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

important.

 

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