Interaction between SO2 and submicron atmospheric particles

In the atmosphere, oxidation of sulfur dioxide (SO2) to sulfate may occur i n the gas phase, in cloud or fog droplets, or in the aerosol phase on the s urface or inside aerosol particles. While aerosol phase reactions have been studied in the case of supermicron sea-salt and crustal particles, very fe w investigations regarding submicron particles are available. In this paper , the importance of aerosol phase sulfate production to the dynamics of sub micron particle populations was examined. The investigation was based on mo del simulations and theoretical evaluations regarding potential SO2 oxidati on reactions. None of the relatively well-quantified aqueous phase reaction s was rapid enough to make small nuclei grow to cloud condensation nuclei ( CCN) size within the particle lifetime in the lower troposphere. This is co nsistent with the few observations showing that the smallest atmospheric pa rticles are enriched in organics rather than sulfate. The amount of submicr on particulate matter could be enhanced significantly by certain aerosol ph ase reactions, but this is likely to require a particle population having a pH close to 7. Aerosol phase reactions could partly explain the apparently too low SO2-to-sulfate conversion rates predicted by several chemical tran sport models over polluted regions. In addition to the bulk aerosol phase, SO2-to-sulfate conversion might involve physical adsorption of SO2 or a com pound reacting with it by the particle surface, or it could take place in a liquid surface layer that usually covers atmospheric particles. Reactions involving physical adsorption seem to have negligible influence on the dyna mics of submicron atmospheric particle populations. Aerosol phase reactions worth future investigation are those occurring in particle surface layers and those occurring in cloud interstitial particles. (C) 2000 Elsevier Scie nce B.V. All rights reserved.