Photodissociation Mechanisms of Major Mercury(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 and its oxidized forms Hg and Hg . The efficient gas-phase photolysis of Hg and Hg has recently been reported. However, whether the photolysis of Hg leads to other stable Hg species, to Hg , or to Hg and its competition with thermal reactivity remain unknown. Herein, we show that all oxidized forms of mercury rapidly revert directly and indirectly to Hg 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 , Hg , and Hg 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 being the main photoproduct of Hg photolysis in the atmosphere, which significantly increases the lifetime of this metal in the environment.