MEASUREMENT AND MODELING OF PARTICLE NITRATE FORMATION

Nighttime measurements of particle number distribution and mass compos ition, and concentrations of NO2, O-3, NH3, and HNO3, were made at nig ht at a rural site in Ontario in August of 1992. A simple model of par ticle growth was constructed to simulate the observed rapid growth of aerosol mass in the 0.2 to 0.5 mu m diameter range. Both measurements and model results indicate that the growth of accumulation mode aeroso l mass was due to condensation of the nitrate radical, HNO3, and NH(3 )onto particles, with the formation of particle ammonium nitrate. The results show that the reaction of O(3 )with NO2 in the isolated noctur nal boundary layer can lead to the production of gas-phase nitric acid . When this occurs in the presence of local ammonia emissions and pree xisting particles, rapid growth of particle ammonium nitrate takes pla ce. The model results show that most of the observed variations can be accounted for by a coupled system of equations including dynamical an d thermodynamic effects. The dynamical approach to equilibrium is suff iciently fast that the gas-phase nitric acid concentrations are more s ensitive to the magnitude of the thermodynamic equilibrium concentrati on than the dynamical time constant. A simple parameterization for the effects of sulphate on particle nitrate formation was developed and s hown to provide a good estimate of the equilibrium concentration of ga s-phase nitric acid.