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
Authors: Behera Abhinna , Boichu Marie , Thieuleux François , Hioki Souichiro , Clarisse Lieven , Khaykin Sergey , Xueref-Remy Irène , Popovici Ioana , Goloub Philippe 
Published: March 2022

&lt;p&gt;Volcanic sulphur dioxide (SO&lt;sub&gt;2&lt;/sub&gt;), 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 SO&lt;sub&gt;2&lt;/sub&gt; 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 SO&lt;sub&gt;2&lt;/sub&gt;, 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 SO&lt;sub&gt;2&lt;/sub&gt; emissions is essential for a thorough evaluation of near-source volcanic hazards and large-scale atmospheric impacts.&lt;/p&gt;&lt;p&gt;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 SO&lt;sub&gt;2&lt;/sub&gt; emissions but with much lower sensitivity. Consequently, there are still gaps in our knowledge of volcanic SO&lt;sub&gt;2&lt;/sub&gt; emissions and SO&lt;sub&gt;2&lt;/sub&gt; to sulphate oxidation rates, notably inside tropospheric plumes, and hence volcanic sulphur-rich compound feedback on the atmosphere.&lt;/p&gt;&lt;p&gt;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 SO&lt;sub&gt;2&lt;/sub&gt; column amounts (CA) improves the retrieval of hourly SO&lt;sub&gt;2&lt;/sub&gt; emissions when compared to the assimilation of OMPS data acquired at approximately the same time and the new SO2 products (both SO&lt;sub&gt;2&lt;/sub&gt; 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 SO&lt;sub&gt;2&lt;/sub&gt; layer height. The inverse modelling is performed utilizing a time series of daily or bi-daily SO&lt;sub&gt;2&lt;/sub&gt; CA snapshots from the TROPOMI, OMPS, and IASI satellite instruments, respectively. Contrary to OMPS, which has 50x50 km&lt;sup&gt;2&lt;/sup&gt; of spatial resolution, and IASI, which has a 12 km circular footprint, TROPOMI has an extraordinary spatial resolution of 5.5x3.5 km&lt;sup&gt;2&lt;/sup&gt; (7x3.5 km&lt;sup&gt;2&lt;/sup&gt; before August 2019). We find that because of their sensitivity to low-level SO&lt;sub&gt;2&lt;/sub&gt; fluxes and thin SO&lt;sub&gt;2&lt;/sub&gt; plumes, and the numerous SO&lt;sub&gt;2&lt;/sub&gt;-rich pixels defining dense parcels, TROPOMI observations enable better evaluation of SO&lt;sub&gt;2&lt;/sub&gt; degassing during paroxysmal eruption phases, offering better-resolved SO&lt;sub&gt;2&lt;/sub&gt; emissions by inverse modelling. However, if meteorological clouds hide the volcanic SO&lt;sub&gt;2&lt;/sub&gt; plumes, the results can be inconsistent, especially if the clouds are near the source. So the additional data, the SO&lt;sub&gt;2&lt;/sub&gt; height product from IASI observations, is used to reconcile and offer more robust SO&lt;sub&gt;2&lt;/sub&gt; emissions. As a second step, we perform inverse modelling using both the SO&lt;sub&gt;2&lt;/sub&gt; 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 SO&lt;sub&gt;2&lt;/sub&gt; 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 SO&lt;sub&gt;2&lt;/sub&gt; plume.&lt;/p&gt;