Hourly SO2 emissions and plume dispersion simulated by inverse modelling using TROPOMI, OMPS, IASI, and ground-based LIDAR observations: case studies of the 2021 Etna and 2018 Ambrym eruptions
Résumé
Volcanic sulphur dioxide (SO2), a precursor of sulphate aerosols, can have a deleterious impact on the atmosphere, ecosystems, and air quality at multiple scales. Knowledge of highly variable volcanic SO2 emissions, i.e., mass flux rates and injection heights, would not only aid comprehension of such atmospheric implications but would also provide information on subterranean volcanological processes of magma transport. Furthermore, volcanic SO2, which frequently co-exists in volcanic plumes with ash and sulphate aerosols, can pose a threat to aviation as ash and acidic aerosols are alarming to aircraft. Therefore, comprehensive knowledge of volcanic SO2 emissions is essential for a thorough evaluation of near-source volcanic hazards and large-scale atmospheric impacts.
Hyper-spectral nadir-viewing UV and infrared satellite instruments record global SO2 mass-loadings on a daily or bi-daily basis. Geostationary sensors, on the other hand, deliver high temporal information on SO2 emissions but with much lower sensitivity. Consequently, there are still gaps in our knowledge of volcanic SO2 emissions and SO2 to sulphate oxidation rates, notably inside tropospheric plumes, and hence volcanic sulphur-rich compound feedback on the atmosphere.
TROPOMI, a hyperspectral UV sensor with increased spatial and spectral resolution than that of the pre-existing UV (OMPS) sensor in the same orbit, was launched in 2017. We discuss how an inverse modelling approach that assimilates TROPOMI SO2 column amounts (CA) improves the retrieval of hourly SO2 emissions when compared to the assimilation of OMPS data acquired at approximately the same time and the new SO2 products (both SO2 CA and layer heights) from IASI. The purpose of using IASI data is to assess the impact of assimilating SO2 data available bi-daily into inverse modelling with additional information on SO2 layer height. The inverse modelling is performed utilizing a time series of daily or bi-daily SO2 CA snapshots from the TROPOMI, OMPS, and IASI satellite instruments, respectively. Contrary to OMPS, which has 50x50 km2 of spatial resolution, and IASI, which has a 12 km circular footprint, TROPOMI has an extraordinary spatial resolution of 5.5x3.5 km2 (7x3.5 km2 before August 2019). We find that because of their sensitivity to low-level SO2 fluxes and thin SO2 plumes, and the numerous SO2-rich pixels defining dense parcels, TROPOMI observations enable better evaluation of SO2 degassing during paroxysmal eruption phases, offering better-resolved SO2 emissions by inverse modelling. However, if meteorological clouds hide the volcanic SO2 plumes, the results can be inconsistent, especially if the clouds are near the source. So the additional data, the SO2 height product from IASI observations, is used to reconcile and offer more robust SO2 emissions. As a second step, we perform inverse modelling using both the SO2 CA and layer heights from IASI. This research investigates the Mount Etna eruption in February 2021, the SO2 plume reaching France, and the 2018 Ambrym eruption, which was the top world-ranking SO2 emitter. In the context of Etna eruption, we use ground-based OHP LIDAR aerosol height measurements to explore the presence of sulphate aerosols and their height in the SO2 plume.