Performance assessment of the IASI-O3 KOPRA product for observing midlatitude tropospheric ozone evolution for 15 years: validation with ozone sondes and consistency of the three IASI instruments

Abstract. The Infrared Atmospheric Sounding Interferometer (IASI) has been monitoring the atmosphere for operational meteorology and atmospheric composition studies since 2007 with a succession of three instruments aboard the Metop-A (2006–2021), Metop-B (2012–Present), and Metop-C (2018–Present) missions. One of the key species monitored is ozone (O3). This study assesses the quality of the regional IASI-O3 KOPRA (Karlsruhe Optimized and Precise Radiative transfer Algorithm) product, version 3.0, and the consistency of the three IASI instruments, IASI-A, IASI-B, and IASI-C, for time series and trend analyses. The IASI-O3 KOPRA products of the three instruments show a very good agreement and consistency, better than 1 %, for the tropospheric ozone column (TrOC) and several partial columns (surface–450 hPa, surface–300 hPa) across the three study domains: Europe, North America, and East Asia. For the quality assessment and trend analyses, we combine the ozone products derived from IASI-A (2008–2018) and IASI-B (2019–2022). The comparison with homogenized ozone sondes for six northern midlatitude stations reveals a small negative bias of about 3 %–6 % in the IASI-O3 KOPRA products in the troposphere for both profiles and columns. A rather good correlation between 0.7 and 0.9 is observed, and an error of about 15 %–17 % (compared to sondes smoothed with averaging kernels, AKs) is estimated. The ozone variability is also well reproduced for all the partial columns with a slight underestimation of about 10 % for the TrOC. Based on the comparison with the ozone sondes, we identified a temporal drift (of about −0.06 ± 0.02 DU yr−1 on average) for three different ozone columns (TrOC, surface–450 hPa, surface–300 hPa). This drift can be more pronounced in summer. However, a significant variability of the estimated drifts depending on the sample of ozone sonde sites is remarked, that does not allow its use for correcting the IASI ozone product time series data over broad domains. While the upper tropospheric ozone trends are mainly positive or undefined, the lower tropospheric ozone trends are mainly systematically negative. The regions most affected by negative trends are the Mediterranean, Western North America, Eastern North America, and East Asia. Compensations between lower and upper tropospheric trends prevent the identification of any specific long-term behavior for TrOCs over the three domains. The negative tropospheric ozone column anomalies observed during the period 2020–2022 (the post-COVID-19 period) only slightly impact the trends already ongoing for the period 2008–2019.