. 

14 http://www.tceq.state.tx.us/policy/ta/am/TexAQS_II.html

15 http://www.ladco.org/index.html

16 Blanchard, C.L.; Lurmann, F.W.; Roth, P.M.; Jeffries, H.E.; Korc, M. 1999. The use of ambient

data to corroborate analyses of ozone control strategies. Atmos. Environ. 33, 369-381.

17 Trainer M., Parrish D.D., Buhr M.P., Norton R.B., Fehsenfeld F.C., Anlauf K.G., Bottenheim J.W.,

Tang Y.Z., Wiebe H.A., Roberts J.M., Tanner R.L., Newman L., Bowersox V.C., Meagher J.F., Olszyna K.J.,

Rodgers M.O., Wang T., Berresheim H., Demerjian K.L., and Roychowdhury U.K. (1993) Correlation of ozone with

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problem from other regions solely through the use of trajectories.13 However, if other analyses

confirm the importance of upwind emissions, trajectories can be useful in determining which

upwind areas, in particular, are contributing to the ozone problem in the OTA. 

2.1.2 Ozone and Ozone Precursor Data: At the simplest level, depending on data

availability, it may be possible in some cases to observe exceedance-levels of 8-hour ozone

being transported into the region of interest. Animations of observed ozone over specific regions

(e.g., as shown on the EPA AIRNow site) may indicate plumes of ozone being generated in

urban areas, then advecting in steady-state over rural areas. It may also be possible to use

animations from fully-evaluated photochemical grid models to detect transport into a particular

region. A more data-intensive approach would be to demonstrate transport by compiling recent

aircraft observations along an upwind boundary. Ozone transport predominantly takes place not

at the surface, but within the entire planetary boundary layer (PBL) during the daytime, and in

the residual layer that exists at night when the surface winds become decoupled from aloft winds

due to the formation of a nocturnal temperature inversion. If aircraft data are available along the

upwind boundaries of an area thought to be affected by significant transport, and those data show

that incoming ozone is equivalent to, or higher than, ozone observed at the surface on days with

exceedances of the NAAQS, that could be construed as evidence of transport. Additionally,

field study analyses such as those completed by the Texas Council of Environmental Quality

(TCEQ) in Houston14 and by the Lake Michigan Air Directors Consortium (LADCO) in the Lake

Michigan region15 can provide the necessary data to quantify the influences of transport on an

area. Finally, it may also be possible through detailed analyses of ozone precursor data to make

some determination about the chemistry of a given airmass that indicates whether the ozone was

formed locally or elsewhere and then advected to the local area. Several studies16,17 have
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NOy in photochemically aged air. J. Geophys. Res. 98, 2917-2925.

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demonstrated cases where ozone exceedances were correlated with air masses in which most of

the chemistry has already taken place, perhaps supporting, in conjunction with other factors, a

determination of overwhelming transport.

2.1.3 Satellite analyses: Recent advances in satellite imagery have made it possible to

track total aerosol in the atmosphere on a periodic basis. For those ozone events that are shown

to be contemporaneous with elevated levels of fine particulate matter, it may be possible to track

the evolution of an ozone event back to a particular source region. In the future, it may be

possible to extract “surface” level ozone from the satellite measurements directly.

2.2 Emissions Analyses

The Phase 1 Rule makes it clear that the local emissions in the area are a key

consideration in determining if an overwhelming transport classification is warranted. If the

NOx and VOC inventories for a particular area are much less than those for other areas for which

there is evidence demonstrating contribution to the ozone nonattainment problem, this provides

support that the transport component is overwhelming the local component of ozone formation.

One approach to assessing the potential importance of local emissions is to compile

county-level emissions inventory estimates for each county over a broad region around the area

being considered. The regional emissions analyses should include 1) the areas affecting the

potential OTA and 2) areas being affected by the potential OTA. These results should be paired

with the information gleaned from the analyses described in Section 2.1. If the emissions from

contributing upwind counties are much larger than what is being emitted locally, this provides

support that the impact of the local emissions may not be significant. EPA recommends that

these emissions inventories should be built using the most current, accurate, and practical

methods available. Several references are available for guidance on building emission

inventories. The first is the “Emissions Inventory Guidance for Implementation of Ozone and

Particulate Matter NAAQS and Regional Haze Regulations” (USEPA, 2005). Additionally,

modelers may also want to consider EPA’s approaches for developing the 2002 National

Emissions Inventory (NEI) to guide the development of the emissions data.

