Experimental evaluation of ion-induced nucleation in nanometer-aerosol formation by alpha-ray radiolysis of SO2/H2O/N-2 mixtures

A new system for experimental investigation of ion-induced nucleation in particle formation by alpha-ray radiolysis is developed. Particles formed by ion-induced nucleation are charged bipolarly, but particles formed by processes such as binary nucleation are electrically neutral. A new irradiation chamber was developed that uses a low electric field to prevent particle neutralization and preserve the charge distribution that results from alpha-ray radiolysis. The effects of the electric field on particle charging and radical generation were evaluated theoretically. From this analysis, it was determined that the optimum conditions for causing particle formation and preserving the charge distribution can be achieved by using a 0.92 MBq(241)Am source and an electric field of 2 kV m(-1) in the ionization chamber. Under these conditions, the charge distributions of particles formed by alpha-ray radiolysis in SO2/H2O/N-2 mixtures were measured for various SO2 and H2O concentrations. The charged-particle fraction decreased with increasing SO2 concentration and was nearly independent of the H2O concentration. It was determined from the results that SO2 mainly activated binary nucleation through SO2-OH radical reactions that produced H2SO4 and formed neutral particles. In contrast, H2O activated both ion-induced and binary nucleation because both water ion clusters and OH radical were produced from H2O molecules.