Nighttime measurements of particle number distribution and mass composition, and concentrations of NO2, O3, NH3, and HNO3, were made at night at a rural site in Ontario in August of 1992. A simple model of particle growth was constructed to simulate the observed rapid growth of aerosol mass in the 0.2 to 0.5 μm diameter range. Both measurements and model results indicate that the growth of accumulation mode aerosol mass was due to condensation of the nitrate radical, HNO3, and NH3 onto particles, with the formation of particle ammonium nitrate. The results show that the reaction of O3 with NO2 in the isolated nocturnal boundary layer can lead to the production of gas-phase nitric acid. When this occurs in the presence of local ammonia emissions and preexisting particles, rapid growth of particle ammonium nitrate takes place. The model results show that most of the observed variations can be accounted for by a coupled system of equations including dynamical and thermodynamic effects. The dynamical approach to equilibrium is sufficiently fast that the gas-phase nitric acid concentrations are more sensitive to the magnitude of the thermodynamic equilibrium concentration than the dynamical time constant. A simple parameterization for the effects of sulphate on particle nitrate formation was developed and shown to provide a good estimate of the equilibrium concentration of gas-phase nitric acid. ¿ 1998 American Geophysical Union