1. Pinal County Air Quality September 11, 2008 PM 10 Natural/Exceptional Event

2. September 11, 2008 Strong outflow winds associated with monsoon storm development caused significant areas of blowing dust. The Maximum winds reached 43 mph and five PCAQCD monitor exceeded the 24-hr PM 10 NAAQS. PM 10 24-hr SiteAQS Site# Conc. (ug/m 3 ) –PCH 04-021-3011(3) 217.1 –Casa Grande04-021-0001(3) 202.5 –Combs04-021-3009(3) 194.7 –Stanfield 04-021-3008(3) 178.2 –Maricopa04-021-3010(3) 170.4

12. Start of wall of dust formation

13. Wall of dust moving East

14. Wall of dust moving East

15. Wall of dust moving East

16. Wall of dust moving East

17. Wall of dust moving East

18. Wall of dust moving East

19. Wall of dust moving East

20. Wall of dust moving East

21. Wall of dust moving East

22. Wall of dust moving East

23. Wall of dust moving East

24. Wall of dust moving East

25. Wall of dust moving East

26. Wall of dust moving East

27. Wall of dust moving East

33. 9/11/08 “But for” Test PCH (TEOM) had the following 24-hr PM 10 concentrations before/after the 9/11/08 natural/exceptional event –9/10/08 – 33.6 ug/m 3 –9/12/08 – 55.9 ug/m 3 Casa Grande –9/10/08 - 25.4 ug/m 3 –9/12/08 – 49.0 ug/m 3 Combs –9/10/08 - 79.5 ug/m 3 –9/12/08 – 75.3 ug/m 3 Stanfield –9/10/08 - 31.4 ug/m 3 –9/12/08 – 55.1 ug/m 3 Maricopa –9/10/08 - 30.8 ug/m 3 –9/12/08 – 60.6 ug/m 3