5.2 What Is The Recommended Modeled Attainment Test For The 24-Hour


Our recommended modeled attainment test for the 24-hour NAAQS for PM2.5 is similar

to the previously described test for the annual NAAQS in that it uses model predictions in a

relative sense to reduce site-specific observations (averaged over 5 years). In the test, we are

interested in reducing the current design values at each site to < 65 :g/m3 49,50.


48In the final step, the future year concentration should be rounded to the tenths digit. A

(rounded) value of 15.0 ug/m3 meets the NAAQS. A value of 15.1 ug/m3 or greater violates the


49PM2.5 nonattainment areas are not specifically designated for the annual or 24-hour

standard. Therefore, the attainment demonstration must show that the area will meet both forms

of the standard. Areas that only violate the annual standard and whose design value is well

below the 24-hr standard may not need to apply the 24-hour modeled attainment test.

Attainment test requirements should be discussed with the appropriate EPA Regional Office.

50The PM2.5 24-hour standard was lowered to 35 ug/m3 in 2006 (71 FR 61224) (40 CFR

50.13) . Planning requirements for areas that violated the old standard are unchanged. We

expect the attainment test for the new standard to be the same or similar to the procedures in this

version of the modeling guidance. As part of the implementation process for the revised

standard, the procedures in the guidance will be re-evaluated. If necessary, we will release a

revised modeling guidance document that addresses any changes to the 24-hour test for the new


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