MODELING OF THE NIGHTTIME NITROGEN AND SULFUR CHEMISTRY IN SIZE RESOLVED DROPLETS OF AN OROGRAPHIC CLOUD

A chemistry module has been incorporated into a Lagrangian type model that computes the dynamics and microphysics of an orographical cloud f ormed in moist air flowing over the summit of Great Dun Fell (GDF) in England. The cloud droplets grow on a maritime aerosol which is assume d to be an external mixture of sea-salt particles and ammonium-sulfate particles. The dry particle radii are in the range 10 nm < r < 1 mum. The gas-phase chemical reaction scheme considers reactions of nitroge n compounds that are important at night. The treatment of scavenging o f gases into the aqueous phase in the model takes into account the dif ferent solubilities and accommodation coefficients. The chemistry in t he aqueous phase focusses on the oxidation of S(IV) via different path ways. Sensitivity analyses have been performed to investigate deviatio ns from gas-liquid equilibria according to Henry's law and also to stu dy the influence of iron and of nitrogen compounds on the aqueous-phas e oxidation of dissolved SO2. When addressing these questions, special attention has been given to the dependence on the droplet size distri bution and on the chemical composition of the cloud condensation nucle i on which the droplets have formed. It was found that the oxidation o f S(IV) via a chain reaction of sulfur radicals can be important under conditions where H2O2 is low. However, major uncertainties remain wit h respect to the interaction of iron with the radical chain. It was sh own that mixing of individual cloud droplets, which are not in equilib rium according to Henry's law, can result in a bulk sample in equilibr ium with the ambient air. The dependence of the aqueous-phase concentr ations on the size of the cloud droplets is discussed for iron, chlori de and NO3.