The PM2.5 NAAQS rulemaking (U.S. EPA 1997a) as well as ambient monitoring rule

language56 indicates that high PM2.5 concentrations occuring in “unique” population oriented

areas should not be compared to the annual NAAQS and should only be compared to the 24-hour

NAAQS (especially when caused by a single source). Furthermore, high PM2.5 concentrations

measured in non-population oriented areas should not be compared to either of the PM2.5

NAAQS. PM2.5 monitor siting guidance (U.S. EPA, 1997b) defines the appropriate scales of

influence for the PM2.5 monitoring network.

High primary PM concentrations can occur at (or near) existing monitors or in

unmonitored areas. The modeled attainment test is primarily a monitor based test. As such, the

focus of the attainment test is whether attainment can be reached at existing monitors. To

address the issue of PM2.5 concentrations in unmonitored areas, we have recommended an

“unmonitored area analysis” (see section 3.4). The unmonitored area analysis is intended to be

the primary means for identifying high PM2.5 concentrations outside of traditionally monitored

locations. The spatial resolution of the modeling that is the underlying basis of the unmonitored

area analysis will determine how well it addresses primary PM hotspot issues. The finer the

resolution of the grid model, the more likely that primary PM hotspots will be recognized.

Based on the monitoring guidance, we believe that an unmonitored area analysis conducted at

12km or finer resolution is sufficient to address unmonitored PM2.5 for the annual NAAQS.

Conducting the unmonitored analysis at 4km or finer resolution will provide an even more

detailed analysis of the spatial gradients of primary PM2.5, especially when evaluating violations

of the 24-hr. NAAQS.


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