After pollutants are identified and standards set by national organizations such as the Environmental Protection Agency (EPA) in the US and the National Institute for Water and Atmosphere (NIWA) in New Zealand, monitoring the atmosphere is done by manual sampling and instrumental sampling. Though each nation may have various sampling protocols, the technologies used are the same, so descriptions of monitoring methods will be similar if not identical from one nation to another.
Using New Zealand for illustration of monitoring methods, manual sampling methods include passive sampling for solids with filters that collect solid pollutants such as Benzene and fluorides from the air. Another is paper tape sampling that collects particulates and H2S. A third manual method uses bubblers in which gases are bubbled up into pollutants in solution to monitor chemicals such as SO2 and NO2.
Some instrumental sampling methods use absorption and spectroscopic technologies. These use infra-red, chemiluminescence, flame photometry, and fluorescence technologies. The monitoring of airborne particles is done with low-energy beta radiation, microbalance recording, and light scattering technologies. Some pollutants monitored by these classes of instrumentation are CO, NO2 and O3, H2S, and SO2.
[Answer drawn from "Air Pollution Monitoring." New Zealand Institute of Chemistry.]