Quantification of low-temperature gas emissions reveals CO2 flux underestimates at Soufrière Hills volcano, Montserrat

: We performed helicopter-borne optical MultiGAS measurements of volcanic gas emissions from Soufrière Hills Volcano, Montserrat, revealing distinct spikes in SO2 and HCl concentrations within a larger CO2-rich plume. Acid-rich concentration spikes matched the distribution of high-temperature fumaroles, whereas CO2 is emitted broadly from high- and low-temperature fumaroles. The CO2 flux was 15 to 41 kilograms per second from hot fumaroles and 61 to 131 kilograms per second for the overall plume. The typical CO2 flux calculation of multiplying CO2/SO2 ratio with SO2 flux underestimates total CO2 flux by at least threefold. We quantified substantial magmatic gas scrubbing by the hydrothermal system, with 56 to 79% of initial HCl and 33 to 68% of initial SO2 lost. This study highlights the importance of precise acid-gas measurements for detecting heterogeneous degassing and suggests that traditional CO2 flux measurements may substantially underestimate contributions from cold CO2 degassing, leading to underestimated global volcanic fluxes.