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 (NOx) combine in the presence of sunlight;
NO2, which is formed as NO combines with oxygen in the air; and acid rain, which
is formed when sulfur dioxide or nitrogen oxides react with water.
