While much of the suspended particulate matter found in the ambient air in urban areas has been emitted directly into the atmosphere, some has been formed there by photochemical reactions from gaseous precursor species. Two major components of this secondary particulate matter have been selected for detailed study in the United Kingdom context. These are particulate sulphate, formed from the precursor, sulphur dioxide, and secondary organic aerosols, formed from oxidation of terpenes and aromatic hydrocarbons. A Lagrangian dispersion model has been used to describe the emissions, transport and transformation of SO2 into particulate sulphate. The origins of the particulate sulphate are delineated in two separate pollution episodes which occurred during 1996. A photochemical trajectory model is used to describe the formation of secondary organic aerosols and to assess the relative contributions from natural biogenic and man-made precursor sources during conditions typical of photochemical pollution episodes.