ACP Atmospheric Chemistry and Physics ACP 1680-7324 Copernicus GmbH Göttingen, Germany 10.5194/acp-11-12317-2011 A method for evaluating bias in global measurements of CO<sub>2</sub> total columns from space Wunch D. 1 Wennberg P. O. 1 Toon G. C. 1 2 Connor B. J. 3 Fisher B. 2 Osterman G. B. 2 Frankenberg C. 2 Mandrake L. 2 O'Dell C. 4 Ahonen P. 5 Biraud S. C. 14 Castano R. 2 Cressie N. 6 Crisp D. 2 Deutscher N. M. 7 8 Eldering A. 2 Fisher M. L. 14 Griffith D. W. T. 8 Gunson M. 2 Heikkinen P. 5 Keppel-Aleks G. 1 Kyrö E. 5 Lindenmaier R. 15 Macatangay R. 8 Mendonca J. 15 Messerschmidt J. 7 Miller C. E. 2 Morino I. 9 Notholt J. 7 Oyafuso F. A. 2 Rettinger M. 10 Robinson J. 12 Roehl C. M. 1 Salawitch R. J. 11 Sherlock V. 12 Strong K. 15 Sussmann R. 10 Tanaka T. 9 16 Thompson D. R. 2 Uchino O. 9 Warneke T. 7 Wofsy S. C. 13 California Institute of Technology, Pasadena, CA, USA Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USA BC Consulting, Ltd., Alexandra, New Zealand Colorado State University, Fort Collins, CO, USA Arctic Research Centre of the Finnish Meteorological Institute, Helsinki, Finland Department of Statistics, The Ohio State University, Columbus, OH, USA University of Bremen, Bremen, Germany University of Wollongong, Wollongong, NSW, Australia National Institute for Environmental Studies, Tsukuba, Japan IMK-IFU, Garmisch-Partenkirchen, Germany Atmospheric & Oceanic Science, University of Maryland, College Park, MD, USA National Institute of Water & Atmospheric Research, Wellington, New Zealand Harvard University, Cambridge, MA, USA Lawrence Berkeley National Laboratories, Berkeley, CA, USA Department of Physics, University of Toronto, Toronto, ON, Canada now at: Japan Aerospace Exploration Agency, Tsukuba, Japan 09 12 2011 11 23 12317 12337 This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. This article is available from http://www.atmos-chem-phys.net/11/12317/2011/acp-11-12317-2011.html The full text article is available as a PDF file from http://www.atmos-chem-phys.net/11/12317/2011/acp-11-12317-2011.pdf

We describe a method of evaluating systematic errors in measurements of total column dry-air mole fractions of CO<sub>2</sub> (<i>X</i><sub>CO<sub>2</sub></sub>) from space, and we illustrate the method by applying it to the v2.8 Atmospheric CO<sub>2</sub> Observations from Space retrievals of the Greenhouse Gases Observing Satellite (ACOS-GOSAT) measurements over land. The approach exploits the lack of large gradients in <i>X</i><sub>CO<sub>2</sub></sub> south of 25° S to identify large-scale offsets and other biases in the ACOS-GOSAT data with several retrieval parameters and errors in instrument calibration. We demonstrate the effectiveness of the method by comparing the ACOS-GOSAT data in the Northern Hemisphere with ground truth provided by the Total Carbon Column Observing Network (TCCON). We use the observed correlation between free-tropospheric potential temperature and <i>X</i><sub>CO<sub>2</sub></sub> in the Northern Hemisphere to define a dynamically informed coincidence criterion between the ground-based TCCON measurements and the ACOS-GOSAT measurements. We illustrate that this approach provides larger sample sizes, hence giving a more robust comparison than one that simply uses time, latitude and longitude criteria. Our results show that the agreement with the TCCON data improves after accounting for the systematic errors, but that extrapolation to conditions found outside the region south of 25° S may be problematic (e.g., high airmasses, large surface pressure biases, M-gain, measurements made over ocean). A preliminary evaluation of the improved v2.9 ACOS-GOSAT data is also discussed.

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