Environmental Context. Elemental mercury (Hg⁰) is converted to divalent mercury (Hg²⁺) in the atmosphere, largely in water droplets. The wet deposition of Hg²⁺ is a major concern to human health. Because it is bio-accumulated through the food chain, consumption of contaminated fish can be particularly dangerous. Currently the budgets of mercury in the atmosphere are poorly understood, due in part to uncertainties in the chemical pathways controlling the speciated forms of mercury. Improved mercury chemistry models are needed to better predict Hg²⁺ levels in water droplets and to estimate wet deposition of Hg²⁺ in order to help assess the potential health risks of mercury.

Abstract. A box model designed to investigate mercury chemical mechanisms in the atmosphere is presented. Aqueous-phase mercury oxidation–reduction, sulfite, and oxygen reactions, along with gas-phase mercury reactions are included in the model. The model is used to evaluate the key reaction steps under several atmospheric conditions. The sensitivity of the results to parameters, initial conditions, and assumptions regarding chemical mechanisms are investigated. Model simulations were performed in closed (liquid only) and two-phase (liquid/gas) systems. In the liquid phase, elemental mercury is oxidized to divalent mercury by ozone at night and by hydroxide radical and ozone during the day. Ozone is shown to play a significant role in mercury mechanisms by oxidizing elemental mercury directly and producing hydroxyl and hydroperoxyl radicals. The effects of sulfite on mercury aqueous chemistry were found to be limited due to its rapid conversion to sulfate. The results of two-phase simulations show that Hg²⁺ concentrations exhibit a diurnal cycle, increasing before the sunrise and decreasing before the sunset due to the aqueous-phase hydrogen–oxygen photochemistry that produces hydroxyl and hydroperoxyl radicals. Gas-phase mercury oxidation reactions significantly enhance the levels of Hg²⁺ in the aqueous phase, and reactions with ozone and hydroxyl radicals play the leading role. The effect of reactive chlorine on mercury chemistry can be important.