component, elemental carbon (EC), and the general category of “other primary particulate

matter” (OPP). In some cases, directly emitted sulfate (and in rare cases nitrate) may also be a

significant component of the local primary PM2.5.

The dispersion modeling results should be evaluated to ensure adequate model

performance. Similar to grid modeling, the dispersion model results should be compared to

ambient data to ensure that the model is working well. Although section 18 of this guidance is

geared towards evaluating grid models, many of the same statistical calculations can be made for

primary PM2.5 and PM2.5 components predicted by a Gaussian dispersion model. Since

secondary PM2.5 is often a large component of total PM2.5 concentrations, it may be difficult to

separate the primary and secondary components of ambient PM2.5. EC and OPP should be

considered to be primary PM2.5. Much of the rest of the primary PM2.5 concentration will be

primary OC.

As part of the analysis, an estimated concentration of primary OC is needed. There are

several methods available for estimating the primary vs. secondary portion of ambient OC.

Among these are the EC tracer method and receptor modeling. The EC tracer method is the

most common method used to estimate secondary and primary OC concentrations (Turpin,

1995), (Strader, 1999) (Cabada, 2004), (Chu, 2005), (Saylor, 2006) . The method uses

measurements of OC and EC and calculated OC to EC ratios to identify periods when OC is

likely to be mostly primary. This information is then used to calculate the secondary

contribution to OC . Receptor models such as CMB and PMF have also been used to estimate

secondary organic concentrations (Na, 2004), (Yuan, 2006).

The following sections discuss a suggested methodology for examining decreases or

increases in PM concentrations due to local primary sources for the Annual and 24-hour PM2.5

NAAQS. Because each nonattainment area has unique emissions sources and source-receptor

relationships, States should work closely with their EPA Regional Office in developing local

area analysis applications.

 

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