Methodology for the 24-Hour NAAQS

The suggested steps outlined below assume that the attainment test has been performed using the

photochemical modeling and that the quarterly DV’s have been computed as discussed in

Section 5.2. As with the annual design value, monitoring data for 5 years are used as the basis

for the projection.

(1) Determine the local primary PM2.5 sources that are thought to be contributing to the monitor

for each of the 4 quarters60.


58Additional receptors should be placed near monitors to examine predicted concentration

gradients. Receptors should only be located in areas that are appropriate for placing FRM

monitors for comparison to the annual NAAQS (see EPA, 1997b).

59RRFs should be calculated as a ratio of the base and future mean concentrations (a ratio

of the means, not a mean of the daily ratios).

60The selection of these sources should be representative of the 5-year period used to

calculate the 5 year weighted average design values. If large emissions changes have occurred

over the 5 year period then it may be necessary to determine the local contribution of identified

(2) For each of the four representative quarters, determine the primary versus secondary

components of PM2.5 at the monitor on the 98th percentile day and all high days.

(3) Identify percentage of total primary PM2.5 at monitor due to the local sources

identified in (1). Convert the percentage to an estimated concentration in ug/m3.


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