EGUsphere EGUsphere EGUsphere EGUsphere 1680-7375 Copernicus Publications Göttingen, Germany 10.5194/egusphere-2024-3736 Global Ground-based Tropospheric Ozone Measurements: Reference Data and Individual Site Trends (2000–2022) from the TOAR-II/HEGIFTOM Project Van Malderen Roeland https://orcid.org/0000-0002-1369-8853 1 Thompson Anne M. https://orcid.org/0000-0002-7829-0920 2 3 Kollonige Debra E. 2 4 Stauffer Ryan M. https://orcid.org/0000-0002-8583-7795 2 Smit Herman G. J. https://orcid.org/0000-0002-2268-4189 5 Maillard Barras Eliane https://orcid.org/0000-0002-6513-8428 6 Vigouroux Corinne 7 Petropavlovskikh Irina https://orcid.org/0000-0001-5352-1369 8 9 Leblanc Thierry 10 Thouret Valérie 11 Wolff Pawel 12 Effertz Peter 8 9 Tarasick David W. 13 Poyraz Deniz https://orcid.org/0000-0002-2692-5621 1 Ancellet Gérard 14 De Backer Marie-Renée 15 Evan Stéphanie https://orcid.org/0000-0003-1014-0907 16 Flood Victoria https://orcid.org/0000-0003-1524-481X 17 Frey Matthias M. 18 Hannigan James W. https://orcid.org/0000-0002-4269-1677 19 Hernandez José L. 20 Iarlori Marco 21 Johnson Bryan J. 9 Jones Nicholas 22 Kivi Rigel https://orcid.org/0000-0001-8828-2759 23 Mahieu Emmanuel https://orcid.org/0000-0002-5251-0286 24 McConville Glen 9 Müller Katrin https://orcid.org/0000-0002-6891-6889 25 Nagahama Tomoo https://orcid.org/0000-0001-7707-978X 26 Notholt Justus 27 Piters Ankie 28 Prats Natalia 29 Querel Richard https://orcid.org/0000-0001-8792-2486 30 Smale Dan https://orcid.org/0000-0003-3385-0880 30 Steinbrecht Wolfgang https://orcid.org/0000-0003-0680-6729 31 Strong Kimberly https://orcid.org/0000-0001-9947-1053 17 Sussmann Ralf https://orcid.org/0000-0002-1970-7538 32 Royal Meteorological Institute of Belgium & Solar-Terrestrial Centre of Excellence, Uccle, Belgium Atmospheric Chemistry and Dynamics Laboratory, NASA Goddard Space Flight Center, Greenbelt, MD, USA GESTAR, University of Maryland, Baltimore County, Baltimore, MD, USA Science Systems and Applications, Inc, Lanham, MD, USA Forschungszentrum Jülich, Germany Federal Office of Meteorology and Climatology MeteoSwiss, Payerne, Switzerland Royal Belgium Institute for Space Aeronomy, Uccle, Belgium Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, CO, USA NOAA Global Monitoring Laboratory, Boulder, CO, USA Jet Propulsion Laboratory, California Institute of Technology, Wrightwood, California, USA Laboratoire d'Aérologie, Université Toulouse III – Paul Sabatier, CNRS, Toulouse, France Observatoire Midi-Pyrénées, Université Toulouse III – Paul Sabatier, CNRS, Toulouse, France Environment and Climate Change Canada, Downsview, ONT Canada LATMOS, Sorbonne Université, Université Versailles St-Quentin, CNRS/INSU, Paris, France Groupe de Spectrométrie Moléculaire et Atmosphérique, Université de Reims, France Laboratoire de l’Atmosphère et des Cyclones (LACy), CNRS, Université de La Réunion, Météo-France,Saint-Denis, France Department of Physics, University of Toronto, Toronto, ON, Canada IMKASF, Karlsruhe Institute of Technology (KIT), Eggenstein-Leopoldshafen, Germany Atmospheric Chemistry, Observations & Modeling, National Center for Atmospheric Research, Boulder, CO, USA State Meteorological Agency of Spain (AEMET), Madrid. Spain CETEMPS Dipartimento di Scienze Fisiche e Chimiche, Università degli Studi dell’Aquila, L’Aquila, Italy School of Physics, University of Wollongong, Australia Finnish Meteorological Institute, Space and Earth Observation Centre, Sodankylä, Finland Institut d'Astrophysique et de Géophysique, Université de Liège, Liège, Belgium Alfred Wegener Institute, Helmholtz-Centre for Polar and Marine Research, Potsdam, Germany Institute for Space‐Earth Environmental Research, Nagoya University, Nagoya, Japan Institute of Environmental Physics, University of Bremen, Bremen, Germany Royal Netherlands Meteorological Institute (KNMI), De Bilt, the Netherlands Izaña Atmospheric Research Center, State Meteorological Agency of Spain (AEMET), Santa Cruz de Tenerife, Spain National Institute of Water & Atmospheric Research Ltd (NIWA), Lauder, New Zealand Deutscher Wetterdienst, Hohenpeissenberg, Germany Karlsruhe Institute of Technology (KIT), IMK-IFU, Garmisch-Partenkirchen, Germany 13 01 2025 2025 1 53 Copyright: © 2025 Roeland Van Malderen et al. 2025 This work is licensed under the Creative Commons Attribution 4.0 International License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/4.0/ This article is available from https://egusphere.copernicus.org/preprints/2025/egusphere-2024-3736/ The full text article is available as a PDF file from https://egusphere.copernicus.org/preprints/2025/egusphere-2024-3736/egusphere-2024-3736.pdf

Tropospheric ozone trends from models and satellites are found to diverge. Ground-based (GB) observations are used to reference models and satellites but GB data themselves might display station biases and discontinuities. Re-processing with uniform procedures, the TOAR-II Working Group Harmonization and Evaluation of Ground-based Instruments for Free-Tropospheric Ozone Measurements (HEGIFTOM) homogenized public data from 5 networks: ozonesondes, In-service Aircraft for a Global Observing System (IAGOS) profiles, solar absorption Fourier-Transform Infrared (FTIR) spectrometer measurements, Lidar observations, and Dobson Umkehr data. Amounts and uncertainties for total tropospheric ozone (“TrOC”, surface to 300 hPa), free and lower tropospheric ozone, are calculated for each network. We report trends (2000 to 2022) for these segments using Quantile Regression (QR) and Multiple Linear Regression (MLR) for 55 datasets, including 6 multi-instrument stations. The findings: (1) Median TrOC trends computed with QR and MLR trends are essentially the same; (2) Pole-to-pole, across all longitudes, TrOC trends fall within +3 ppbv/decade to -3 ppbv/decade, equivalent to (-4 % to + 8 %)/decade depending on site. (3) The greatest fractional increases occur over most tropical/subtropical sites with decreases at northern high latitudes but these patterns are not uniform. (4) Post-COVID trends are smaller than pre-COVID trends for Northern Hemisphere mid-latitude sites. In summary, this analysis conducted in the frame of TOAR-II/HEGIFTOM shows that high-quality, multi-instrument, harmonized data over a wide range of ground sites provide clear standard references for TOAR-II models and evolving tropospheric ozone satellite products for 2000–2022.