Water Vapor Detection in Volcanic Plumes: Near-Infrared Cameras applications at Lascar, Chile, and Litli Hrútur, Iceland

Abstract. Water vapor (H₂O) dominates volcanic gas emissions globally with > 70 mol% of total volatile discharge, yet accurate H₂O flux measurements remain challenging due to high atmospheric background and H₂O’s spectroscopic and physical complexities. We developed a multi-band near-infrared (NIR) camera system, calibrated with an in-situ Multi-GAS instrument to quantify volcanic H₂O flux. By combining plume speed measurement with H₂O absorption data, we derived the H₂O fluxes under favorable atmospheric conditions. We tested our approach at two contrasting volcanic settings: the passively degassing, high altitude, arid atmosphere Lascar volcano (Chile), and at the actively erupting, sea-level, humid atmosphere Fagradalsfjall volcano (Iceland) during the Litli Hrútur 2023 eruption. In November 26–29, 2022, and December 29, 2024, Lascar emitted 23,115 ± 10,694 t d^-1 and 46,891 ± 18,863 t d^-1 of H₂O, respectively, higher than previous estimates using traditional SO₂-based methods. In July/August 2023, the Litli Hrútur eruption averaged 19,108 ± 7,560 t d^-1 of H₂O emissions, matching petrological estimates and steadily declining towards the end of the eruption. The simultaneous deployment of NIR camera, miniDOAS, and a UV camera prove that H₂O and SO₂ emissions vary independently, with Multi-GAS H₂O/SO₂ ratios fluctuating over time. This variability challenges traditional measurements and demonstrates that independent direct measurements of major gases (H₂O, CO₂, and SO₂) are essential for accurate volatile budgets and understanding volcanic degassing processes. Our work shows that the NIR camera approach provides a high-rate near-real time and direct method to obtain and visualize H₂O emission rates.