Retrieval of vertical concentration profiles of SO2 using the IASI satellite instrument

The development of MUSICA IASI processing was initiated as part of the MUSICA (Multi-platform remote Sensing of Isotopologues for investigating the Cycle of Atmospheric water) project funded by the European Research Council. This processor works with IASI (Infrared Atmospheric Sounding Interferometer) radiances measured under cloud-free conditions, and, using optimal estimation, derives vertical distributions of water vapor, ratios between water vapor isotopologues, and several trace gases including greenhouse gases, methane and nitrous oxide. In accordance with the FAIR (findable, accessible, interoperable, reusable) principles, all MUSICA IASI products are freely available together with their observation specific averaging kernels and uncertainty covariances. Recently, retrievals of sulfur dioxide (SO2), peroxyacetyl nitrate (PAN), acetic acid and acetone have been included, in order to account for their important spectroscopic signals in case of volcanic eruptions and biomass burning events, respectively.    Here, we present the MUSICA IASI SO2 retrieval setup. A particularity is the use of a logarithmic SO2 concentration scale, which facilitates a reliable detection of the altitude where the SO2 plume is situated (retrieved from the spectral signal, no a priori assumptions of an SO2 plume height required). We document the superiority of this retrieval setup compared to a retrieval on a linear SO2 concentration scale. In addition, we empirically demonstrate the quality of the SO2 profile data in the preliminary results of three different volcano eruption events at three different latitude regions: 2019 Raikoke (48°N), 2021 La Palma (Cumbre Vieja, 29°N), and 2022 Hunga Tonga-Hunga Ha’apai (20°S). The amount of SO2 and the SO2 plume height are different for the three events. We show that the MUSICA IASI product is able to correctly capture these differences.   The availability of individual averaging kernels allows a quantitative inter-comparison to other SO2 data products. For SO2 plumes in the low and middle troposphere, we will explore respective comparison possibilities to data obtained from Ticosonde in-situ measurements. For SO2 plumes in the upper troposphere or stratosphere, we plan to use data provided by MLS (Microwave Limb Sounder instrument aboard the NASA/Aura satellite). IASI SO2 products generated by other processors and TROPOMI/S5P SO2 products will also be potential candidates for inter-comparison studies.