Progress and Problems in Modeling Chemical Processing in Cloud Droplets and Wet Aerosol Particles

Water plays an important role in the atmosphere in form of cloud, fog or rain droplets and associated with aerosol particles because unique chemical reactions can occur in the aqueous medium. These reactions often lead to different products and take place on different time scales than in the surrounding air. Examples include the oxidation of sulfur dioxide (SO2) to sulfate (SO42-), the major contributor to 'acid rain', or the aqueous formation of secondary organic aerosol mass (aqSOA). Implementing chemical aqueous phase processes into large-scale models is a great challenge as often the resolution of such models is much coarser than the size of individual clouds and/or than the spatial and temporal scales, on which particle composition and water content change. This chapter summarizes the basics and most recent findings in the formation of sulfate and organic aerosol mass and how they are implemented in models. As many oxidation reactions of organic are initiated by OH radical, its source and sink reactions in the aqueous phase are also discussed.