Photochemical air pollution

From Glossary of Meteorology

photochemical air pollution

Type of air pollution, such as Los Angeles smog, associated with the buildup of oxidation products formed from the degradation of hydrocarbons, etc.

The term arises because sunlight is required to initiate photolysis reactions. Ozone and nitrogen dioxide are always present in photochemically produced mixtures; often other species such as peroxyacetyl nitrate and formaldehyde are also produced.