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@ARTICLE{Keppens:1048411,
      author       = {Keppens, Arno and Hubert, Daan and Granville, José and
                      Nath, Oindrila and Lambert, Jean-Christopher and Wespes,
                      Catherine and Coheur, Pierre-François and Clerbaux, Cathy
                      and Boynard, Anne and Siddans, Richard and Latter, Barry and
                      Kerridge, Brian and Di Pede, Serena and Veefkind, Pepijn and
                      Cuesta, Juan and Dufour, Gaelle and Heue, Klaus-Peter and
                      Coldewey-Egbers, Melanie and Loyola, Diego and
                      Orfanoz-Cheuquelaf, Andrea and Maratt Satheesan, Swathi and
                      Eichmann, Kai-Uwe and Rozanov, Alexei and Sofieva, Viktoria
                      F. and Ziemke, Jerald R. and Inness, Antje and Van Malderen,
                      Roeland and Hoffmann, Lars},
      title        = {{H}armonisation of sixteen tropospheric ozone satellite
                      data records},
      journal      = {Atmospheric measurement techniques},
      volume       = {18},
      number       = {22},
      issn         = {1867-1381},
      address      = {Katlenburg-Lindau},
      publisher    = {Copernicus},
      reportid     = {FZJ-2025-04622},
      pages        = {6893 - 6916},
      year         = {2025},
      abstract     = {The first Tropospheric Ozone Assessment Report (TOAR,
                      2014–2019) encountered several observational challenges
                      that limited the confidence in estimates of the burden,
                      short-term variability, and long-term changes of ozone in
                      the free troposphere. One of these challenges is the
                      difficulty to interpret the consistency of satellite
                      measurements obtained with different techniques from
                      multiple sensors, leading to differences in spatiotemporal
                      sampling, vertical smoothing, a-priori information, and
                      uncertainty characterisation. This motivated the Committee
                      on Earth Observation Satellites (CEOS) to initiate a
                      coordinated activity VC-20-01 on improving the assessment
                      and harmonisation of tropospheric ozone measured from space.
                      Here, we report on work that contributes to this CEOS
                      activity, as well as to the ongoing second TOAR assessment
                      (TOAR-II, 2020–2025). Our objective is to harmonise the
                      spatiotemporal perspective of (sixteen) satellite ozone data
                      records, thereby accounting as much as possible for
                      differences in vertical smoothing and sampling. Four
                      harmonisation methods are presented to achieve this goal:
                      two for ozone profiles obtained from nadir sounders
                      (UV-visible, IR, and combined UV-IR), and two for
                      tropospheric ozone column products derived by one of the
                      residual methods (Convective Cloud Differential or
                      Limb–Nadir Matching). We discuss to what extent
                      harmonisation may affect assessments of the spatial
                      distribution, seasonal cycle, and long-term changes in free
                      tropospheric ozone, and we anchor the harmonised profile
                      data to ozonesonde measurements recently homogenised as part
                      of TOAR-II. We find that approaches that use global ozone
                      fields as a transfer standard (here the Copernicus
                      Atmosphere Monitoring Service ReAnalysis, CAMSRA) to
                      constrain the harmonisation generally lead to the largest
                      reduction of the inter-product dispersion (IPD) between
                      satellite datasets. These harmonisation efforts, however,
                      only partially account for the observed discrepancies
                      between the satellite datasets, with a reduction of about
                      $10 \%–40 \%$ of the IPD upon harmonisation, depending
                      on the products involved and with strong spatiotemporal
                      dependences. This work therefore provides evidence that it
                      is not only the differences in spatiotemporal smoothing and
                      sampling, but rather the differences in measurement
                      uncertainty that pose the main challenge to the assessment
                      of the spatial distribution and temporal evolution of free
                      tropospheric ozone from satellite observations.},
      cin          = {JSC},
      ddc          = {550},
      cid          = {I:(DE-Juel1)JSC-20090406},
      pnm          = {5111 - Domain-Specific Simulation $\&$ Data Life Cycle Labs
                      (SDLs) and Research Groups (POF4-511)},
      pid          = {G:(DE-HGF)POF4-5111},
      typ          = {PUB:(DE-HGF)16},
      doi          = {10.5194/amt-18-6893-2025},
      url          = {https://juser.fz-juelich.de/record/1048411},
}