Two-phase model of mercury chemistry in the atmosphere

A hybrid kinetic and equilibrium model simulating the conditions in clouds is developed to assess mercury chemistry in the atmosphere. Simulation of an air parcel containing cloud droplets is used to examine the effects of a number of physical and chemical parameters on the evolution of dissolved divalent mercury [Hg(II)] concentration profiles. Sensitivity analysis on each parameter is performed and the resulting Hg(II) profiles are plotted. The modeled steady-state Hg(II) levels are then compared to the measured values in precipitation. It is found that gaseous-phase oxidation of elemental mercury by ozone contributes a significant fraction of dissolved Hg(II) in the droplets, and that aqueous phase radical (i.e. OH and HO2) play an important role in mercury transformations. S(IV) is an important reductant for aqueous-phase Hg(II). However, the contribution of S(IV) in mercury chemistry is limited to the cases when aqueous phase S(IV) is present at a concentration to maximize HgSO3 formation. After the system reaches a steady state, HO2 is the only reductant to balance the Hg(II) production by various oxidation pathways in both phases. Based on the simulation results, it is suggested that more oxidation pathways should be identified to better describe mercury chemistry in the atmosphere.