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

Nonattainment Area

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

Prescribed Date

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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