Near-source observations of bromine oxide indicate oxidation of magmatic gases at the magma-atmosphere interface

Volcanic gases carry information about subsurface processes. Decoding this information in gas measurements is essential for volcano hazard monitoring and fundamental geochemical research. Volcanic gas measurements are often interpreted under the assumption that these gases remain chemically inert upon emission to the atmosphere. Measurements of bromine oxide (BrO) in young volcanic plumes however contradict this interpretation since it is an oxidation product of hydrogen bromide (HBr) emissions.

We present model simulations of the high-temperature interface between magmatic gases and the atmosphere and simulate the plume evolution from emission into the first hours downwind. We refer to the early-stage plume evolution where magmatic gases enter the atmosphere and cool and dilute to atmospheric temperatures as the magma-atmosphere interface. For that purpose, we use a two stage box model based on chemical kinetics and a physical mixing and dilution parameterization. The mechanism simulates C-H-O-N-S chemistry and has sub-mechanisms for reactive halogens (Cl-Br) and mercury. The first stage exploits the analogs between combustion chemistry and early hot volcanic plumes and the second stage focusses on the multi-phase formation of BrO in the young plume (up to several kilometers downwind distance from the emission source).

The model is able to reproduce BrO observations in minutes old plumes from Mt Etna, which crucially depends on radical formation in the high-temperature plume stage only milliseconds after magmatic gas release to the atmosphere. The magma-atmosphere interface, also affects oxidation chemistry of other reduced trace gases emitted by the volcano such as molecular hydrogen (H₂) and carbon monoxide (CO), modifying thereby the magmatic gas redox state. This process is critically controlled by the magmatic gas emission temperature upon entering the atmosphere. The model furthermore explains the co-existence of reduced gases (H₂ and CO) with reactive halogens such as BrO as it is observed for example in the plume of Mt Etna. This evidences that magmatic gases are likely emitted several hundred Kelvin below the magma temperature.