Step 2. Identify the species composition for each quarter for each monitor.

These quarterly PM2.5 concentrations are then separated into their component species by

multiplying the quarterly maximum daily concentration at each site by the estimated fractional

composition of PM2.5 species, by quarter and year, based on observed species fractions for the

days > 65 ug/m3 or from the days in the high end of the distribution of PM2.5 concentrations

(using the same methodology as used in the annual average calculations54) (e.g., 30% nitrate x

70.0 ug/m3 PM2.5 = 21.0 ug/m3 nitrate).

to model the full year. But for areas that have monitored violations (or high values that are close

to the NAAQS) in all 4 seasons, the entire year should be evaluated.

53The test should be performed for each monitoring site that meets the data completeness

criteria for calculating a 98th percentile value under the 24-hr NAAQS. The highest

concentration from the other 3 quarters (less than or equal to the 98th percentile) is taken from

any values that are measured at the site. It is necessary to examine the concentrations in other

quarters which are less than the design value because PM species may have different responses

to emissions controls in different parts of the year. Basecase concentrations that are less than the

design value concentrations may end up being higher than the design value concentration in the

future (after RRFs are applied). There may not always be data available for all four quarters.

54The ambient speciation data for the 24-hour attainment test should use the same

adjustments that were recommended for the annual test. The adjusted data should be carefully

examined. Adjustments to daily concentrations of speciation data (used for the 24-hr test) may

in some cases be large compared to adjusted quarterly average data.

 

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