-and passively collected mass

To apply the test, States must first have run an air quality model at least twice--to

simulate current emissions and to simulate future year emissions. We recommend a modeled

attainment test which has 4 basic steps. Additional details are provided later in this section.

Step 1. Compute observed quarterly mean PM2.5 and quarterly mean composition for each


Derive current quarterly mean concentrations37 for each of the major components of PM2.5. This

is done by multiplying the monitored quarterly mean concentration of Federal Reference Method

37Concentrations should be calculated based on calendar quarters for two reasons. First,

the NAAQS is calculated for a calendar year, so the quarters need to fit evenly within a year.

Second, the monitored data used to calculate design values is averaged on a calendar quarter

basis before calculating annual averages.

(FRM) (EPA, 1997a) derived PM2.5 by the monitored fractional composition of PM2.5 species for

each quarter. (e.g., 20% sulfate x 15.0 ug/m3 PM2.5 = 3.0 ug/m3 sulfate).

Step 2. Using model results, derive component-specific relative response factors (RRF) at

each monitor for each quarter.

For each quarter, apply an air quality model to estimate current and future concentrations for

each of the components of PM2.5. Take the ratio of future to current predictions for each

component. The result is a component-specific relative response factor (RRF).

The relative response factor for component j at a site i is given by the following expression:

(RRF)ij = ([Cj, projected]/[Cj, current])i

where Cj, current is the quarterly mean concentration predicted at or near the monitoring site

with emissions characteristic of the period used to calculate the baseline design value for

annual PM2.5.

Cj, projected is the future year quarterly mean concentration predicted at or near the

monitoring site.

(e.g., given model predicted base year sulfate of 10.0 ug/m3 and future year concentration of 8.0

ug/m3, then RRF for sulfate is 0.800).

Step 3. Apply the component specific RRFs


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