[en] The source-depletion method, which is used to account for dry deposition in gaussian plume models of the atmospheric dispersion of pollutants, is investigated mathematically on the basis of a diffusivity model. The results are used to derive simple criteria for the validity of the method, which can be applied within the framework of the gaussian model without explicit recourse to diffusivity theory. A method for estimating long-range transport is similarly assessed. It is shown that the source-depletion method can normally be used to an accuracy better than a factor of two for distances out to 100 km from the source in all but the most stable weather conditions, and that even in Pasquill category F stability, fairly high deposition velocities are required for the method to fail. An alternative method, based on an asymptotic solution of the diffusion equation, is suggested for such rare cases. (author)