Determine which area(s) should be used to characterize estimated changes in emissions.
If there are consistent, major contrasts between downwind and upwind sites, States may focus
their emission estimates on those emissions which occur within the nonattainment area. If
there is a contrast using some measures, but a less apparent or small difference using other
measures, States should focus emission estimates on the nonattainment area and on a larger
geographical area determined as described in the last paragraph in Section 2.1. If there are
only small or indiscernible differences between measures at upwind and downwind sites,
States should focus their emission estimates on a larger geographical area, determined as
described in Section 2.1.
2.3 Use Air Quality Model Estimates
In Section 2.1, we noted that air quality models may be used in a relatively non-resource
intensive fashion to determine whether the mid-course review should focus on changes in
VOC emissions, changes in NOx emissions or changes in both. In Section 2.3, we describe
other uses of model-generated results which we believe would increase the credibility of a
9If per chance States have conducted regional scale modeling for 2002, we recommend
consideration of this information, especially it’s viable use in assessing the role of transport.
Using model projections as a target against which to compare air quality observations.
Model results, obtained prior to the mid-course review, may be used to check whether
changes in air quality observed at the time of the mid-course review are consistent with likely
attainment. We suggest the following procedure. Model most recent available emissions
present during a base period (i.e., 1996 emissions9), projected emissions for the year of the
mid-course review (e.g., 2004 assuming the mid-course review is to be completed by the end
of 2004) and emissions projected for two years prior to the required attainment date (e.g.,
2005 for a 2007 attainment date). Select a modeled air quality metric to use as predicted
values corresponding to each of these three emission scenarios. For the 1-hour ozone
NAAQS, this metric would ordinarily be the highest 1-hour daily maximum ozone
concentration predicted within the non-attainment area(s) or at locations clearly impacted by
the area(s) being reviewed. Other metrics can be considered on a case by case basis.
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