While terrestrial volcanism is thought to be an important natural primary source of mercury (Hg) to the atmosphere, the magnitude of volcanic Hg fluxes remains highly uncertain. Estimates 1 range from 45 Mg y -1 to 2000 Mg y , with the most recent study 2 suggesting Hg emissions of 179 ± 39 Mg y -1 and 20 ± 20 Mg y -1 (mean ± standard deviation) for passive and eruptive volcanic degassing, respectively. These estimates were mostly obtained by combining the estimated global volcanic SO2 flux with a Hg/SO2 emission ratio. Observed Hg/SO2 emission ratios, however, can vary by several orders of magnitude between different volcanic systems .