There is growing evidence that the changing global atmosphere is impacting urban and regional air quality and that emissions from great urban conurbations are significantly influencing global change. This book presents the topics needed to understand possible changes in regional and global atmospheric air quality. Air quality measurements reveal the influence of anthropogenic emissions on the background atmosphere, especially at regional and local scales, superimposed on a background of biogenic emissions. The book also deals with possible changes in atmospheric chemistry in remote areas due to the impact of halogens, halogen oxides, and organic sulphur compounds. Important areas that need to be known in detail for the modelling of these influences are ozone formation, radical mechanisms in the gas phase, oxidation processes in the aqueous phase, and the formation of aerosols. All these are discussed on the basis of both laboratory studies and field measurements. The various observational data, emission data and chemical mechanisms are needed as inputs for modelling the regional impact of anthropogenic emissions and global change.