Pre- and post-eruptive gas composition measurements at Nyiragongo Volcano, East Africa, using a portable Multi-GAS

Nyiragongo Volcano is located in the western branch of the East African Rift, in eastern Democratic Republic of Congo (DRC). Nyiragongo has gained fame thanks to its long-lived lava lake and the significant hazards it presents to the >1.5 million people living within <30 km, particularly the inhabitants of Goma (DRC) and Gisenyi (Rwanda).&#160; These hazards include fast-moving lava flows produced during effusive eruptions (e.g., in 1977, 2002, and 2021) and regional environmental and health impacts from the persistent volcanic gas plume.Continuous gas emissions at Nyiragongo present an opportunity for geochemical monitoring of the volcano during changes in activity, such as the recent 2021 flank eruption.&#160; From late 2019 to the present (early 2023), we monitored plume H2O-CO2-SO2-H2S compositions at the summit and at the May 2021 flank vent using a portable multi-GAS (multiple Gas Analyzer System). Pre-eruption measurements from 2019 on the crater rim of Nyiragongo reveal two geochemical end-members: the lava lake gas plume characterized by CO2/SO2 ratios of 50-60 and crater fumaroles characterized by CO2/SO2 ratios up to 300 and a trace of H2S. Overall, the bulk composition of the summit gases was 39.31-70.34% H2O, 29.52-59.93% CO2 and 0.14-0.76% SO2, within the range of plume compositions reported by Gerlach (1979). At the May 2021 eruptive vent, located ~3 km from the summit on the volcano&#8217;s southern flank, measurements in August 2021 (after the eruption had ceased) reveal more C-poor gases with a CO2/SO2 ratio of 21.90 and a bulk composition of 95.41% H2O, 4.35% CO2 and 0.32% SO2. The reappearance of the lava lake in the main summit crater in late September 2021 was accompanied by a decrease in the CO2/SO2 ratio to 5 at the May 2021 eruptive vent.&#160; These results show that measuring the changes in plume composition by multi-GAS improves monitoring of Nyiragongo. In the future, we plan to deploy a permanent multi-GAS station at the summit of Nyiragongo to carry out much needed continuous geochemical monitoring.&#160;Key words: Nyiragongo Volcano, volcanic gases, volcano monitoring, multi-GAS, Plume chemistry