| 01 Apr 2026
An 18-year record of atmospheric sulphur dioxide (SO2) derived from IASI measurements
Abstract. Over the past decades, satellite measurements of atmospheric sulphur dioxide (SO2) have served a wide range of applications, from volcanology to air quality monitoring and climate assessment. In this paper, we present a 2007–2025 record of twice-daily global SO2 vertical column abundances and SO2 plume altitudes derived from measurements by the three Infrared Atmospheric Sounding Interferometer (IASI) instruments onboard the Metop platforms. Building upon earlier work, the algorithm has been refined and complemented with several new components. Most notably, the sensitivity of the algorithm to low SO2 abundances and low-altitude plumes has been improved and the plume altitude retrieval now features enhanced performance for large SO2 columns. Special care was taken to maximize consistency of the product over time and across the IASI instruments. In addition to SO2 at the retrieved altitude, the dataset also includes SO2 column retrievals assuming plume altitudes ranging from 1 to 60 km. These allow the construction of averaging kernels that can be used in model assimilation or to obtain more accurate column estimates when the plume altitude is constrained by independent information. The first part of the paper details the retrieval methodology and presents sensitivity and uncertainty analyses. The latter indicate that the retrieval uncertainty is smallest for plumes above 8 km and increases gradually toward lower altitudes, particularly within the first few kilometres and in atmospheres with a high water vapour content. Detailed comparisons with measurements from the CALIOP lidar indicate that plume altitudes are generally accurate to within 1–2 km. Column comparisons with TROPOMI in fresh volcanic plumes indicate reasonable agreement. In the second part of the paper, we present an overview of the 18-year dataset, providing detailed time series of SO2 column abundances and mass-altitude profiles, with a focus on volcanic SO2. For each eruption with a plume mass exceeding 30 kt, we report the maximum atmospheric SO2 mass, the mass below and above 8 km altitude, the peak plume altitude, and the altitude range containing 75 % of the mass. When available, the maximum mass is compared with estimates reported in the literature. The SO2 altitude and column data from IASI/Metop-A, -B, and -C are available at https://dx.doi.org/10.25326/870 (Clarisse and Franco, 2026a), https://dx.doi.org/10.25326/869 (Clarisse and Franco, 2026b), and https://dx.doi.org/10.25326/868 (Clarisse and Franco, 2026c), respectively.
The paper provides a thorough record of volcanic SO2 column abundances and plume altitudes from 2007 to 2025, utilizing data from the three IASI instruments aboard Metop platforms. The first section details enhancements to SO2 retrieval methods, including sensitivity and uncertainty analyses. Notably, the refined retrievals demonstrate increased sensitivity to low SO2 concentrations at plume edges and improved plume altitude estimates for large columns. The authors have ensured consistency across instruments and throughout the time series. Uncertainty is minimized for SO2 plumes above 8 km, but increases at lower altitudes, especially in conditions of elevated atmospheric water vapor, and clouds and ash presence. Comparisons with CALIOP and TROPOMI reveal that plume altitude retrievals are accurate within 1–2 km, except for the case of high stratospheric Hunga plum, where explicit height constraint was necessary (figure 8). Overall, there is reasonable agreement with TROPOMI SO2 column retrievals for volcanic plumes above 8 km. The latter part of the manuscript introduces the 18-year IASI dataset, emphasizing volcanic SO2 mass, altitude distribution, and comparisons with prior IR and UV SO2 mass retrievals. The presentation is clear, and the manuscript is ready for publication.
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