As discussed in Section 2, EPA has determined that the best approach to using models to

demonstrate attainment of the NAAQS is to use a model in a relative mode. However, for some

model applications there may be strong evidence from the performance evaluation that the

model is able to reproduce detailed observed data bases with relatively little error or bias.

Particularly for cases such as these, some types of “absolute” modeling results may be used to

assess general progress towards attainment from the baseline inventory to the projected future

inventory86. There are several metrics that can be considered as part of this type of additional


! Percent change in total amount of ozone or PM2.5 >= NAAQS87 within the nonattainment


! Percent change in number of grid cells >= NAAQS within the nonattainment area

! Percent change in grid cell-hours (days) >= NAAQS within the nonattainment area

! Percent change in maximum modeled 8-hour ozone within the nonattainment area

86 Care should be taken in interpreting absolute metrics if the model evaluation shows a

large underprediction or overprediction of ozone or PM2.5 concentrations. An underprediction of

observed concentrations will make it artificially easy to show progress towards absolute

attainment levels and an overprediction will make it artificially difficult to show progress

towards attainment.

87For each of these metrics, the appropriate comparison to the level of the NAAQS is 85

ppb for 8-hour ozone; 65 ug/m3 for 24-hour PM2.5; and 15 ug/m3 for annual PM2.5.


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