Ground-based Tropospheric Ozone Measurements: Regional tropospheric ozone column trends from the TOAR-II/ HEGIFTOM homogenized datasets

Abstract. The quantification of long-term free-tropospheric ozone trends is essential for understanding the impact of human activities and climate change on atmospheric chemistry, but is challenged by the diversity between satellite tropospheric ozone records and the sparse temporal and spatial sampling of ground-based measurements. Here, we explore if a more consistent understanding of the geographical distribution of tropospheric ozone column (TrOC) trends can be obtained by focusing on regional trends calculated from ground-based measurements. Regions were determined with a correlation analysis between modelled TrOCs at the site locations. For those regions, TrOC trends were estimated with Quantile Regression and Dynamical Linear Modelling for the Trajectory-mapped Ozonesonde dataset for the Stratosphere and Troposphere (TOST), and with a linear mixed-effects modelling (LMM) approach to calculate synthesized trends from the homogenized HEGIFTOM (Harmonization and Evaluation of Ground-based Instruments for Free-Tropospheric Ozone Measurements) individual site trends. For different periods (1990–2021/22, 1995–2021/22, 2000–2021/22), both approaches give increasing (partial) tropospheric ozone column amounts over almost all Asian regions (median confidence), and negative trends over the Arctic regions (very high confidence). Trends over Europe and North America are mostly weakly positive (LMM method) or negative (TOST). For both approaches, the 2000–2021/22 trends decreased in magnitude compared to the 1995–2021/22 for most of the regions, and for all time periods and regions, the pre-COVID trends are larger than the post-COVID trends. Our results enable the validation of global satellite TrOC trends, and assessment of the performance of atmospheric chemistry models to represent the distribution and variation of TrOC.