3.0 Using Weight Of Evidence To Estimate If A SIP Is On Track
In this Section, we describe how to combine information from the previous analyses to
determine if the weight of available air quality monitoring, meteorological, emissions and
modeling evidence supports a hypothesis that a SIP is on track toward attainment.
3.1 Compute Normalized (For Meteorology) Trend For Ozone Within And Nearby A
10We recognize that the design value in 2004 may not accurately reflect current air quality at
that time. The design value at a monitoring site is ordinarily the 4th highest daily maximum concentration
observed over a continuous 3-year period. However, if a major change in emissions occurs during this
3-year period (e.g., in Spring, 2004) the 4th highest value may well be observed before these changes
(e.g., in 2002 or 2003). If none of the observed top 4 daily maximum ozone concentrations at a
monitor occurs in 2004, we also recommend that States compare the observed 2nd high daily
maximum concentration observed in 2004 with the mid-course review target, and present supporting
information that 2004 is not an unusually low year due to benign meteorological conditions.
Select appropriate ozone trend parameters. If available, use robust measures of ambient
precursor trends to help confirm emissions reductions. Normalize ozone trends for
meteorological differences using a curve fitting approach like the one described in Section 2.1
or other approaches like the Cox, et al. (1993), CART or filtering approach described by
Milanchus, et al. (1998).
3.2 Compare The Observed Trend With A Target Consistent With Attainment By The
This comparison may be performed in several ways.
Comparison with photochemical grid model estimates
For example the first way may be to compare the monitored observations with modeled
estimates previously obtained for a base period, mid-course and attainment date. This
method is illustrated in Section 2.3. If a fitted curve and residuals are calculated, actual
observations may be compared with model estimates, as described. If another method is used
in which residuals are not estimated, the normalized, rather than actual observations should
be compared with the target. Use of model results for this purpose is advantageous, because
it does not require a State to make a number of explicit assumptions about which emissions
trends to consider or to make assumptions about the role of transport. The disadvantages are
resources needed to do the modeling and the likelihood that the model results will be based on
a relatively limited sample (i.e., few days).
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