An explicit microphysical cloud model with size-resolved chemistry is utilized to assess the impact of an externally mixed, marine aerosol on the magnitude of sulfate production in-cloud and how the sulfate is distributed over dry particle size. Consequences of the distribution for the light-scattering efficiency of the sulfate and its efficiency as cloud condensation nuclei (CCN) are evaluated. The model results suggest that the presence of alkaline sea-salt particles has a significant impact on the magnitude of sulfate produced in-cloud, its size distribution, light-scattering efficiency, and CCN efficiency. ¿ American Geophysical Union 1992