Field data from recent studies on volcanic aerosols are used to propose a model for the dynamic evolution of a volcanic plume from aerosol generation to stabilization of the plume cloud after the first minute of plume travel time. The aerosol has a large number of ultrafine particles with radii of 10--20 nm and a large mass of acid droplets with radii of several hundred nanometers. We investigate the size and concentration profile of these particles along the plume travel path, incorporating coagulation, dilution by turbulent mixing, and growth as function of relative humidity. The light-scattering properties of the plume cloud are expressed in terms of the extinction coefficient, which is clearly dominated by the presence of acid droplets. The hygroscopic behavior of the droplets determines the rapidly changing visual appearance of the plume from a transparent haze to a bright, dense cloud with increasing humidity. When the chemistry of the magmatic gas does not favor the formation of acid droplets, Rayleigh scattering by high number concentrations of ultrafine particles explains the bluish color of the plume; the observability is restricted to special meteorologic and volcanologic conditions. ¿ American Geophysical Union 1993