Hi @lacnob, PurpleAir sensors use Plantower laser counters to read the air quality. The laser counter sucks in air using a small fan at a consistent rate. The air flows through the laser counter and passes a very small laser. As the particles cover the laser, the light from the laser is reflected onto a detection plate. This detection plate inside the laser counter measures how long the particle has covered the laser. With this measurement, the size of the particle can be determined. The sensor will also count the number of particles that have gone through.
The laser counters use these values to calculate raw particulate matter values. We use these raw particulate matter values to calculate the AQI.
Another explanation is available here: PurpleAir Sensors Functional Overview.