A photochemical box model with size-segregated aerosol model, which explicitly accounts for size-dependent chemical properties of sea-salt particles, has been developed to simulate multi-phase chemistry of chlorine and bromine in the marine boundary layer (MBL). Simulated results are compared with those obtained by a model that only implicitly accounts for particle size dependence by applying the quasi-size-dependent approach (bulk model). It is found that the bulk model fails to capture the decreased volatilization rates of bromine from the small-sized sea-salt particles caused by rapid depletion of bromide. This leads to substantial overestimation of gas phase bromine mixing ratios by the bulk model, affecting the behavior of other species such as ozone and NOx that are sensitive to bromine chemistry. The findings warrant the use of size-segregated model in simulating MBL halogen chemistry. ¿ 2001 American Geophysical Union