Atmospheric monomethylmercury: Inferred sources constrained by observations and implications for human exposure.

Monomethylmercury (MMHg) is a potent neurotoxin that poses a threat to human health. MMHg cycles in all spheres of the Earth but the sources and fate of atmospheric MMHg are unclear. Here, we develop a global model for atmospheric MMHg, which integrates the presently available data and indicates the limitations of the current study. Constrained by the observations in the atmosphere, the global atmospheric MMHg from all sources is 1009 (205-2474 as an uncertainty range) Mg/yr, with the largest sources from the in-cloud methylation of divalent mercury (475 Mg/yr) and MMHg sea spray (395 Mg/yr). MMHg has a short lifetime of 1.9 days in the troposphere due to rapid photo-demethylation. Our model indicates a net loss of marine MMHg to the atmosphere and thus a detoxifying effect on MMHg contamination in marine fish. However, it suggests additional MMHg deposition to the land, particularly in densely populated coastal areas, introducing a new risk pathway that needs to be considered in mercury exposure assessment. The atmosphere plays a non-negligible role in the biogeochemical cycle and human health, which requires further study and consideration in implementing the global Minamata Convention.