51The top 25% of days may seem like too many days per quarter to use for a 98th

percentile based standard, but for sites with a 1 in 6 day sampling schedule meeting minimal

completeness criteria (assuming 11 samples per quarter), the top 25% of days is only 3 days per

quarter. For most sites, the top 25% of monitored days per quarter will represent between 3 and

8 days.

52In some areas it may not be necessary to model and evaluate the NAAQS for all

quarters. For example, if PM2.5 concentrations only exceed the NAAQS in the 1st and 4th

quarters, and concentrations in the 2nd and 3rd quarters are very low, then it may not be necessary

Similar to the annual PM2.5 attainment test, we recommend interpolation techniques for

FRM monitors that do not have co-located speciation data. Because the 24-hour standard is a

98th percentile based value, the component composition on high concentration days may be

highly variable from day to day and from site to site. Therefore, while interpolation techniques

may need to be used, we strongly recommend collecting speciation data at all FRM sites that

violate the 24-hour NAAQS. A precise estimate of the PM2.5 components at violating sites will

help reduce uncertainty in the future year concentration estimates.

We recommend a modeled attainment test for the 24-hour PM2.5 NAAQS with 6 steps.

Step 1. Compute observed 98th percentile values for each year and next highest

concentrations for each quarter.

The first step in projecting the daily design value is to identify the maximum daily average

PM2.5 concentration in each quarter that is less than or equal to the annual 98th percentile value

over the entire year. This results in data for each year (for five years) for each site which

contains one quarter with the 98th percentile value and three quarters with the maximum values

from each quarter which are less than or equal to the 98th percentile value53.


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