Reactive halogens drive rapid mercury oxidation in hot volcanic emission-plumes

Alexander Nies , Jonas Kuhn, Bastien Geil, Jeroen Sonke , Nicole Bobrowski, Guillaume Dayma, Thorsten Hoffmann, Luke Surl, Tjarda J. Roberts

Volcanoes release mercury (Hg) alongside gases and particles through continuous passive degassing and by explosive and effusive eruptions. The global dispersion and environmental impact depend on volcanic Hg oxidation state, which remains poorly constrained: thermochemical calculations of magmatic gas composition and crater-rim sampling indicate Hg to be predominantly in the reduced Hg0 form that is relatively inert and disperses into the global atmosphere. However, several near-source plume observations report HgII forms that can be deposited locally and regionally, associated with important HgII uptake by volcanic ash. Oxidation of near-source volcanic plume Hg0 into HgII is largely assumed to occur via photolytic atmospheric halogen chemistry. We tested this hypothesis in the fumarole field of Vulcano (Italy) by sampling Hg0 and HgII at multiple distances from the fumarole source during day and night. Substantial (up to ~65%), near-source (< 50 cm, < ~1 s downwind) HgII detected during day and night rules out a significant role of photochemistry. Reanalysis of near-downwind (~minutes) ash-bound Hg datasets from explosive eruptions of Mt. Redoubt, Mt. Spurr, and Augustine volcano (Alaska, USA) supports fast oxidation during both day and night. Model simulations of chemical reactions in the hot, rapidly cooling volcanic plume identify high-temperature chemistry as a viable non-photolytic Hg oxidation mechanism that generates ~10% to ~80% HgII/Hgtot within seconds following emission (but < 6% for halogen-poor emissions). Rapid high-temperature chemical processing therefore determines the oxidation state - and hence environmental fate - of volcanic Hg emissions into the atmosphere today and in the geological past.