5.3.2 Analysis of Primary PM2.5 Impacts at Monitors using a Gaussian

Dispersion Model

To apply a dispersion model in an attainment test, it becomes important to determine the

local component of primary PM2.5 at the monitor and the sources that are contributing to that

component. There is no single, simple method for quantifying this contribution, but detailed

analysis of ambient data and advanced data analysis techniques, such as receptor modeling, may

help quantify the contribution. The simplest method for identifying the local component of

PM2.5 is to examine local ambient PM2.5 concentrations. For this analysis, it is important to

identify the local contributions from as small an area as possible. This will make it easier to

identify contributing sources. It is most appropriate to examine monitored concentration

differences between urban monitors (with the highest concentrations) and more suburban

measurements. This is likely to be representative of the excess contribution from a relatively

local area. “Urban excess” calculations which pair an urban monitor with a rural background

monitor (U.S. EPA, 2004b) are more likely to indicate “local” contributions that may be more

representative of an entire metropolitan area. In most cases, the local component will include

contributions from more than one source.


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