Assessment of 16-year tropospheric ozone trends from the IASI Climate Data Record

<p>Assessing tropospheric ozone O<span class="inline-formula">₃</span> 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 O<span class="inline-formula">₃</span> concentrations since 2007. This study presents the first comprehensive analysis of the 16-year O<span class="inline-formula">₃</span> Climate Data Record (CDR) from IASI/Metop (2008–2023), a homogeneous dataset offering valuable insights into the variability and long-term trends of tropospheric O<span class="inline-formula">₃</span>. The IASI-CDR ozone product is evaluated against TROPESS (TRopospheric Ozone and its Precursors from Earth System Sounding) O<span class="inline-formula">₃</span> retrievals from the Cross-track Infrared Sounder (CrIS). The comparison shows excellent agreement for total ozone biases <span class="inline-formula">&lt;1.2 <i>%</i></span>, correlations <span class="inline-formula">&gt;0.97</span> 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. Drift analysis indicates the long-term temporal stability of IASI tropospheric ozone, with values below 3 % per decade globally and regionally, satisfying the stability criterion requirement. IASI data reveal a global decline in tropospheric O<span class="inline-formula">₃</span> <span class="inline-formula">−</span>0.08 <span class="inline-formula">±</span> 0.05 DU yr<span class="inline-formula">⁻¹</span>, <span class="inline-formula"><i>p</i>=0.01</span>, strongest in the tropics and Europe. The comparison with ozonesonde data, shows high-certainty decreases consistently observed in the tropics across all datasets (IASI, smoothed sonde, and raw sonde), supporting the robustness of the findings in this region. Vertical analysis reveals that negative trends dominate in the lower troposphere, while positive trends in the upper troposphere align with ultraviolet (UV) satellite observations. This vertical contrast highlights the importance of separating lower and upper tropospheric layers when comparing IR and UV datasets. Although discrepancies remain when considering the full tropospheric column, 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.</p>