A novel simplified ground-based thermal infrared (TIR) system for volcanic plume geometry, SO ₂ columnar abundance, and flux retrievals

Abstract. In the last few decades, volcanic monitoring using remote sensing systems has become an essential tool to investigate the effects of volcanic activity on environment, climate, human health, and aviation, as well as to give insights into volcanic processes. Compared to satellite measurements, ground-based instruments offer continuous spatial and temporal coverage capable of providing high-resolution and high-sensitivity data. This work presents a new simplified prototype of a thermal infrared (TIR) system (named “VIRSO2”). The instrument comprises three cameras, one working in the visible spectrum and two in the TIR (8–14 µm). In front of one of the two TIR cameras, an 8.7 µm filter is placed. The system is designed for the detection of volcanic emission, geometry estimation, determining the columnar content of SO2 and ash, and SO2 flux retrievals. The retrieval procedures developed are detailed starting from the geometric characterisation with wind direction correction, the calibration by considering the effects of filter multireflections and temperature, and the SO2 mass by exploiting MODTRAN radiative transfer model (RTM) simulations. The SO2 flux is then computed by applying the traverse method, with the plume speed obtained from the wind speed at the crater altitude. As test cases, the measurements collected at Etna Volcano (Italy) on 1 April 2021 during a lava fountain episode and 30 August 2024 during a quiescent phase have been considered. The results show that the system can provide reliable information on plume detection, altitude, and SO2 flux. Its simplicity, its low cost, and the possibility of carrying out measurements at a safe distance from the vent during both the day and the night make this system ideal for real-time monitoring of volcanic emissions, thus helping to provide information on the state of activity of the volcano and therefore to mitigate the effect that these natural phenomena have on humans and the environment.