A three-dimensional, size-resolved and chemically resolved aerosol model is developed. Gas-to-particle conversion is represented by dynamic mass transfer between the gas and aerosol phases. Particle-phase thermodynamics is computed by a new thermodynamic model, Simulating Composition of Atmospheric Particles at Equilibrium 2. The aerosol model is applied to simulate gas and aerosol behavior in the August 27--29, 1987, episode in the South Coast Air Basin of California. The assumption that volatile inorganic species such as NH4NO3 are at instantaneous, local equilibrium is examined. ¿ 1998 American Geophysical Union