Abstract
Studies of the chemistry of the urban atmosphere provide special challenges which arise from the high density of emissions, strong concentration gradients and relatively high pollutant concentrations. In contrast to the regional and global atmosphere, local dispersion processes play a much larger role in determining atmospheric concentrations and also have a substantial effect on chemical transformations. On the other hand, residence times in the urban atmosphere are relatively short, hence limiting the range of chemical reaction processes which are significant. Some key species such as the hydroxyl radical have different predominant source processes in the urban and the regional atmosphere. A case is made that the differences are so large that urban atmospheric chemistry needs to be given special treatment and cannot simply be considered as subtlely different from regional and global atmosphere studies.