Effect of NO2 on particle formation in SO2 /H2O/air mixtures by ion-induced and homogeneous nucleation

Effects of NO2 on particle formation in SO2/H2O/Air mixtures by alpha-ray irradiation as a source of hydroxyl radicals are investigated to evaluate the contribution of homogeneous and ion-induced nucleation in the nanometer-sized aerosol particle generation. The total particle number concentration, the charged-particle fraction, and the electrical mobility distribution of particles and hydrated and solvated ions generated in NO2/SO2/H2O/Air mixtures were measured at NO2 levels of 0-1.98 ppm, SO2 levels of 0-3.87 ppm, and H2O levels of 6480-9889 ppm. Total particle number concentration first tends to increase with increasing NO2 concentration up to a certain NO2 level and then decreases for NO2 concentrations beyond this point. The charged-particle fraction, indicative of particle formation by ion-induced nucleation, tends to decrease with increasing NO2 concentration, reaching a minimum value. The peak intensity of the electrical mobility distribution for particles significantly exceeded that in the absence of NO2 at a relatively low concentration of NO2. The intensity decreased with increasing NO2 concentration and fell below that in the absence of NO2. The results indicate that 2 opposing mechanisms that enhance and suppress the particle formation work simultaneously in NO2/SO2/H2O/Air mixtures, and the dominant mechanism depends on the NO2 concentration.