I believe the PurpleAir map is using the wrong type of particulate matter concentration values from the Plantower modules in its indoor sensors, and consequently the particulate matter and air quality index numbers shown for indoor sensors are inaccurate. PurpleAir can correct this by using different values from the sensors.
The Plantower PMS5003, PMS6003, and PMS1003 modules used in all PurpleAir sensors report two different types of PM1.0, PM2.5, and PM10 values: CF=ATM and CF=1. PurpleAir’s FAQ had described it:
[The Plantower sensors] provide 2 different mass concentration conversion options; CF_1 uses the “average particle density” for indoor particulate matter and CF_ATM uses the “average particle density” for outdoor particulate matter.
My understanding is that the PurpleAir map has always used CF=ATM values for outdoor sensors, and it’s been reported that since Nov. 21, 2019 the map uses CF=1 for indoor sensors. My own analysis of the map suggests that it is still using CF=1 values for indoor sensors.
I believe that PurpleAir is using the wrong values for indoor sensors, and that the description in the FAQ was incorrect. The problem may come down to a misinterpretation of the information in Plantower’s data sheets. The English language data sheets for the PMS1003, PMS5003 and PMS6003 provide scant information about the differences between CF=ATM and CF=1, stating merely that CF=ATM readings are for the “atmospheric environment”, and CF=1 is a “standard particle”, and stating that “CF=1 should be used in the factory environment”.
However the Chinese data sheets provide much more information. Please see the Chinese data sheet for the PMS5003, v2.3, page 12. I provide here an English language translation furnished by a Chinese speaker:
Note: The standard particle mass concentration value refers to the mass concentration value obtained by density conversion using industrial metal particles as equivalent particles, which is suitable for environments such as industrial production workshops. The mass concentration of particulate matter in the atmospheric environment is calculated by using the main pollutants in the air as equivalent particles for density conversion, which is suitable for ordinary indoor and outdoor atmospheric environments.
So you can see that CF=1 is not for ordinary indoor environments. It is just for factory environments with industrial metal particles. The CF=ATM values are for ordinary outdoor and indoor environments.
PurpleAir’s use of CF=1 for indoor sensors means that the reported values are higher than they should be for readings above 25 μg/m³. (From my own testing of the PMS5003, there is no difference between CF=ATM and CF=1 readings below 25 μg/m³.) The use of CF=1 also means that indoor and outdoor sensor readings cannot be directly compared, which is an obvious limitation, as outdoor air is often the source of indoor air pollution.
I hope that PurpleAir can update its map to use CF=ATM sensor values for indoor sensors by default, perhaps using CF=1 only for a sensor if the owner has indicated it is in an environment with industrial metal particle pollution (which I would expect to be a very small fraction of sensor locations). The use of CF=ATM sensor values from indoor sensors should be applied to the map’s raw PM1.0, PM2.5, and PM10 data layers, and to all of the various air quality index data layers that are computed from PM2.5 sensor data.