A simplified model of the dynamics of photochemical aerosols is developed, and initial applications are presented. The model, which can be incorporated in or coupled to a photochemical air quality simulation model, takes into consideration the emissions of particles with diameters of 0.01 to 1.0 micrometer and their growth in this size range by the formation of secondary aerosol. The formation of particulate matter is assumed to occur in two stages--condensable material first forms in the gas phase and then diffuses to the particles. A pseudo-steady-state is assumed in which the rate of formation of condensable material in the gas phase is set equal to the rate at which condensation occurs on particles. Model simulations are presented for a batch system both with and without sources of primary aerosol and for a trajectory system. Model simulations and smog chamber experiments are in qualitative agreement.