Exceptional Events

Exceptional events are unusual or naturally occurring events that can affect air quality but are not reasonably controllable using techniques that agencies implement in order to attain and maintain the National Ambient Air Quality Standards (NAAQS). Exceptional events are events for which the normal planning and regulatory process established by the Clean Air Act (CAA) is not appropriate. The Exceptional Events Rule (EER), codified at 40 Code of Federal Regulations (CFR) Parts 50.1, 50.14, and 51.930, allows states to petition EPA to exclude air quality monitoring data influenced by exceptional events from applicable regulatory determinations.

DAQ submits Exceptional Event Demonstrations to EPA when ambient air monitoring data is affected by an exceptional event, in accordance with the EER. These events can include natural events such as high-winds, wildfires, stratospheric ozone intrusions, and volcanic and seismic activities. Federal regulations also allow states to petition to exclude data when exceedances or violations are caused by emissions from fireworks displays on a case-by-case basis. 

Mitigation of Exceptional Events

40 CFR 51.930(a) requires that states requesting to exclude air quality data due to exceptional events must take appropriate and reasonable actions to protect public health from exceedances or violations of the NAAQS. In its 2016 EER revision, EPA implemented a requirement for the development of mitigation plans for areas with historically documented or known seasonal events. Clark County was identified as an area subject to the new mitigation plan requirements for wildfire (ozone) and high wind (PM10) events.

In October 2018, DAQ adopted the Clark County Mitigation Plan for Exceptional Events, which addresses the EPA-required plan components and encompasses wildfire (ozone and PM2.5), high wind (PM10), and fireworks (PM2.5) events. The plan outlines the programs, systems, and control measures in place to protect the public and mitigate the impact of these types of events in Clark County. EPA Region 9 determined that the plan was complete for PM10 due to high wind dust and ozone due to wildfires in a letter dated December 17, 2018. The mitigation plan was approved by the Clark County Board of County Commissioners (BCC) on January 21, 2020, and supersedes the Natural Events Action Plan approved by the BCC on March 15, 2005.

Exceptional Event Demonstrations
Demonstrations Available for Public Comment
Please visit our planning notices webpage to view demonstrations currently out for public comment.

Demonstrations Submitted to EPA for Action

Date Pollutant Monitor Value Type Status
April 15, 2013  PM10   Multiple   267 µg/m3  High Winds  Deferred  Review
July 3, 2011  PM10  Multiple   242 µg/m3  High Winds      EPA Concurrence 6/25/14 
May 23, 2012  Ozone  Multiple  80 ppb  Wildfire  Deferred Review
May 10, 2012  PM10  Multiple  314 µg/m  High Winds  EPA Review
May 21, 2008  PM10  Craig Road  168 µg/m3  High Winds  EPA Review
Feb 13, 2008  PM10  Craig Road  203 µg/m3  High Winds  EPA Review
June 29-30, 2005  Ozone  Valley Wide  85 ppb  Wildfire  EPA Review