Mercury Physicochemical and Biogeochemical Transformation in the Atmosphere and at Atmospheric Interfaces: A Review and Future Directions

IntroductionMercury is a fascinating fluid metal that exists as liquid at 25 °C (0 °C = 273 K).This silver colored d-block heavy metal exhibits noble-gas-like characteristics and solidifies at a temperature of -38.83 °C.(1) Elemental mercury is the only metal that is liquid at room temperature and has an “ideal” high-energy surface. The average surface tension value of mercury based on different experimental measurements is 466 ± 33 mN m -1 .(2) Due to the versatility of mercury, it is used as a catalyst, as an electrode material in electrochemistry, in optical spectroscopy, as reflective liquid in liquid mirror telescopes, and in medicine, e.g., as amalgam in dental fillings. Yet, due to the varying, but significant, acute and chronic toxicity of mercury compounds, its usage has been of recent concern. Among single events with a large number of human casualties were the Minamata incident due to the release of highly toxic MeHg in the industrial wastewater from the 1930s to the 1960s in Minamata, Japan, and, in 1971 the Iraqi disaster, where organo-mercury fungicide treated seed grains were consumed by humans. Medical symptoms of mercury poisoning vary from parathesia to ataxia, loss of vision, insanity, paralysis coma, congenital deformities affecting fetuses, and death.(3) Because of the urgency to limit mercury related disasters, in 2013 an international treaty was signed by 128 countries, to control and “where feasible” reduce atmospheric emissions from point source categories (e.g., coal fired power plants and smelters). The treaty also called for additional mercury research.(4)The atmosphere is the fastest moving fluid in the Earth’s environment.Many mercury species have appreciable vapor pressures at Earth surface temperatures, allowing them to enter the atmosphere and, depending on the mercury species involved, undergo long-and short-range transport, atmospheric physical and chemical transformation, as well as feedback with Earth’s surface (see Figure 1), which is the main subject of this review paper.