Quantification of low-temperature gas emissions reveals CO 2 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 SO 2 and HCl concentrations within a larger CO 2 -rich plume. Acid-rich concentration spikes matched the distribution of high-temperature fumaroles, whereas CO 2 is emitted broadly from high- and low-temperature fumaroles. The CO 2 flux was 15 to 41 kilograms per second from hot fumaroles and 61 to 131 kilograms per second for the overall plume. The typical CO 2 flux calculation of multiplying CO 2 /SO 2 ratio with SO 2 flux underestimates total CO 2 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 SO 2 lost. This study highlights the importance of precise acid-gas measurements for detecting heterogeneous degassing and suggests that traditional CO 2 flux measurements may substantially underestimate contributions from cold CO 2 degassing, leading to underestimated global volcanic fluxes.