Useful Air Quality Link (AirNow) – 
The U.S. Environmental Protection Agency, National Oceanic and Atmospheric Administration, National Park Service, tribal, state, and local agencies developed the AirNow system to provide the public with easy access to national air quality information. State and local agencies report the air quality index (AQI) for cities across the US and parts of Canada and Mexico.

Airnow is available at: https://www.airnow.gov/