calculated to the extent that they provide meaningful information. Wherever possible, these types

of performance measures should also be calculated for ozone precursors and related gas-phase

oxidants (NOx, NOy, CO, HNO3, H2O2, VOCs and VOC species, etc.) and ozone (and

precursors) aloft.

Thresholds: EPA recommends that the three metrics above be calculated two ways: 1)

for pairs in which the 1-hour or 8-hour observed concentrations are greater than 60 ppb46, and 2)

for all pairs (no threshold)47. This will help to focus the evaluation on the models ability to

predict NAAQS-relevant ozone and minimize the effects of the normalization. In terms of pairing

model predictions with monitored observations, EPA recommends that the grid cell value in

which the monitor resides be used for the calculations. It would also be acceptable to consider

bi-linear interpolation of model predictions to specific monitoring locations48. States/Tribes

should recognize that, even in the case of perfect model performance, model-observed residuals

are unlikely to result in exact matches due to differences between the model predictions which are

volume averages and the observations which are point values.

Averaging: The statistics should initially be calculated for individual days (averaged

over all sites) and individual sites (averaged over all days). As appropriate, States/Tribes should

then aggregate the raw statistical results into meaningful groups of subregions or subperiods. For

example, examination of model performance within and near the non-attainment area(s) of

interest may be more important than examination of performance in other parts of the modeling

domain. Similarly, priority may be placed on examination of the days that are potentially used in

the attainment test (base period days with 8-hour ozone > 70 or 85 ppb). That is not to say that

model performance evaluations should ignore performance on lower ozone days or in areas

outside of the nonattainment areas. Simply, additional factors should be considered when

prioritizing more detailed analyses.


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