In this section, we identify several means of analysis for addressing the issues identified
in section 1.2. States should perform the set of analyses that is most appropriate based on
available data bases for the area under review.
(1) What do air quality trends (normalized for meteorological differences) at midcourse
review suggest about the likelihood of meeting the NAAQS on time?
(2) What does aerometric analysis of the role of transport vs. local emissions suggest
about the likelihood of meeting the NAAQS?
(3) Are aerometric data observed at mid-course review time consistent with modeled
projections used earlier to conclude that the most recent SIP revision is sufficient to
meet the NAAQS on time?
2.1 Use Air Quality Trends To Assess Whether A SIP Is On Track Toward Attainment
These analyses require decisions to be made about four factors to make well informed
judgments about whether timely attainment is likely: (1) choosing one or more trend
parameters, (2) adjusting (i.e., normalizing) observed ozone trends in the selected trend
parameters for meteorological differences, (3) deciding whether ozone trends are likely to be
most responsive to changes in VOC or NOx emissions or changes in both, and (4) selecting
the area for which trends in emissions will be estimated.
Choosing trend parameters. There are several things to consider when choosing trend
parameters in a mid-course review. Closeness of the parameter to the definition (i.e., form) of
the NAAQS, susceptibility to extreme or unusual meteorological conditions, stability of the
parameter, dependence on the number of sites with measured values. Table 2.1 presents
some potential trend parameters along with comments about the suitability of each. Table 2.1
is not comprehensive. Other trend parameters may be used, so long as a rationale for doing
so is presented. Since trend parameters could behave differently from one another, it is
advisable to select several parameters to get a better overall sense of how air quality is
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