A box model has been developed to study the formation of cloud condensation nuclei from DMS, in the unpolluted marine boundary layer (MBL). Chemical and microphysical processes include gas phase chemistry of DMS and SO2, formation and growth of H2SO4-H2O aerosols, in-cloud SO2 oxidation, and scavenging of SO2 and H2SO4 molecules by sea salt, cloud drops, and precipitation. Meoteorological processes include exchange between the MBL and the overlying free troposphere (FT) and horizontal mixing after particle removal by precipitation. The description of the FT aerosol is based on the notion of new particle production in the upper troposphere (for which there is experimental evidence) and the notion that these particles evolve into a self-preserving aerosol (for which there is no clear experimental evidence yet). We argue that FT aerosols are expected to be self-preserving in areas of large-scale subsidence such as the subtropics. The model results are in reasonable agreement with summer observations of DMS, SO2, non-sea-salt (nss) SO3, CN, and CCN at Cape Grim. They show that entrainment of FT aerosols in the MBL quenches new particle formation within the MBL. A sensitivity/uncertainty analysis shows that most of the variation in the number concentration of MBL aerosols is due to the variation in parameters linked to the FT aerosol, whereas most of variations in the MBL nss-sulphate mass is due to variations in the DMS flux. We conclude that FT-MBL exchange is likely to be an important mechanism that can explain both the observed levels of CN and CCN (active in stratiform clouds) in the MBL and their lack of short-term variability. ¿ American Geophysical Union 1995