Model assessment and verification - Air Pollution Modeling and Its Application - page 373

Applying an ozone threshold strongly impacts the relative importance of the
regional emissions. A large portion of the grid cells modeled to exceed the ozone
thresholds are within the Atlanta region. Thus, a concentration-response metric
that includes a threshold would place much greater importance on reducing Atlanta
emissions. Although strong regional differences appear when thresholds are applied,
source categories within a region (e.g., mobile and non-EGU point sources in
Atlanta) continue to have nearly identical per-ton impacts.
Integrating the sensitivity impacts over population (Year 2000 Census) further
accentuates the regional differences while maintaining parity of source categories
within a region. The Atlanta region has the highest population density in Georgia,
so Atlanta emissions have the greatest per-ton impact on a population-weighted
basis. Note that whereas Atlanta and Macon NO x have roughly equal per-ton
impacts on spatially-averaged, no-threshold bases, they differ by an order of
magnitude when both a threshold and population-weighting are considered.
4. Conclusions
For the scenario considered here, the temporal duration of a concentration-response
metric would likely have little bearing on the prioritization of control strategies for
SIP development. Application of a threshold or population weighting enhances the
importance of controls in the most polluted and populated regions, but does not
substantially affect prioritization within a region.
This scenario was somewhat unusual in that ozone was almost entirely NO x -
limited and insensitive to VOC. It is possible that temporal metrics could play a
bigger role in ranking NO x and VOC control measures in more transitional
regions. The sensitivity of ozone to NO x typically turns strongly negative at night,
so 24-h metrics would likely enhance the importance of VOCs relative to NO x .
Acknowledgments Funding for this research was provided by EPA STAR Grant R833665.
Photochemical model inputs were provided by the Georgia Environmental Protection Division.
Neither U.S. EPA nor GA EPD has reviewed this manuscript.
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