From: Robert Coats/RTP/USEPA/US

To: AQS Users 1, AQS Users 2

Date: 08/02/2012 03:15 PM

Subject: Reversal of July 23, 2012 change to Ozone 8-Hour Design Value Calculations

Dear AQS Stakeholders,

In the "AQS Software Notes for July 23, 2012" we described a change in the AQS calculations for the 8hour ozone summaries. The software change meant that "8-hour ozone summaries now allow all days with at least one valid 8-hour average to be used when selecting the 4th maximum for the year." After further consideration of comments from AQS stakeholders regarding this interpretation of 40 CFR Part 50 Appendix P, and of technical and policy consistency concerns raised within EPA, we are reversing this change. Therefore, only valid days (i.e.,days with 18 or more valid 8-hour averages) will be used when selecting the 4th maximum.

AQS will return to using the original calculations for 8-hour ozone summaries on Friday, August 3, 2012 at 9:00 PM EDT, and any affected ozone Design Values will revert to their original values by Monday, August 6, 2012 at 7:00 AM EDT.

If you have questions, please respond to

Thank You,

Robert Coats

National Air Data Group OID/OAQPS/OAR/EPA

109 TW Alexander Dr. Mail Drop C339-04

Research Triangle Park, NC 27711

Phone: (919) 541-5448 Fax: (919) 685-3684