Exceptional events are unusual or naturally occurring events that can affect air quality but are not reasonably controllable or preventable by the District.  Because these events are outside normal regulatory control, they may be excluded from certain air quality planning and compliance determinations under the National Ambient Air Quality Standards (NAAQS).  Exceptional events can include wildfires, high wind dust events, stratospheric ozone intrusions, and volcanic or seismic activity.

In the Mojave Desert region, wildfire smoke and high wind dust events are among the most common exceptional events affecting air quality.

The Exceptional Events Rule

In September 2016, the U.S. Environmental Protection Agency (EPA) finalized revisions to the Exceptional Events Rule, which establishes the criteria and procedures for determining whether air quality monitoring data has been influenced by an exceptional event.

The rule:

• applies to all exceptional event types and all National Ambient Air Quality Standards (NAAQS);
• ensures that air quality measurements are properly evaluated and characterized based on their causes;
• identifies reasonable actions the District should take to address air quality and public health impacts caused by these events;
• avoids unreasonable planning requirements for air agencies when violations of the NAAQS are caused by exceptional events; and
• ensures that the use of air quality data, whether afforded special treatment or not, is subject to full public disclosure and review.

Without an exceptional event determination, naturally occurring exceedances could negatively affect designation and attainment status.  After reviewing flagged monitoring data, the demonstration package supporting exclusion, and public comments, EPA may concur that an event qualifies as exceptional.  If approved, the associated data may be excluded from certain regulatory determinations, helping the District accurately demonstrate progress toward attaining and maintaining the National Ambient Air Quality Standards.