We present an aerosol size distribution model summarizing measurements of sulfate, sea salt, and dust aerosol obtained in the marine boundary layer and marine free troposphere. Stratospheric aerosol cases are not included here. Aerosols were sampled in both both clean background conditions and aged anthropogenic aerosol (>3 days) which had advected over the ocean. Model size distributions are developed for the sulfate accumulation mode, sea-salt coarse mode, and dust coarse mode. The data demonstrate that both the accumulation and coarse mode aerosol gradually shift to larger diameters as the aerosol mass increases. Based on this relationship, it is possible to estimate the size distribution based on aerosol mass. Comparisons with other aerosol models show significant differences, which suggest further measuring and modeling efforts are needed.¿ 1997 American Geophysical Union