The gas-phase chemistry of urban atmospheres

This paper reviews the chemistry of urban atmospheres, using recent measurement data to highlight the key concepts. We briefly summarise historical reports of air pollution and the impact that human activities have had on urban atmospheres since the Industrial Revolution. Although pollution events in the first half of the 20th century were caused by high concentrations of smoke and sulphur dioxide, photochemical pollution has become the major problem in most of the major cities around the world. The chemistry of photochemical pollution episodes is discussed in some detail, particularly the crucial role played by volatile organic carbon and nitrogen oxides. Issues to be considered when modelling the chemistry of urban areas are briefly summarised, such as the uncertainties in chemical mechanisms and emission inventories, as well as the complexities of dynamical processes. Finally, we present some recent issues in urban chemistry, including the discovery that the amount of volatile organic carbon in urban atmospheres may be grossly under-estimated. We also use modelling results to show the importance of the reaction of ozone with reactive hydrocarbons as a radical source in urban atmospheres. Finally, the use of NOX-NO2 relationships to predict annual mean NO2 concentrations is discussed.