Chlorine chemistry in urban atmospheres: a review

Environmental context Atmospheric chlorine radicals can affect the chemical composition of the atmosphere through numerous reactions with trace species. In urban atmospheres, the reactions of chlorine radicals can lead to effects such as increases in ozone production, thus degrading local and regional air quality. This review summarises the current understanding of atmospheric chlorine chemistry in urban environments and identifies key unresolved issues. Abstract Gas phase chlorine radicals (Cl•), when present in the atmosphere, react by mechanisms analogous to those of the hydroxyl radical (OH•). However, the rates of the Cl•-initiated reactions are often much faster than the corresponding OH• reactions. The effects of the atmospheric reactions of Cl• within urban environments include the oxidation of volatile organic compounds and increases in ozone production rates. Although concentrations of chlorine radicals are typically low compared to other atmospheric radicals, the relatively rapid rates of the reactions associated with this species lead to observable changes in air quality. This is particularly evident in the case of chlorine radical-induced localised increases in ozone concentrations. This review covers five aspects of atmospheric chlorine chemistry: (1) gas phase reactions; (2) heterogeneous and multi-phase reactions; (3) observational evidence of chlorine species in urban atmospheres; (4) regional modelling studies and (5) areas of uncertainty in the current state of knowledge.