ADEQ has notified the U.S. Environmental Protection Agency (EPA) of information regarding exceptional events that resulted in exceedances of the 24-hour PM₁₀ National Ambient Air Quality Standards (NAAQS) in the Greater Phoenix area during 2008. After preliminary investigations of the extreme natural conditions that caused the exceedances, ADEQ determined that the aforementioned high PM₁₀ concentrations were caused by exceptional events. ADEQ staff worked with U.S. EPA staff to add certain components to the assessment reports to improve clarity and better document the conditions that caused the exceedances. This was available for a formal 30-day review and comment period.

For definitions of exceptional events see 40 CFR 50.1(j) and (k), and for "treatment of air quality monitoring data influenced by exceptional events" see 40 CFR 50.14.

To strengthen the request for U.S. EPA concurrence on exceptional/natural events in Arizona, ADEQ researched threshold wind speeds that suspend dust into the air and special circumstances that lead to elevated dust levels. ADEQ also examined the effectiveness of PM₁₀ control measure for high wind events in Arizona. This research is described in the white papers listed below. The white papers are included as supplements to the exceptional event assessments.

• Impact of Exceptional Events 'Unusual Winds' on PM₁₀ Concentrations in Arizona
• High Wind Exceptional Events And Control Measures for PM₁₀ Areas