1. Pinal County Air Quality October 27, 2008 PM 10 Exceptional/Natural Event

2. October 27, 2008 Winds associated with abnormally tight pressure gradient caused by high pressure and a back door cold front to the east. The tight pressure gradient caused significant areas of blowing dust. The Maximum winds reached 28 mph and three PCAQCD monitors exceeded the 24-hr PM 10 NAAQS. PM 10 24-hr SiteAQS Site# Conc. (ug/m 3 ) –Stanfield04-021-3008(3) 212.6 –Stanfield04-021-3008(1) 201.0 –Maricopa04-021-3010(3) 159.4

3. Strong high pressure & “back door” cold front Result in a strong pressure gradient

12. NOTE: Stanfield had 2 invalid hours at 1 and 2 pm.

14. Casa Grande Airport NWS Observations Sustained wind speed/gust & Visibility

15. 10/27/08 “But for” Test Stanfield (TEOM) had the following 24-hr PM 10 concentrations before/after the 10/27/08 exceptional/natural event –10/26/08 – 86.2 ug/m 3 –10/28/08 – 87.8 ug/m 3 Maricopa (TEOM) had the following 24-hr PM 10 concentrations before/after the 10/27/08 exceptional/natural event –10/26/08 – 84.0 ug/m 3 –10/28/08 – 72.6 ug/m 3