Primary Pollutants and Secondary pollutants
Primary pollutants are directly emitted from the sources.
Examples of primary pollutants are carbon monoxide, carbon dioxide, sulphur dioxide and benzene etc.
Secondary pollutants are not emitted directly. They are formed from the combination of primary pollutants with some other compound.
Examples of secondary pollutants are Ozone, Formaldehyde, PAN (peroxy acetyl nitrate) and Smog etc.
