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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