Air pollution is characterized by a complex mixture of compounds emitted by various pollution sources (primary pollutants), as well as those formed through chemical reactions occurring in the atmosphere (secondary pollutants). This mixture includes so-called indicator pollutants or tracers, whose levels are intended to reflect overall air pollution and the various pollution sources present. The most commonly monitored pollutants are fine particles with a diameter of less than 10 µm (PM10) or 2.5 µm (PM2.5), ozone (O3), nitrogen dioxide (NO2), and sulfur dioxide (SO2). In Lower Normandy, monitoring is carried out by Air C.O.M., an accredited air quality monitoring association (AASQA). (R.A.)