How it is studied
Air enters an opening and particles impact on a moving adhesive surface. Position on the surface corresponds to sampling time. Microscopy and the counting protocol convert these impacts into concentrations.
An example to interpret it
Volume example: at a constant flow of 10 L/min, one hour represents 600 litres, or 0.6 m³ of air. If the instrument stops, that volume no longer describes the full hour; record the interruption before calculating concentration.
What to check
Flow calibration, adhesive condition and time alignment matter as much as grain counts. Rooftop data describes that sampling location; it does not automatically represent every street or indoor space.