What are primary and secondary pollutants ?

Primary pollutants are emitted directly into the atmosphere from their source and remain in the same form.
Examples : dust, ash, smoke, fumes, hydrocarbons and oxides of sulphur and nitrogen.

Secondary pollutants are formed by chemical interactions between primary pollutants and other reagents present in the atmosphere.
Example : peroxyacyl nitrate (PAN) and ozone are formed when primary pollutants such as oxides of nitrogen and hydrocarbons react in the presence of sunlight.