PurpleAir devices use Plantower laser counters to estimate the number of particles that pass through them. They also make estimations regarding the size of these particles, allowing us to find the volume that these particles occupy in any given space. However, the calculation of mass (which is the standard global measurement for particulate matter) requires volume and density. This is where _cf_1, _atm, and _alt are utilized.
CF=1 and ATM
CF=1 and CF=ATM (appearing as _cf_1 and _atm in the PurpleAir API) are formulas used in Plantower laser counters. One of the things that these formulas do is estimate the density of the particles passing through the laser counters. With both the volume and density of the particles, the laser counters can generate a mass measurement (i.e. µg/m3).
CF1 and ATM data are shown by default on the PurpleAir Map as follows:
- Indoor sensors* display CF1 data
- Outdoor sensors* display ATM data
*Indoor and outdoor sensors, in this context, are determined by how the sensor is registered. Thus, a PurpleAir Classic, while typically being used outdoors, can be registered as an indoor sensor, and the data from the sensor will be CF1 data on the map.
It should be noted that both the CF=1 and ATM algorithms are unknown to us; the manufacturer, Plantower, considers these formulas to be proprietary and hasn’t provided them to us. We follow their instructions for how to use the data, which comes directly out of the laser counters as CF=1 and ATM data.
Until recently, the CF=1 algorithm was a black box as it is considered a proprietary algorithm by Plantower. However, a researcher in our community named Lance Wallace (@Lance) was able to recreate the algorithm. You can learn more about his study here: API History Fields Descriptions - #6 by Lance.
ALT cf=3
Speaking of Lance Wallace, ALT cf=3, or ALT-CF3 (appearing as _alt in the PurpleAir API), is an alternate formula that he created. It has been found, in various situations, to be more accurate than the previous two factors. More information on the ALT cf=3 algorithm can be found in this community article.
Additionally, you can view _alt on the Map by choosing the ALT cf=3 option in the Apply conversion field of the configuration menu.
The screenshot shows the Apply conversion dropdown menu.
Learn More
SD Card File Headers
API History Fields
Making API Calls
What Do PurpleAir Sensors Measure?