A substance, gaseous material or aerosol which has been introduced into the air (either by human activity or by natural processes) in sufficient concentration to produce a measurable effect on humans, animals, vegetation or materials (monuments, etc.):
SO2,
NO2,
H2S,
CO, hydrocarbons, etc.
Source:
PAC, 1990, 62, 2167. 'Glossary of atmospheric chemistry terms (Recommendations 1990)' on page 2172 (https://doi.org/10.1351/pac199062112167)