Bibliography: leaves 75-84.Photochemical smog is formed by the interaction of sunlight with nitrogen oxides and hydrocarbons. These precursors are principally emitted by anthropogenic sources. As the major component of the photochemical oxidants is ozone, it is used as the indicator of photochemical smog and air quality standards today are therefore based on ozone. Another important photochemical oxidant is peroxyacetyl nitrate (PAN). Many authors believe that PAN is a better indicator of photochemical activity than ozone, because PAN has, unlike ozone, no large natural sources. Thus, the occurrence of high PAN concentrations is unequivocally related to anthropogenic pollution. The objectives of this work are to summarise the current research on the formation of photochemical smog with emphasis on PAN and to investigate the photochemical smog situation in South Africa using PAN as an indicator
