5.3.1 Analysis of Primary PM2.5 Impacts at Monitors

Most PM2.5 monitors are located at “urban scale” sites. These sites tend to represent

relatively large spatial scales and do not have large gradients compared to other monitors in the

area. Some sites are classified as “neighborhood” scale and may be influenced by more local

sources. For reasons stated above, local influences creating spatial gradients are likely to consist

mostly of primary PM2.5 (OC, EC, and other primary particulates). These sources may be local

point sources, or they may be nearby roads or other mobile or area sources.

It may be necessary to evaluate the impact of local primary PM sources for contributions

to both the 24-hour and annual NAAQS. As stated earlier, it may not be appropriate to compare

population oriented FRM sites that are dominated by point sources, to the annual NAAQS. But

there are numerous cases where the impact from local sources is not dominant, but a sizable

contributor to total PM2.5 (~10-30% of total annual average PM2.5). In these cases, a more

refined analysis of the contribution of local primary PM2.5 sources to PM2.5 at the monitor(s) will

help explain the causes of nonattainment at the monitor and may lead to the more efficient ways

to attain the standard by controlling emissions from local sources.


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