Abstract
An explanation is offered for the similarities observed experimentally among size distributions of natural aerosols measured at different places and times. This explanation is based on a similarity transformation of the equation describing the kinetics of a coagulating, settling aerosol. On dimensional grounds, an approximate shape is derived for the upper end of the spectrum which compares well with the available experimental data. An estimate is given for the rate at which matter is transferred from the lower to the upper end of the spectrum.