ABC - DEF - GHI - JKL - MNO - PQRS - TUV - WXYZ Primary & Secondary pollutant
Definition:
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.
Source & ©: GreenFacts
More:
Source & ©: EPA
Sources of Pollutants in the Ambient Air 
Related words: Ozone To read about this term in context: GreenFacts Digest on Air PollutionParticulate Matter, Ozone, and Nitrogen Dioxide:
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