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 are unknown. In this Research article, we show that all oxidized forms of mercury rapidly revert directly and indirectly to Hg(0) via photolysis. Results are based on non-adiabatic dynamics simulations, in which the photoproduct ratios are 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 compete with thermal oxidation being Hg(0) the ultimate fate of Hg in the atmosphere, which significantly increases the lifetime of this metal in the environment.