Impacts of Volcanic Emissions on the Global Biogeochemical Mercury Cycle: Insights From Satellite Observations and Chemical Transport Modeling

Volcanism is the largest natural source of mercury (Hg) to the biosphere. However, past Hg emission estimates have varied by three orders of magnitude. Here, we present an updated central estimate and interquartile range (232 Mg a-1; IQR: 170-336 Mg a-1) for modern volcanic Hg emissions based on advances in satellite remote sensing of sulfur dioxide (SO2) and an improved method for considering uncertainty in Hg:SO2 emissions ratios. Atmospheric modeling shows the influence of volcanic Hg on surface atmospheric concentrations in the extratropical Northern Hemisphere is 1.8 times higher than in the Southern Hemisphere. Spatiotemporal variability in volcanic Hg emissions may obscure atmospheric trends forced by anthropogenic emissions at some locations. This should be considered when selecting monitoring sites to inform global regulatory actions. Volcanic emission estimates from this work suggest the pre-anthropogenic global atmospheric Hg reservoir was 580 Mg, 7-fold lower than in 2015 (4,000 Mg). Volcanism is widely recognized as the most important natural source of mercury (Hg) globally, but existing emissions estimates contain substantial uncertainty. This study combines satellite observations of sulfur dioxide (SO2) in volcanic plumes and measured Hg:SO2 ratios to quantify the magnitude and spatiotemporal variability of global volcanic Hg emissions. Using a global model, we show that the spatial pattern of volcanic releases and atmospheric dynamics result in greater concentrations of volcanic Hg in the mid-latitude Northern Hemisphere compared to the mid-latitude Southern Hemisphere. Modeling results suggest that variability in volcanic Hg emissions at some locations may obscure trends in atmospheric Hg concentrations driven by human emissions. The influence of volcanic Hg emissions should therefore be considered during selection of global monitoring sites used to track the progress of regulatory actions designed to mitigate Hg pollution. Volcanic release estimates from this work suggest the natural atmospheric Hg reservoir was similar to 7 times smaller than in 2015, reinforcing that humans have profoundly disrupted the global biogeochemical Hg cycle. Volcanic mercury emissions of 232 Mg a-1 (IQR: 170-336 Mg a-1) are estimated by indexing to sulfur dioxide from satellite remote sensingOver 90% of volcanic mercury emissions occur in the tropics and mid-latitude Northern HemisphereVolcanic emissions support a pre-anthropogenic atmospheric mercury reservoir of approximately 580 Mg (7-fold lower than in 2015)