Application of nucleation to the study of the gas phase photo-oxidation of sulfur dioxide

The photo-oxidation of SO2 by disproportionation has been investigated by excitation in the singlet absorption band (λ=305 nm). The experiments were conducted in a diffusion cloud chamber in which the water, used as the working fluid, was maintained at a maximum activity of 2.65. The rate of SO3 production was directly related to the observed rate of nucleation through our previous study on binary nucleation in H2SO4–H2O mixtures. Our results lead to a rate constant of SO2 disproportionation k1a= (8.4±2) ×109 l mole−1s−1, value which agrees with that of the recent study by Marvin and Reiss (if one corrects their results to take into account the pressure saturation quenching effect), but is 200 times larger than the previous values determined in the absence of water vapor.