18.5.4 Retrospective Analyses

A retrospective analysis is intended to examine the ability of the model to respond to

emissions changes by comparing recent trends in observed ozone or PM2.5 concentrations to the

model-predicted trend over the same time period. The approach is a direct assessment of what we

are most interested in---does the model accurately predict changes in air quality as a result of

changes in emissions? As part of this analysis the model is run for current episodes or time

periods and episodes in one or more historical time periods using the emissions and

meteorological inputs appropriate for each time period modeled. While retrospective analyses

may be useful, it may be difficult to obtain meteorological and emissions inputs for the historical

time period(s) that are calculated using techniques and assumptions which are consistent with the

calculation of these same inputs for the current time period. Using inconsistent inputs will

confound the interpretation of the predicted trend.

Because differences in meteorology between years can confound the apparent change in

pollutants, EPA recommends that, in most cases, retrospective analyses use constant meteorology

and backcasted emissions. The model should respond in a predictable way if the emissions

changes are large enough. Hence, backcasting an emissions change of only a few percent would

not be viable. However, if NOx or SO2 emissions have been reduced by a large percentage (e.g.

30% or more), then that may be a good test of the response of the model. In Section 8, we noted

that a retrospective analysis can be a useful tool for diagnosing why an area has not attained the

NAAQS. Therefore, retrospective analyses are one of the few tools that can be used to determine


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