Volcanic vents – OH mixing ratios as in a Bunsen burner flame?

Volcanic gas emissions influence the composition of the atmosphere and therefore also our climate. For some gas species, volcanoes represent even the most important natural source. In atmospheric research, however, volcanoes are often neglected as an important source of many gas species and are still little studied, partly also because of the challenges in terms of the necessary technology and logistics.In volcanic gas mixtures near the source, very high OH mixing ratios (ppb-ppm) are often assumed, usually based on thermodynamic equilibrium calculations (e.g. Gerlach, 2004). However, no OH measurements have been successfully performed in such environments.Here we report on CO/CO2 measurements in the downwind volcanic gas plume of Mount Etna, which were obtained by taking air core samples with an UAV and analysed immediately afterwards with Cavity Ring-Down Spectroscopy in July 2024 (T. Schuck et al., 2025). We relate these results to previous near-source CO/CO2 emission measurements and to calculated CO/CO2 emission ratios from petrological studies. The observed change of more than two orders of magnitude in the CO/CO2 ratio can only be explained by the oxidation of CO and therefore allows us to estimate the amount of OH necessary to explain the high proportion of CO oxidation. Our estimate based on kinetic chemistry, happening after the first seconds of the gas release, leads indeed to results of unusual high amounts of OH in the source region.Gerlach, T. M. (2004). Volcanic sources of tropospheric ozone‐depleting trace gases. Geochemistry, Geophysics, Geosystems, 5(9).Schuck, T., Degen J., Bobrowski, N., Metais-Bossard M., Boucher L., Chen, H., Geil, B.H., Giuffrida, G.B. van Heuven, S.,  Hoffmann, T., Ortenzi G., and Engel A. (2025). First deployment of a drone-borne active AirCore in a volcanic plume at Mount Etna, submitted to EGU