2.3 Photochemical Grid Modeling Analyses

Photochemical grid models (PGMs) are powerful tools to assess the impacts of emissions on air

quality over a particular domain of interest. However, in many cases PGM-based analyses can

be resource-intensive to establish and complete. Thus, in light of other available analyses to
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18 ENVIRON, 2002. User's Guide for a Comprehensive Air Quality Model with Extensions (CAMx).

ENVIRON Corp, Novato, CA 94945. Also see, http://www.camx.com.

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confirm whether an area is affected by transport, EPA does not believe such modeling would be

necessary to support all such petitions. However, EPA is encouraging the use of modeling as an

element of the weight of evidence to support an OTA petition, where such use is feasible and/or

necessary.

2.3.1 Source apportionment: One modeling tool that can be used to evaluate the

downwind contributions of emissions in upwind States is source apportionment. The source

apportionment technique was developed to provide a means of estimating the contributions of

many different source areas/categories to ozone formation in a single model run. This is

achieved by using multiple tracer species to track the fate of ozone precursor emissions (VOC

and NOx) and the ozone formation caused by these emissions within a simulation. The

methodology is designed so that all ozone and precursor concentrations are attributed to the

selected source areas/categories at all time steps. Thus, for all receptor locations and times, the

ozone concentrations predicted by the model are attributed to various source areas/categories

selected for analysis. Additional information on the source apportionment technique can be

found in the CAMx User’s Guide.18 Results that indicate large contributions from sources

outside the local area could support a determination of overwhelming transport.

2.3.2 Sensitivity analyses / Zero out modeling: The zero-out modeling technique provides

another technical approach to quantifying the downwind impact of emissions in upwind States. 

The zero-out modeling provides an estimate of downwind impacts by calculating the difference

between the model estimates from a base case run and the estimates from a simulation in which

the base case man-made emissions of NOx and VOC are removed from a specific area. This

approach is useful for showing both the effects of transported ozone on a particular area and the

effects of the emissions within a local area on ozone locally and downwind. If the response of

the model is small (i.e., even with zero local emissions, there is still a local ozone problem due to

transport), when emissions from the OTA are removed, it would support a determination that

local ozone is strongly influenced by transport.

Additional modeling-based tools beyond those mentioned here may also be used if they

can be shown to represent sound approaches to assessing the origin and transport of ozone and

its precursors.

3.0 Summary

Most subpart 1 ozone areas are not subject to classification. However, an overwhelming

transport classification is available for rural nonattainment areas whose ozone problem is the

result of transport of ozone and ozone precursors and which do not have emissions that
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significantly contribute to ozone concentrations in other areas. The EPA will propose that an

area will be classified as an overwhelming transport area if:

(1) it meets the definition of a rural area under 40 CFR 51. 904(a)(1),

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

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

NAAQS is relatively minor; and

(3) it is determined that emissions within the area do not make a significant contribution

to the ozone concentrations measured in the area or in other areas.

In general, EPA believes the rural limitations will restrict the number of areas eligible for

such classification. This guidance document outlines the types of analyses available for those

subpart 1 areas that meet the location test in 51.904(a)(1) to compile a sufficient body of

evidence to demonstrate whether the area meets the other two components of the test for the

OTA classification. States requesting this classification for a subpart 1 ozone area are

encouraged to conduct the technical analyses discussed in Section 2, as well as any other

relevant technical analyses. Documentation which describes each analysis performed (e.g., data

bases used, modeling methodology, results, etc.) and explains how strongly it supports a

determination of overwhelming transport should be submitted to the appropriate EPA regional

office. States/Tribes seeking the OTA classification are encouraged to work closely with the

appropriate Regional Office to develop a plan for performing the analyses for such a

demonstration. Obtaining early review of approaches being applied can help assure efficient use

of resources.