A harmonized OMI/MLS–IASI record of tropospheric ozone over China during 2005–2025

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Tropospheric ozone (O 3 ) is both a greenhouse gas and a harmful air pollutant, yet its evolution over China after 2020 has not been consistently documented because the widely used OMI/MLS satellite record ends in that year. Here we construct a continuous 21-year (2005–2025) record of tropospheric column ozone (TCO) over China by harmonizing OMI/MLS and IASI observations within a month-aware machine-learning framework. Leave-one-month-out cross-validation over the 13-month sensor-overlap period yielded an R 2 of 0.85 and an RMSE of 2.59 DU. A reduced-predictor configuration that excluded all meteorological and emission covariates reproduced the main seasonal structure, trend estimates, and the sustained high levels of the Full configuration, indicating that the 2021–2025 extension and the resulting full-period trend estimates were only weakly sensitive to the inclusion of these predictors; the 13-month overlap nevertheless limits independent validation of post-2020 interannual variability and the exact magnitude of the full-period trend. China-mean TCO increased significantly at 1.93 DU per decade during 2005–2025 ( p < 0.001), with the strongest increase in summer (2.79 DU per decade) and the weakest in winter (1.29 DU per decade). TCO remained at sustained high levels during 2021–2025: the 2021–2025 mean (36.16 DU) exceeds the 2005–2020 mean by 1.73 DU, four of the five highest annual means of the record occurred in this period, and these high levels are spatially widespread across the country and quantitatively consistent with the long-term trend, indicating a continuation of the established upward trajectory rather than an anomalous enhancement above it. Exploratory analysis of ERA5 temperatures shows no robust association between detrended TCO anomalies and regional lower- or mid-tropospheric temperatures, and the associations at 225 hPa are scale-dependent and interpreted only as diagnostics. The record therefore does not support attributing either the long-term increase or the sustained high levels during 2021–2025 to regional warming or to any single driver. Recent chemistry–climate model studies emphasize the roles of changes in precursor-emission magnitude and spatial distribution, together with internal variability, in shaping multidecadal tropospheric ozone trends; this broader context indicates that the high TCO levels over China cannot be interpreted from domestic emission totals alone. Quantifying the relative contributions of precursor emissions, hemispheric background ozone, and meteorological variability remains a priority for chemistry–climate model studies, for which this harmonized record provides an evaluated observational basis.