A pollutant emitted directly into the air from identifiable sources (e.g.
SO2,
NO, hydrocarbons, etc.). Secondary pollutants, such as ozone, are generated within the atmosphere through chemical changes which occur in primary pollutants.
Source:
PAC, 1990, 62, 2167. 'Glossary of atmospheric chemistry terms (Recommendations 1990)' on page 2208 (https://doi.org/10.1351/pac199062112167)