What are Channel A and Channel B?

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.

Other Fields That Use A and B

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

So what does it mean when I have a very low confidence score and the sensors report vastly different values? My confidence score is oven way below 30% and the A & B sensors are vastly different (see graph below).

I don’t see any cobwebs or other detritus in my Classic monitor. Is it near death? Do I need to clean it somehow (suggestions appreciated so I don’t damage it) or do I need to replace it?

Please advise!

Hi Bill,

It’s possible that there is debris inside the laser counter (channel A) that is blocking particles from moving through or covering the lens. We recommend cleaning the sensor: Sensor Maintenance. As blowing air through the air channel can can the fan to move too quickly, causing damage, we recommend that you apply short bursts of air or use a toothpick to hold the fan in place.

If this doesn’t work, you can also replace the laser counters.

Hi Andrew -

I did all that and did further diagnosis.

I decided to take Purple Air up on the offer to trade-in my two sensors (outdoor PA-II and indoor PA-I, as arranged with Jacqui Hunt) so they can be refurbished and resold.

I’ve ordered two new Zen sensors. They should be delivered tomorrow.

I’m all set. Thanks!