A model is described which accepts meteorological input data from marine areas and calculates vertical profiles of size distributions and infrared extinction coefficients in the marine boundary layer under clear sky, fog, and precipitation conditions. For the clear case, aerosols consist of a mixture of dust, water soluble and sea salt particle components represented by the sum of four log-normal size distributions having meteorological and airmass dependent amplitude and modal radius parameters. These were derived by empirical fits to actual ship measurements recorded over the Atlantic Ocean. The calculations account for particle growth using empirical formulations which take particle chemistry into account, while extinction coefficients are determined using Mie theory. Continental component contributions are inferred using a meteorological interpreter model which also corrects for errors or uncertainties in the input meteorological data by ensuring consistency between calculated visibility and observed visibility. Vertical aerosol profiles are modelled considering the production, mixing, and deposition of aerosols in the marine boundary layer, with simpler parameterizations for fog and precipitation cases.