emissions. The procedure is the same as the ozone example presented above, except the spatial

mean is used instead of the highest concentration near the monitor.

For the 24-hour PM2.5 NAAQS and for annual PM2.5 NAAQS sites which exhibit strong

spatial concentration gradients of primary PM2.5, we recommend calculating an RRF based on

the single value in the grid cell which contains the monitor. Particularly in areas with strong

primary PM concentration gradients, the use of a single grid cell is more likely to accurately

capture the local change in PM2.5 components.

3.4 Estimating design values at unmonitored locations: what is an unmonitored

area analysis and why is it needed?

An additional review is necessary, particularly in nonattainment areas where the ozone or

PM2.5 monitoring network just meets or minimally exceeds the size of the network required to

report data to Air Quality System (AQS). This review is intended to ensure that a control

strategy leads to reductions in ozone or PM2.5 at other locations which could have baseline (and

future) design values exceeding the NAAQS were a monitor deployed there. The test is called

an “unmonitored area analysis”. The purpose of the analysis is to use a combination of model

output and ambient data to identify areas that might exceed the NAAQS if monitors were located



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