(4) Model with a dispersion model the current and future concentrations of these

local sources using actual emissions that are representative of emissions on high PM2.5 days61.

-Place receptors at and near the monitor location(s)62.

-The receptor spacing should be fine enough to get an accurate picture of the

concentration at the monitor.

-Model the base and future year.

-For both base and future year use the same 1 year of met data used for the

photochemical modeling (if available, use more than 1 year of met data).

-Calculate the 24-hour average for each day. Average the high days63 within each quarter

to determine a modeled high concentration for each quarter in the base and future year.

(5) From the modeling in (4), calculate quarterly RRFs64 for total primary PM2.5 modeled for the

selected sources.

(6) Multiply the primary PM2.5 identified in (3) by the RRFs from (5).

sources for each year. The identification of the contribution to the monitor should be most

influenced by the modeling year (the middle year of the 5 year period) since it is the year being

modeled and it has the strongest weighting in the design value calculations.

61Because the test is relative, in most cases, actual emissions should be used. The actual

emissions should be representative of emissions on high PM2.5 days (days that exceed the

NAAQS). Since the absolute predicted concentrations are not used directly, allowable emissions

may overestimate the changes in concentrations due to the identified sources. This should be

evaluated on a case-by-case basis. Allowable emissions may be appropriate if there are sources

that often emit above their typical levels on high PM2.5 days.

62Additional 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 24-hour NAAQS (see EPA, 1997b).

63High days may be all days > 65 ug/m3 or the high end of the distribution of modeled

days (e.g top 25% days)

64RRFs 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).


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