Photochemical smog is composed of primary and secondary pollutants. Primary pollutants, which includes nitrogen oxides and volatile organic compounds are introduced into the atmosphere via vehicular emissions and industrial processes.
Secondary pollutants, like ozone and PAN (peroxyacyl nitrates), result from the reaction of primary pollutants with ultraviolet light. All of these harsh chemicals are usually highly reactive and oxidizing. Photochemical smog is therefore considered to be a problem of modern industrialization. It is present in all modern cities, but it is more common in cities with sunny, warm, dry climates and a large number of motor vehicles.