Understanding PurpleAir vs. AirNow.gov Measurements of Smoke Pollution

You may have noticed PurpleAir and AirNow.gov giving different Air Quality Index (AQI) values for your zip code. For example, on August 21st, when I checked these sites, the AQI given on AirNow.gov for the 94720 zip code of Berkeley was 107. By contrast, PurpleAir gave values that are in the 130s to 150s.

With the help of Brett Singer, an air quality scientist at Lawrence Berkeley Labs, Arian Dybwad, CEO of Purple Air, and various folks at the EPA, I was able to mostly piece together what’s going on.

First, let’s discuss correction factors. Empirical studies have found that PurpleAir monitors tend to yield larger AQI values than EPA monitors like those used by AirNow (see end of this article for more on why) when measuring wood smoke. To correct for this, PurpleAir provides several correction factors you can use to get a better estimate.

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