On the multimodality of density functions of pollutant aerosols

Abstract

Numerous observations support the view that continental tropospheric aerosols, particularly urban aerosols, have surface and mass density functions which are multimodal. We examine here briefly the origins of such multimodality. One obvious mechanism producing multimodality involves the mixing of various dissimilar primary sources. The production of multimodal mass densities by this mixing process and their smoothing by coagulation is discussed briefly. A second, hitherto unexplored, mechanism is the production of additional modes in the mass density function by condensation processes in the atmosphere involving secondary aerosol sources. These additional modes are shown to be essentially independent of the size distributions of the primary sources.