Abstract:New particle formation phenomena in the troposphere have received considerable attention, but the exact mechanism controlling the early stages of aerosol nucleation and growth is still not understood. In this paper, soft X-ray radiolysis of SO2/H2O/Air gas mixtures is carried out to investigate the competitive processes of ion-induced and binary homogeneous nucleation, experimentally. The electrical mobility distributions of newly formed particles were measured with a scanning mobility particle sizer (SMPS). Charged particles in the system are indicative of ion-induced nucleation; uncharged particles are produced by homogeneous nucleation. Ion, which mainly contributed to aerosol nucleation process, had nearly no effect on aerosol growth process. In contrast, SO2 enhanced both aerosol nucleation and growth process. The electrical mobility distribution of nanometer-sized particles appeared to be bimodal peaks. The position of big ones would shift with the flow rate of the gas mixture. At low SO2 concentrations, ion-induced nucleation mechanism may be a dominant factor; however, at increased SO2 concentrations the contribution of binary homogeneous nucleation relative to that of ion-induced nucleation increases accordingly.