Effect of NO 2 on Particle Formation in SO 2 /H 2 O/Air Mixtures by Ion-Induced and Homogeneous Nucleation

Abstract

Effects of NO₂ on particle formation in SO₂/H₂O/Air mixtures by f-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.