Effects of NO2 gas on the particle formation in SO2/H2O/Air mixture by a-ray radiolysis are studied experimentally. When NO2 gas is added to the SO2/H2O/Air mixture, particle number concentrations increase and the charged-particle fractions of newly borne particles are higher than 0.85. These results suggest that HNO3 is not produced by the reaction between NO2 and OH radical and the particle generation is not reduced by the production of HNO3. In order to elucidate whether HNO3 is produced or not, NH3 gas is added to NO2/H2O/Air mixture. Particles appear upon the addition of NH3, but they are not generated in its absence. All particles generated upon the addition of NH3 have a charge. It is found from the results that particles are formed by the ion-induced nucleation in the NH3/NO2/H2O/Air mixture, but not by the homogeneous nucleation of NH4NO3 produced by the reaction between HNO3 and NH3. The above results suggest that particle generation in the NO2/SO2/H2O/Air mixture by a-ray radiolysis is caused by ion-induced nucleation following the ion-molecule reactions, but HNO3 is not produced by the reaction of NO2 and OH radical.