On the PurpleAir map, you may have noticed two different sets of data points, denoted as “A” and “B.” We call these “channels.” In the example below, you will see a graph that shows data from a sensor in the Salt Lake City area (UT, USA).
You can find this sensor on our map here: Real-Time Air Quality Map | PurpleAir
You will notice blue and black lines on this graph. The blue line represents channel A, and the black represents channel B. Each channel is connected to a laser counter in a sensor. Our outdoor sensors include two laser counters, which measure air particulate individually. We compare the correlation of these two channels to create a measure of confidence—called the Confidence Score—that serves as an assurance of data quality.
A and B data are not exclusive to PM measurements; they are a theme with PurpleAir data. Below, you’ll see an example of field descriptions in our API documentation. You’ll see that even the environmental sensor inside a PurpleAir sensor can report A and B data.
This will be the case for all components with multiple outputs. If only one output is reported, such as the single laser counter in indoor sensors and the environmental sensor in certain PurpleAir sensors, only channel A will return data. The average will still be present, but the value will match the base output.
Whenever two data channels are present, we also include a field that reports the average of the two channels. This is simply calculated with the formula used in the following example:
pm2_5_atm = (pm2_5_atm_a + pm2_5_atm_b) / 2