> Laser light is emitted from the sensor and focused by the sensor lens at approximately 5 mm from the top of the sensor’s lens surface.

> Particles traveling in free space due to the natural ambient airflow are detected when passing through the laser focal (sensitive) region.

> Due to the interaction between particles and light, the light scatters in different directions; a fraction is back-scattered towards the sensor, where the integrated photo-detectors detect it.

> The back-scattered signal is processed by unique algorithms (based on particle counts, particle relative velocity, probed air volume during measurement) to derive the particulate matter mass concentration.

You can detrend for high humidities, but once water condenses, the only way round this would be to add a drying instrument.