Pollutants may be of primary origin, emitted directly from the source, or secondary, being formed in the atmosphere by chemical interactions of primary pollutants and other atmospheric constituents. Carbon monoxide and nitric oxide (NO) are primary pollutants produced during the combustion of carbon-based fuels.
The formation of nitrogen dioxide (N02), by further oxidation of NO, is a secondary process. Ozone (03) is another secondary pollutant, formed in the atmosphere by the action of sunlight on N02. In urban areas 03 forming reactions are enhanced by the presence of hydrocarbons from vehicle exhausts.
In addition, other photochemical reactions produce oxidants such as peroxyacety nitrate (PAN), hydrogen peroxide and nitric acid as well as aldehydes, formic acid and fine particulates.
During hot, calm weather the formation of this cocktail of photochemical air pollutants leads to photochemical smog, which can persist for several days before dispersion. This effect, first recognised in Los Angeles in the 1940s, occur in many cities throughout the world where high temperatures are the norm