The cause of predicted violations of the unmonitored area analysis may not be obvious.

While careful analysis of the emissions in the area may reveal likely sources of an ozone

violation, elimination of the violation may not be simple or straightforward. Alternatively,

predicted violations of the unmonitored area analysis for PM2.5 may, in some cases be simple to

diagnose. They may occur in areas with high emissions of primary PM2.5. Elimination of the


23The metric should approximate the measured design values at monitoring sites.

Depending on the days modeled, other metrics, such as the 2nd or 3rd highest predicted ozone

values or an average of several days may be a better proxy for the design value.

predicted violation may be possible by placing controls on one or more sources. Additional

dispersion modeling may be helpful in diagnosing the source(s) of high primary PM2.5 in a

particular grid cell. The results of a more refined dispersion modeling analysis may help identify

emissions controls or locations where additional monitoring may be appropriate.

It is recommended that predicted violations of the unmonitored area analysis are carefully

scrutinized to determine whether they are likely to exist in the ambient air or whether they may

be caused by an error or uncertainty in the modeling system. At a minimum, it may be

appropriate to commit to additional deployment of ambient monitors in areas where the

unmonitored area analysis predicts future violations24. This monitoring would allow a better

assessment in the future of whether the NAAQS is being met at currently unmonitored locations.

Violations of the unmonitored area analysis should be handled on a case by case basis.

As such, additional analyses and/or tracking requirements may be needed depending on the

nature of the problem and the uncertainty associated with the potential violation(s).


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