The AirBeam2 uses a light-scattering technique to detect particulate matter (PM) in the air. A fan inside the AirBeam2 pulls air in and a laser light shines on the air. PM in the air scatters the laser light. The AirBeam2 measures the light scattered by the PM and calculates the number and size of the PM. The data is then sent to the AirCasting app.
You can read more about how the AirBeam2 works at the Habitat Map website, https://www.habitatmap.org/blog/airbeam2-technical-specifications-operation-performance