DRAFT 6-29-05

NOy in photochemically aged air. J. Geophys. Res. 98, 2917-2925.


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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