The role of photo-induced solution reactions in determining atmospheric aerosol concentrations in discussed and is investigated with theoretical estimates. The main photo-induced solution reaction considered is the conversion of dissolved O3, into H2O2 which can then react with dissolved SO2 and NO2 to form sulfate and nitrate. The rate of this reaction is compared to the rate of H2O2 formation in solution from diffusion and absorption of H2O2 gas to the aerosol surface. The influences of H2O2 concentration and aerosol size on the relative rates are investigated. It is found that the photo-induced solution reaction rate is much lower than the gas diffusion rate but becomes more competitive as particle size increases.