Photodissociation mechanisms of major Hg(II) species in the atmospheric chemical cycle of mercury.

Mercury is a contaminant of global concern that is transported throughout the atmosphere as elemental mercury Hg-0 and its oxidized forms Hg-I and Hg-II. The efficient gas-phase photolysis of Hg-II and Hg-I has recently been reported. However, whether the photolysis of Hg-II leads to other stable Hg-II species, to Hg-I, or to Hg-0 and its competition with thermal reactivity remain unknown. Herein, we show that all oxidized forms of mercury rapidly revert directly and indirectly to Hg-0 by photolysis. Results are based on non-adiabatic dynamics simulations, in which the photoproduct ratios were determined with maximum errors of 3%. We construct for the first time a complete quantitative mechanism of the photochemical and thermal conversion between atmospheric Hg-II, Hg-I, and Hg-0 compounds. These results reveal new fundamental chemistry that has broad implications for the global atmospheric Hg cycle. Thus, photoreduction clearly competes with thermal oxidation, with Hg-0 being the main photoproduct of Hg-II photolysis in the atmosphere, which significantly increases the lifetime of this metal in the environment.