Halogen chemistry of the marine boundary layer

Abstract
This chapter describes the atmospheric chemistry governing the behaviour of reactive inorganic halogen compounds in the marine boundary layer (MBL). Particular emphasis is placed on the influence of the halogens on ozone and oxidising processes. The introduction indicates the lifecycle of reactive halogen species, their sources and sinks, as well as the chemical interactions with the background photochemistry of the troposphere. The gas-phase chemistry of the chlorine, bromine and iodine species is then described, indicating their differences in reactivity. The reactions which lead to ozone loss and coupling reactions with NO and HO species present in the MBL are also discussed. The next section deals with the heterogeneous reactions involving halogen species, in particular those which lead to activation of Cl ^- and Br ^- contained in sea-salt aerosols. A brief description of the properties of marine aerosols is given. The processes involved in chemical modification (`ageing') of sea-salt aerosol are described. The chemical mechanisms for halogen release by acid displacement, redox reactions involving NO and redox reactions involving HOX (X = halogen) are then discussed in detail. A brief description of the evidence for halogen catalysed ozone loss and VOC degradation from atmospheric measurements and modelling studies is given.