1. Pinal County Air Quality October 29, 2007 PM 10 Exceptional Event

2. October 29, 2007 Winds associated with a surface gradient caused by a strong area of high pressure to the north of Pinal County on October 29th. Max winds reached 22 mph which caused the Maricopa monitor to exceed the 24-hr PM 10 NAAQS. Site24-hr PM 10 Conc. (ug/m 3 ) Maricopa156.2

8. 10/29/07 “But for” Test Had it not been “but for” the winds associated with the strong pressure gradient which caused Maricopa to have a 1-hr PM 10 concentration of 313 ug/m 3 at 1 p.m., Maricopa would have NOT exceeded the PM 10 24-hr NAAQS. –If that elevated hour was replaced with the average of the other hours then the 24-hr PM 10 concentration would be 149 ug/m 3

9. Additional 10/29/07 “But for” Test Maricopa had the following 24-hr PM 10 concentrations before/after the 10/29/07 event –10/28/07 – 104.0 ug/m 3 –10/30/07 – 103.3 ug/m 3

10. Questions/Comments Contact person at PCAQCD –Scott DiBiase – Planning Manager (520)866-6969 Mailing address: 31 N. Pinal St. P.O. Box 987 Florence, AZ. 85232