Comment on amt-2021-31
Abstract
The thermal infrared nadir spectra of IASI (Infrared Atmospheric Sounding Interferometer) are successfully used for retrievals of different atmospheric trace gas profiles. However, these retrievals offer generally reduced information about the lowermost tropospheric layer due to the lack of thermal contrast close to the surface. Spectra of scattered solar radiation observed in the near and/or short wave infrared, for instance by TROPOMI (TROPOspheric Monitoring Instrument) offer higher sensitivity near ground and are used for the retrieval of total column averaged mixing ratios of a variety of atmospheric trace gases. Here we present a method for the synergetic use of IASI profile and TROPOMI total column data. Our method uses the output of the individual retrievals and consists of linear algebra a posteriori calculations (i.e. calculation after the individual retrievals). We show that this approach is largely equivalent to applying the spectra of the different sensors together in a single retrieval procedure, but with the substantial advantage of being applicable to data generated with different individual retrieval processors, of being very time efficient, and of directly benefiting from the high quality and most recent improvements of the individual retrieval processors. We demonstrate the method exemplarily for atmospheric methane (CH4). We perform a theoretical evaluation and show that the a posteriori combination method yields a total column averaged CH4 product (XCH4) that conserves the good sensitivity of the corresponding TROPOMI product while merging it with the upper tropospheric and lower stratospheric (UTLS) CH4 partial column information of the corresponding IASI product. As consequence, the combined product offers in addition sensitivity for the tropospheric CH4 partial column, which is not provided by the individual TROPOMI nor the individual IASI product. The theoretically predicted synergetic effects are verified by comparisons to CH4 reference data obtained from collocated XCH4 measurements at six globally distributed TCCON (Total Carbon Column Observing Network) stations, CH4 profile measurements made by 24 individual AirCore soundings, and lower tropospheric CH4 data derived from continuous ground-based in-situ observations made at two nearby Global Atmospheric Watch (GAW) mountain stations. The comparisons clearly demonstrate that the combined product can reliably detect XCH4 signals and allows to distinguish between tropospheric and UTLS CH4 partial column averaged mixing ratios, which is not possible by the individual TROPOMI and IASI products. We find indications of a weak positive bias of about +1 % of the combined lower tropospheric data product with respect to the references. For the UTLS CH4 partial columns we find no significant bias.
Authors 23
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Benjamin Ertl corresponding
Karlsruhe Institute of Technology
Affiliation as printed
Institute of Meteorology and Climate Research (IMK-ASF), Karlsruhe Institute of Technology, Karlsruhe, Germany
Steinbuch Centre for Computing (SCC), Karlsruhe Institute of Technology, Karlsruhe, Germany
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Christopher J. Diekmann corresponding
Karlsruhe Institute of Technology
Affiliation as printed
Institute of Meteorology and Climate Research (IMK-ASF), Karlsruhe Institute of Technology, Karlsruhe, Germany
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Farahnaz Khosrawi corresponding
Karlsruhe Institute of Technology
Affiliation as printed
Institute of Meteorology and Climate Research (IMK-ASF), Karlsruhe Institute of Technology, Karlsruhe, Germany
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Amelie N. Röhling corresponding
Karlsruhe Institute of Technology
Affiliation as printed
Institute of Meteorology and Climate Research (IMK-ASF), Karlsruhe Institute of Technology, Karlsruhe, Germany
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Karlsruhe Institute of Technology
Affiliation as printed
Institute of Meteorology and Climate Research (IMK-ASF), Karlsruhe Institute of Technology, Karlsruhe, Germany
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Karlsruhe Institute of Technology
Affiliation as printed
Institute of Meteorology and Climate Research (IMK-ASF), Karlsruhe Institute of Technology, Karlsruhe, Germany
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Omaira E. GarcÃa corresponding
Agencia Estatal de Meteorología
Affiliation as printed
Izaña Atmospheric Research Center, Agencia Estatal de Meteorología (AEMET), Santa Cruz de Tenerife, Spain
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Agencia Estatal de Meteorología
Affiliation as printed
Izaña Atmospheric Research Center, Agencia Estatal de Meteorología (AEMET), Santa Cruz de Tenerife, Spain
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Tobias Borsdorff corresponding Aachen Earth science group SRON Netherlands Institute for Space Research
Space Research Organisation Netherlands
Affiliation as printed
Earth Science Group, SRON Netherlands Institute for Space Research, Utrecht, The Netherlands
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Space Research Organisation Netherlands
Affiliation as printed
Earth Science Group, SRON Netherlands Institute for Space Research, Utrecht, The Netherlands
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Space Research Organisation Netherlands
Affiliation as printed
Earth Science Group, SRON Netherlands Institute for Space Research, Utrecht, The Netherlands
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Huilin Chen corresponding
Affiliation as printed
Center for Isotope Research, University of Groningen, Groningen, The Netherlands
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Rigel Kivi corresponding
Finnish Meteorological Institute
Affiliation as printed
Space and Earth Observation Centre, Finnish Meteorological Institute, Sodankylä, Finland
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Thomas Laemmel corresponding
Commissariat à l'Énergie Atomique et aux Énergies Alternatives · Laboratoire des Sciences du Climat et de l'Environnement
Affiliation as printed
Laboratoire des Sciences du Climat et de l’Environnement (LSCE), CEA, 91191 Gif sur Yvette, France
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Michel Ramonet corresponding
Commissariat à l'Énergie Atomique et aux Énergies Alternatives · Laboratoire des Sciences du Climat et de l'Environnement
Affiliation as printed
Laboratoire des Sciences du Climat et de l’Environnement (LSCE), CEA, 91191 Gif sur Yvette, France
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Centre National de la Recherche Scientifique · École Polytechnique
Affiliation as printed
LMD/IPSL, CNRS, Ecole polytechnique, University Paris-Saclay, Palaiseau, France
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Centre National de la Recherche Scientifique · École Polytechnique
Affiliation as printed
LMD/IPSL, CNRS, Ecole polytechnique, University Paris-Saclay, Palaiseau, France
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Martin Steinbacher corresponding
Swiss Federal Laboratories for Materials Science and Technology
Affiliation as printed
Swiss Federal Laboratories for Materials Science and Technology (EMPA), Dübendorf, Switzerland
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Affiliation as printed
Air Monitoring Network, Federal Environment Agency (UBA), Langen, Germany
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Nicholas Michael Deutscher corresponding
Affiliation as printed
Centre for Atmospheric Chemistry, School of Earth, Atmospheric and Life Sciences, Faculty of Science, Medicine and Health, University of Wollongong, Wollongong, Australia
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DAVID W. T. GRIFFITH corresponding
Affiliation as printed
Centre for Atmospheric Chemistry, School of Earth, Atmospheric and Life Sciences, Faculty of Science, Medicine and Health, University of Wollongong, Wollongong, Australia
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Voltaire A. Velazco corresponding
Affiliation as printed
Centre for Atmospheric Chemistry, School of Earth, Atmospheric and Life Sciences, Faculty of Science, Medicine and Health, University of Wollongong, Wollongong, Australia
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David Frank Pollard corresponding
National Institute of Water and Atmospheric Research
Affiliation as printed
National Institute of Water and Atmospheric Research Ltd (NIWA), Lauder, New Zealand
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