An Automatic Procedure for Volcanic SO2 Flux Retrieval using TROPOMI L2 Products

In remote areas, ground-based measurements are often scarce or non-existent. Therefore, accurate global spatial coverage is necessary for effective volcanic surveillance, making satellite measurements using Remote Sensing (RS) systems crucial resources for real-time monitoring of volcanoes and for the analysis of their activity. This work presents the development of an automatic procedure for the retrieval of sulphur dioxide (SO₂) fluxes, exploiting Near Real-Time (NRT) Level 2 (L2) products from the TROPOspheric Monitoring Instrument (TROPOMI), an imaging spectrometer on board the Sentinel-5P polar satellite. The core processing, implemented in Python, involves the vertical interpolation of SO₂ Vertical Column (VC) products (provided at 1, 7 and 15 km), based on the mean plume altitude, extracted from the TROPOMI L2 layer height data product. The raw satellite data are resampled in a uniform grid, and georeferenced using the Universal Transverse Mercator (UTM) projection, to correct for spatial distortions. The SO₂ flux is then computed by integrating the VCs with the wind speed profiles acquired from the nearest available radiosonde station on the day of the event. This modified version of the standard traverse method uses concentric circular transects to ensure independence from the wind direction. The developed procedure is applied to three different 2025 test-case eruptions: Etna (Italy), Nyamulagira (Democratic Republic of the Congo) and Hayli Gubbi (Ethiopia), with the aim of reconstructing the time series of their emission. The method is validated by comparing, for Etna, the satellite-derived fluxes to the ground-based measurements acquired by the FLux Automatic MEasurement (FLAME) network, a series of Ultraviolet (UV) ground-based scanning spectrometers installed around the volcano. The results confirm the validity of the approach and demonstrate the tool capability to perform a quick and automatic assessment of volcanic activity around the world, providing reliable information that can be used to mitigate the impact of these natural phenomena.