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.