In section 17 we recommend using 2002 as the baseline inventory year for the current

round of ozone, PM2.5, and Regional Haze SIPs. If States/Tribes model the full year of 2002 (or

episodes from 2002) then the base case and baseline inventory years will be the same25. But if

States/Tribes model other years or episodes or full seasons from other years, then the base case

inventories should be projected (or “backcasted”) to 2002 to provide a common starting point for

future year projections.

Alternatively, the baseline emissions year could be earlier or later than 2002, but it

should be a relatively recent year (preferably within the 5 year design value window). In order

to gain confidence in the model results, the emissions projection period should be as short as

possible. For example, projecting emissions from 2002 to 2009 (with a 2000-2004 baseline

average design value) should be less uncertain than projecting emissions from 1995 to 2009

(with a 1993-1997 baseline average design value). Use of an older baseline average design value

period is discouraged.

It is desirable to model meteorological time periods occurring during the period reflected

by the baseline design value (e.g., 2000-2004). However, modeling time periods need not be

selected from the period corresponding to the baseline design value, provided they are

representative of meteorological conditions which commonly occur when exceedances of the

ozone or PM2.5 standard occur (or represent typical annual concentrations of PM2.5) . The idea is

to use selected representative time periods to capture the sensitivity of predicted ozone or PM2.5

to changes in emissions during commonly occurring conditions. There are at least three reasons

why using time periods outside the period with the baseline design value may be acceptable: (1)

availability of air quality and meteorological data from an intensive field study, (2) the desire to


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