Photochemical smog contains a mixture of primary and secondary pollutants. Primary pollutants are those which have been released directly into the photochemical smog present in the atmosphere, by the combustion of fuels; example: CO. Secondary pollutants are those which have been formed when the primary pollutant of the photochemical smog undergoes further reaction, by the heat of the sun; example: ozone. NOx and NO2 are the main primary and secondary pollutants produced respectively from motor vehicles.
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