Variation of volcanic gas composition at a poorly accessible volcano: Sakurajima, Japan

• Published in: Journal of volcanology and geothermal research 2020-12, Vol.407, p.107098, Article 107098
• Main Authors: Shinohara, H., Kazahaya, R., Ohminato, T., Kaneko, T., Tsunogai, U., Morita, M.
• Format: Article
• Language: English
• Subjects: Degassing; Multi-GAS; Sakurajima volcano; Volcanic gas composition
• ISSN: 0377-0273; 1872-6097
• DOI: 10.1016/j.jvolgeores.2020.107098

Volcanic gas compositions are estimated based on volcanic plume measurement with Multi-GAS at a frequently erupting Sakurajima volcano by application of airborne methods and automatic monitoring station on the flank during 2013–2019. The airborne measurements of CO2 and H2O concentrations often suffer from large fluctuation of background likely due to entrainment of the atmosphere derived from different altitude. By removing the background effect, a representative volcanic CO2/SO2 mole ratio is estimated as 0.4 ± 0.1 and we did not observe clear temporal variation. The volcanic H2O/SO2 ratios are estimated as 30–70 with a large uncertainty because of the strong atmospheric perturbation. The SO2/H2S ratios show quite a large variation ranging from 1 to 800. The large SO2/H2S ratios larger than 30 are observed during ash eruption and are suggested as a result of oxidation by atmosphere on hot ashes. The small ratios less than 10 are observed when frequency of explosions is low. Variation of the SO2/H2S ratios of the small values can be caused by variation of degassing pressure, however, the small variation of the CO2/SO2 ratio suggests a limited range of the pressure variation. •Multi-GAS volcanic gas measurements performed at the inaccessible Sakurajima volcano.•Cessna-borne, UAV-borne and automatic station measurements are combined.•Low altitude atmosphere entrainment largely perturbed the results.•Volcanic CO2/SO2 and H2O/SO2 ratios are estimated as 0.4 and 30.•Large SO2/H2S ratio variations of 1–800 with large values during ash eruptions.