The United States Environmental Protection Agency (EPA) records and collects air quality measurements on U.S. territory. Data for CO, NO2, O3, PM2.5 and PM10 are published as indicators of air pollution.
daily
usually hourly
6524 Stations 19034 Sensors
Unless otherwise noted, all data created by the U.S. EPA is public domain by default and is not subject to U.S. copyright protection under 17 U.S.C. § 105. For more details on the U.S. public domain license, click here: http://www.usa.gov/publicdomain/label/1.0/
https://www.epa.gov/outdoor-air-quality-data/about-air-data-reports