How is Air Quality Index (AQI) computed?
In Europe since 2017 (and what CAMS use for Euronews AQI products), the following source of AQI is used:
http://airindex.eea.europa.eu/
The computation of AQI is explained under "About European Air Quality Index" link (see in the Country and Stations selection box on the right-hand side at the top of the page).
In the United States, the U.S. Environmental Protection Agency (EPA) has published a document on the reporting of daily air quality:
https://www3.epa.gov/airnow/aqi-technical-assistance-document-sept2018.pdf
but there are almost as many computation formulas as countries!