Mechanisms of sulphate aerosol production in clouds: effect of cloud characteristics and season in the Indian region

Abstract

Measurements made during the Indian Ocean Experiment (INDOEX) in 1998, indicate likely regional atmospheric effects of sulphate aerosol over India including the potential for cloud processing of SO₂ to sulphate. Sulphate aerosol production in clouds was examined for cloud characteristics and pollutant concentrations typical of the Indian region, to assess the contribution of different mechanisms to sulphate formation. A simple model was formulated incorporating gas-liquid equilibria, gas-phase mass transfer, SO₂ oxidation in the gas phase by OH• radicals and in the aqueous phase by H₂O₂, O₃ and O₂ catalysed by Fe³⁺ and Mn²⁺ in a cloud with uniformly sized drops. Sulphate formation was simulated in St/Sc, Cu and Cb clouds using an initial pH of 6.5, likely to occur in this region, for cloud drop diameters of 10, 50 and 100 μm. The clouds were assumed to have nucleated on ammonium sulphate aerosols and contained reported background concentrations of these ions in clean rainwater over the Indian ocean. The simulations produced final cloud pHs of 4.5–5 and final sulphate concentrations of 25–90 μM, in agreement with the range of reported preliminary measurements. An examination of mass transfer effects showed that the characteristic diffusion times of H₂O₂ and SO₂ were lower by a factor of 10 or more than the respective reaction times, indicating that mass transfer effects would not limit the rate of S_VI formation under likely atmospheric conditions. Sulphate formation in St/Sc clouds was shown to likely be SO₂ limited in July and H₂O₂ limited in January. In all cases, the gas phase reaction and the O₂ reaction catalysed by Mn²⁺ and Fe³⁺ contributed less than 1% to the SVI produced. In St/Sc and Cu clouds, 85–96% of the S_VI was from the H₂O₂ reaction. In Cb clouds about 60–75% of the S_VI was from the H₂O₂ reaction and 25–41% from the O₃ reaction. In Cb clouds, the short residence and the persistence of alkaline conditions in the earlier part of the cloud cycle, enhance the contribution of the O₃ reaction to the greatest extent. If alkaline conditions were to persist throughout the cloud cycle (from cation chemistry not simulated in this model), the contribution of the O₃ reaction to S_VI could be further enhanced.