Pinal County Air Quality July 23, 2007 PM 10 Exceptional Event

July 23, 2007 Strong winds associated with a monsoon cell which developed close to the Combs monitor. Max winds reached 24 mph which caused one PCAQCD monitor to exceed the 24-hr PM 10 NAAQS. Site24-hr PM 10 Conc. (ug/m 3 ) Combs 160.9

9 p.m. 7/22/07 to 8 p.m. 7/23/07 local time Location of Combs monitor

and strong cell

Location of Combs monitor and strong cell

Location of Combs monitor and strong cell

Location of Combs monitor and strong cell

Location of Combs monitor and strong cell

Location of Combs monitor and strong cell

Location of Combs monitor and strong cell

Location of Combs monitor

7/23/07 “But for” Test Had it not been “but for” the high winds between 5-6 p.m. associated with the cell development near Combs which caused it to have a 1-hr PM 10 concentration of 2,158 ug/m 3, Combs would have NOT exceeded the PM hr NAAQS. –If the 2,158 ug/m 3 was replaced with the average of the other 23 hours then the 24-hr PM 10 concentration would be 70 ug/m 3

Additional 7/23/07 “But for” Test Combs had the following 24-hr PM 10 concentrations before/after the 7/23/07 event –7/22/07 – 23.2 ug/m 3 –7/24/07 – 49.3 ug/m 3