The Fire and Smoke Map has an FAQ document which details the difference between the AirNow Fire and Smoke Map and the PurpleAir map on page 7.
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 several factors including:
- Data resolution
- The US EPA conversion formula for PurpleAir data
- AirNow and AirNow’s Fire and Smoke map both display the NowCast AQI, which doesn’t use averages in the same way the PurpleAir Map does.
How to More Closely Match the PurpleAir Map With the Fire And Smoke Map
Selecting the following map settings will make the PurpleAir Map match the Fire and Smoke Map much more closely. However, due to a variety of reasons they still won’t line up perfectly.
Map Settings:
- Data layer: US EPA PM2.5 AQI
- Apply conversion: US EPA
- Averaging period: 1-hour
You can apply these settings manually, or click here to open the map with them already applied.
Data Resolution and AQI
AirNow displays the NowCast AQI. According to AirNow, “The NowCast calculation uses longer averages during periods of stable air quality and shorter averages when air quality is changing rapidly”.
The PurpleAir Map allows you to select the averaging period used. Selecting an Averaging period of “1-hour” will typically cause the PurpleAir Map to more closely match the NowCast AQI.
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 “Apply conversion” setting 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 apply the same conversion that AirNow uses by changing this to “US EPA.”
Learn More
PurpleAir Map Guide
The Apply Conversion Field
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