National Air Quality: Status and Trends of Key Air Pollutants | US EPA

There are links to two types of summaries provided on this page – air quality trends and air quality design values.

The air quality trends summaries are intended to reflect actual air quality and therefore include concentrations that may have been impacted by episodic events like wildfires and dust storms.  Air Quality Design Values are used to designate and classify nonattainment areas and intended to reflect air quality that is not impacted by exceptional events. Therefore if concentrations are shown to be impacted by an exceptional event they are not included in design value calculations. The EPA defines exceptional events as unusual or naturally-occurring events that affect air quality but are not reasonably controllable using techniques that tribal, state, or local air agencies may implement.