applicable operating or construction permit for that source. We define “actual emissions”as the
best estimate of the actual emissions that occurred during the baseline period being modeled41.
Modeling with allowable emissions is sometimes warranted. For example, for permit
modeling, we generally compare the absolute predicted modeled concentrations against the
NAAQS or the PSD increments. In these cases we want the modeling to be conservative. That
is, we want to be sure that a maximum permitted level of emissions cannot cause of violation of
the NAAQS or PSD increment. Therefore, in the case of permit modeling, it is sometimes
appropriate to model with allowable emissions. But for a local area analysis, we are trying to
determine the actual (or typical) contribution from sources to a monitor and we are using the
model results in a relative sense. Therefore, it is only appropriate (and in fact conservative) to use
actual emissions. Using actual emissions should lead to a reasonable estimate of of air quality
improvements from reducing emissions at the flagged sources. Using allowable emissions might
lead to an overestimate of benefits from emissions controls at flagged sources.
41In the emissions baseline modeling, the actual emissions may sometimes be replaced
with “typical” emissions (e.g. EGU and fire emissions).
18.0 What are the Procedures for Evaluating Model Performance and
What is the Role of Diagnostic Analyses?
The results of a model performance evaluation should be considered prior to using
modeling to support an attainment demonstration or regional haze assessment. The performance
of an air quality model can be evaluated in two ways: (1) how well is the model able to replicate
observed concentrations of PM2.5 components, ozone and/or precursors (surface and aloft), and
(2) how accurate is the model in characterizing the sensitivity of ozone and/or PM2.5 to changes in
emissions? The first type of evaluation can be broadly classified as an "operational evaluation"
while the second type of evaluation can be classified as a "diagnostic evaluation". The modeled
attainment tests recommended in Sections 3-6 use models to predict the response of ozone and
PM2.5 to changes in emissions and then applies the resulting relative response factors to observed
(rather than modeled) ambient data. Thus, while historically, most of the effort has focused on
the operational evaluation, the relative attainment test makes the diagnostic evaluation even more
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