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Discussion papers | Copyright
https://doi.org/10.5194/amt-2018-257
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 16 Aug 2018

Research article | 16 Aug 2018

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Measurement Techniques (AMT).

Improved Retrievals of Carbon Dioxide from the Orbiting Carbon Observatory-2 with the version 8 ACOS algorithm

Christopher W. O'Dell1, Annmarie Eldering2, Paul O. Wennberg3, David Crisp2, Michael R. Gunson2, Brendan Fisher2, Christian Frankenberg3, Matthäus Kiel3, Hannakaisa Lindqvist4, Lukas Mandrake2, Aronne Merrelli5, Vijay Natraj2, Robert R. Nelson1, Gregory B. Osterman2, Vivienne H. Payne2, Thomas R. Taylor1, Debra Wunch6, Brian J. Drouin2, Fabiano Oyafuso2, Albert Chang2, James McDuffie2, Michael Smyth2, David F. Baker1, Sourish Basu7,8, Frédéric Chevallier9, Sean M. R. Crowell10, Liang Feng11,12, Paul I. Palmer11,12, Mavendra Dubey13, Omaira E. García14, David W. T. Griffith15, Frank Hase16, Laura T. Iraci17, Rigel Kivi18, Isamu Morino19, Justus Notholt20, Hirofumi Ohyama19, Christof Petri20, Coleen M. Roehl3, Mahesh K. Sha21, Kimberly Strong6, Ralf Sussmann22, Yao Te23, Osamu Uchino19, and Voltaire A. Velazco15 Christopher W. O'Dell et al.
  • 1Colorado State University, Fort Collins, CO, USA
  • 2Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USA
  • 3California Institute of Technology, Pasadena, CA, USA
  • 4Finnish Meteorological Institute, Helsinki, Finland
  • 5SSEC, University of Wisconsin-Madison, Madison, WI, USA
  • 6University of Toronto, Toronto, Canada
  • 7NOAA Earth System Research Laboratory, Global Monitoring Division, Boulder, CO, USA
  • 8Cooperative Institute for Research in Environmental Sciences, University of Colorado Boulder, Colorado, USA
  • 9Laboratoire des Sciences du Climat et de l’Environnement, IPSL, CEA-CNRS-UVSQ, Gif-sur-Yvette, France
  • 10College of Atmospheric and Geographic Sciences, University of Oklahoma, Norman, OK, USA
  • 11National Centre for Earth Observation, University of Edinburgh, UK
  • 12School of GeoSciences, University of Edinburgh, UK
  • 13Los Alamos National Laboratory, Los Alamos, NM, USA
  • 14Izaña Atmospheric Research Center, Meteorological State Agency of Spain (AEMet), Santa Cruz de Tenerife, Spain
  • 15Centre for Atmospheric Chemistry, University of Wollongong, Wollongong, Australia
  • 16Karlsruhe Institute of Technology, IMK-ASF, Karlsruhe, Germany
  • 17NASA Ames Research Center, Moffett Field, CA, USA
  • 18Finnish Meteorological Institute, Sodankylä, Finland
  • 19National Institute for Environmental Studies (NIES), Tsukuba, Japan
  • 20University of Bremen, Bremen, Germany
  • 21Royal Belgian Institute for Space Aeronomy, Brussels, Belgium
  • 22Karlsruhe Institute of Technology, IMK-IFU, Garmisch-Partenkirchen, Germany
  • 23LERMA-IPSL, Sorbonne Université, Observatoire de Paris, Université PSL, CNRS, 75005, Paris, France

Abstract. Since September 2014, NASA's Orbiting Carbon Observatory-2 (OCO-2) satellite has been taking measurements of reflected solar spectra and using them to infer atmospheric carbon dioxide levels. This work provides details of the OCO-2 retrieval algorithm, versions 7 and 8, used to derive the column-averaged dry air mole fraction of atmospheric CO2,/sub> (XCO2) for the roughly 100,000 cloud-free measurements recorded by OCO-2 each day. The algorithm is based on the Atmospheric Carbon Observations from Space (ACOS) algorithm which has been applied to observations from the Greenhouse Gases Observing SATellite (GOSAT) since 2009, with modifications necessary for OCO-2. Because high accuracy, better than 0.25%, is required in order to accurately infer carbon sources and sinks from XCO2, significant errors and regional-scale biases in the measurements must be minimized. We discuss efforts to filter out poor quality measurements, and correct the remaining good-quality measurements to minimize regional-scale biases. Updates to the radiance calibration and retrieval forward model in version 8 have improved many aspects of the retrieved data products. The version 8 data appear to have reduced regional-scale biases overall, and demonstrate a clear improvement over the version 7 data. In particular, error variance with respect to TCCON was reduced by 20% over land and 40% over ocean between versions 7 and 8, and nadir and glint observations over land are now more consistent. While this paper documents the significant improvements in the ACOS algorithm, it will continue to evolve and improve as the CO2 data record continues to expand.

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