Fifty years of volcanic mercury emission research: Knowledge gaps and future directions.

Volcanism is a potentially important natural source of mercury (Hg) to the environment. However, its impact on the global Hg cycle remains poorly understood despite advances over the last five decades. This represents a major uncertainty in our understanding of the relative contributions of natural and anthropogenic Hg sources to the global atmosphere. This uncertainty, in turn, impacts evaluation of the effectiveness of policies to mitigate the impact of anthropogenic Hg on the environment. Here we critically review recent progress in volcanic Hg emission research, including advances in sampling methods and understanding of the post-emission behavior of Hg in the atmosphere. Our statistical analysis of the limited available data shows that the plumes of non-arc volcanoes exhibit significantly higher Hg concentrations than arc volcanoes, yet the latter emit 3-fold higher Hg fluxes on average. Arc volcanism also dominates volcanic gas emissions globally, indicating that arc volcanoes should be a priority for future Hg emission research. We explore several methodological challenges that continue to hinder progress in quantifying global volcanic Hg emissions, and discuss the importance of longer time-frame data collection to capture temporal variations in emissions. Recommendations are proposed for working toward a more accurate assessment of the global volcanic Hg flux. A detailed summary of all published volcanic Hg emissions data worldwide is also presented as a reference tool for future work.