A chemical box model of the marine boundary layer has been developed. It treats reactions in the gas phase and in deliquesced sea-salt aerosol particles. A quasi-size-dependent approach was used to obtain mean values of those aerosol properties that depend on the droplet radii. A residence time of 2 days was assumed for the return of the aerosol particles to the sea surface. Emission and deposition fluxes simulate exchange processes with seawater and air masses surrounding the air parcel. Apart from the well-known reactions, the chemical reaction mechanism includes a large set of reactions of halogen compounds that are of potential importance for the ozone budget. Photochemical reactions are switched on during the day, assuming a semisinusoidal diurnal cycle of the photolysis rates. In our model runs, a heavily polluted urban air mass (in which O3 has been formed by photochemical smog reactions over land) is advected over the ocean. We found two processes that convert aqueous phase bromide into reactive bromine compounds. First, when concentrations of nitrogen oxides are still high, NO3 is scavenged during nighttime by the aerosol particles and oxidizes bromide: Br-+NO3→Br+NO-3. The second process is a cycle in which the aqueous phase reaction H++HOBr+Br-→Br2+H2O plays a central role. Br2 is only slightly soluble, volatilizes, and is dissociated into Br atoms. Subsequently, rapid destruction of ozone takes place via the gas phase reactions Br+O3→BrO+O2, BrO+HO2→HOBr+O2, and HOBr+h&ngr;→Br+OH. Sensitivity analyses have been performed to investigate how our results are influenced by input parameters that are not well known. ¿ American Geophysical Union 1996