Modeling multi-phase halogen chemistry in the marine boundary layer with size-segregated aerosol module: Implications for quasi-size-dependent approach

A photochemical box model with size-segregated aerosol module, which explic itly accounts for size-dependent chemical properties of sea-salt particles, has been developed to simulate multi-phase chemistry of chlorine and bromi ne in the marine boundary layer (MBL). Simulated results are compared with those obtained by a model that only implicitly accounts for particle size d ependence by applying the quasi-size-dependent approach (bulk model). It is found that the bulk model fails to capture the decreased volatilization ra tes of bromine from the small-sized sea-salt particles caused by rapid depl etion of bromide. This leads to substantial overestimation of gas phase bro mine mixing ratios by the bulk model, affecting the behavior of other speci es such as ozone and NOx that are sensitive to bromine chemistry. The findi ngs warrant the use of size-segregated model in simulating MBL halogen chem istry.