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 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.