Comparing map data to card data

I’m trying to compare data I pulled from PurpleAir SD card to data I download from the PurpleAir map.

Should i be using the “pm2.5_aqi_cf_1” or “pm2.5_aqi_atm” fields to compare to data I download from the PurpleAir map?


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Hi Matt,

“cf_1” and “_atm” are calibration factors that apply to outdoor sensors and indoor sensors. If your sensor is outdoors, use “cf_1.” If your sensor is indoors, use “_atm.” The map automatically applies one of these fields depending on if you registered your sensor indoors or outdoors.

Hope this helps.


Thanks! That’s really helpful. Are there documents of why the calibration is different for inside vs outside? Preferably in layman’s terms. I’m assuming a large warehouse environment would use the indoor calibration.

Hi Matt,

PurpleAir sensors use lasers to scan particles. They record particle size, however, they are unable to determine the mass of these particles. “cf_1” and “_atm” are calibration factors that estimate the mass of particles passing through the lasers. They use different algorithms for inside and outside because the mass of particles is different on average between indoor and outdoor.

Yes, any sensor that is inside, no matter the size of the space, would use “cf_1:” large warehouses included.