Tropospheric Ozone Assessment Report (TOAR): 16-year ozone trends from the IASI Climate Data Record

Abstract. Assessing tropospheric ozone (O₃) variability is essential for understanding its impact on air quality, health, and climate change. The Infrared (IR) Atmospheric Sounding Interferometer (IASI) mission onboard the Metop platforms, has been providing global measurements of O₃ concentrations since 2007. This study presents the first comprehensive analysis of the 16-year O₃ Climate Data Record (CDR) from IASI/Metop (2008–2023), a homogeneous dataset offering valuable insights into the variability and long-term trends of tropospheric O₃. The IASI-CDR ozone product is evaluated against TROPESS (TRopospheric Ozone and its Precursors from Earth System Sounding) O₃ retrievals from the Cross-track Infrared Sounder (CrIS). The comparison shows excellent agreement for total ozone (biases < 1.2 %, correlations > 0.97) and good agreement for tropospheric ozone (biases 10–12 %, correlations 0.77–0.91). Comparisons with ozonesonde data indicate that IASI underestimates tropospheric ozone by 2 % in the tropics and by up to 10 % in mid and high latitudes. Spatiotemporal analysis of IASI data from 2008 to 2023 reveals a global negative trend in tropospheric O₃ (‑0.40 ± 0.10 % year^-1), with the most pronounced decreases observed in the tropics and in Europe. Despite differing from positive trends in ultraviolet (UV) satellite data, both UV and IR satellite instruments show a significant drop in tropospheric ozone starting in 2020, partly due to pandemic-related emission reductions. This study emphasizes the importance of long-term, consistent datasets for tracking ozone trends and the need for improved data retrieval and integration to address regional and temporal discrepancies.