Atmospheric mercury: Recent advances in theoretical, computational, experimental, observational, and isotopic understanding to decipher its redox transformations in the upper and lower atmosphere and interactions with Earth surface reservoirs

Abstract

Mercury is a volatile heavy element with no known biological function. It is present in trace amounts (on average, ∼ 80 ppb) but is not geochemically well blended in the Earth's crust. As a result, it occurs in extremely high concentrations (up to a few percent) in certain locations. It is found along tectonic plate faults in deposits of sulfide ores (cinnabar), and it has been extensively mobilized during the Anthropocene. Mercury is currently one of the most targeted global pollutants, with methylmercury compounds being particularly neurotoxic. Over 5000 t of mercury is released into the atmosphere annually through primary emissions and secondary re-emissions. Much of the re-emitted mercury resulting from exchanges with surface reservoirs is related to (legacy) human activities, such as direct releases. Understanding the dynamics of the global Hg cycle is critical for assessing the impact of emission reductions under the UN Minamata Convention, which became legally binding in 2017. This review of atmospheric mercury focuses on fundamental advances in field, laboratory, and theoretical studies, including six stable Hg isotope analytical methods, which have contributed recently to a more mature understanding of the complexity of the atmospheric Hg cycle and its interactions with the Earth's surface ecosystem.