(a) A primary pollutant is an air pollutant emitted directly from a source. A secondary pollutant is not directly emitted as such, but forms when other pollutants (primary pollutants) react in the atmosphere.Examples of a secondary pollutant include ozone, which is formed when hydrocarbons (HC) and nitrogen oxides (NO) combine in the presence of sunlight; , which is formed as NO combines with oxygen in the air; and acid rain,which is formed when sulphur dioxide or nitrogen oxides react with water along with smog.