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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