How do I match PurpleAir's Map with AirNow's Map?

If you have visited AirNow’s Fire and Smoke Map, you may have noticed that it displays PurpleAir sensors alongside FRM/FEM monitors. However, you may have also noticed that the data displayed on AirNow’s map doesn’t always match the data on the PurpleAir Map.

The discrepancies between the two are influenced by a couple of factors:

  1. ​Data resolution
  2. The US EPA conversion formula for PurpleAir data

Data Resolution

​AirNow displays hourly averaged PurpleAir data that updates every hour. Conversely, the PurpleAir map displays 10-minute averaged data by default. However, you can change the data resolution on the PurpleAir Map to match the averaging period on the AirNow map, which will bring the data more in line. You can do so by changing the averaging period in the PurpleAir map’s configuration menu to 1-hour.

In order to fully align the data, you’ll also need to use the formula AirNow applies to raw PurpleAir data.


US EPA Conversion Formula

​The US EPA has created a formula that alters PurpleAir data and is meant for wildfire conditions. This formula is applied to the PurpleAir data that appears on the previously mentioned AirNow Fire and Smoke map. You can learn more about the US EPA conversion formula here: The "Apply Conversion" Field.

​​The field “Apply conversion” on the PurpleAir Map is set to “No” by default. This means that no formulas are applied and you are seeing raw PurpleAir data. You can replicate the conversion that AirNow uses by changing the value in the “Apply Conversion” field on the map’s configuration menu to “US EPA.”


Learn More

PurpleAir Map Guide
The Apply Conversion Field
What is the Difference Between CF=1, ATM, and ALT?

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