Chlorine chemistry in urban atmospheres: a review

Gas phase chlorine radicals (Cl-center dot), when present in the atmosphere, react by mechanisms analogous to those of the hydroxyl radical (OH center dot). However, the rates of the Cl-center dot-initiated reactions are often much faster than the corresponding OH center dot reactions. The effects of the atmospheric reactions of Cl-center dot 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.