Articles | Volume 11, issue 23
Atmos. Chem. Phys., 11, 12317–12337, 2011
Atmos. Chem. Phys., 11, 12317–12337, 2011

Research article 09 Dec 2011

Research article | 09 Dec 2011

A method for evaluating bias in global measurements of CO2 total columns from space

D. Wunch1, P. O. Wennberg1, G. C. Toon1,2, B. J. Connor3, B. Fisher2, G. B. Osterman2, C. Frankenberg2, L. Mandrake2, C. O'Dell4, P. Ahonen5, S. C. Biraud14, R. Castano2, N. Cressie6, D. Crisp2, N. M. Deutscher7,8, A. Eldering2, M. L. Fisher14, D. W. T. Griffith8, M. Gunson2, P. Heikkinen5, G. Keppel-Aleks1, E. Kyrö5, R. Lindenmaier15, R. Macatangay8, J. Mendonca15, J. Messerschmidt7, C. E. Miller2, I. Morino9, J. Notholt7, F. A. Oyafuso2, M. Rettinger10, J. Robinson12, C. M. Roehl1, R. J. Salawitch11, V. Sherlock12, K. Strong15, R. Sussmann10, T. Tanaka9,*, D. R. Thompson2, O. Uchino9, T. Warneke7, and S. C. Wofsy13 D. Wunch et al.
  • 1California Institute of Technology, Pasadena, CA, USA
  • 2Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USA
  • 3BC Consulting, Ltd., Alexandra, New Zealand
  • 4Colorado State University, Fort Collins, CO, USA
  • 5Arctic Research Centre of the Finnish Meteorological Institute, Helsinki, Finland
  • 6Department of Statistics, The Ohio State University, Columbus, OH, USA
  • 7University of Bremen, Bremen, Germany
  • 8University of Wollongong, Wollongong, NSW, Australia
  • 9National Institute for Environmental Studies, Tsukuba, Japan
  • 10IMK-IFU, Garmisch-Partenkirchen, Germany
  • 11Atmospheric & Oceanic Science, University of Maryland, College Park, MD, USA
  • 12National Institute of Water & Atmospheric Research, Wellington, New Zealand
  • 13Harvard University, Cambridge, MA, USA
  • 14Lawrence Berkeley National Laboratories, Berkeley, CA, USA
  • 15Department of Physics, University of Toronto, Toronto, ON, Canada
  • *now at: Japan Aerospace Exploration Agency, Tsukuba, Japan

Abstract. We describe a method of evaluating systematic errors in measurements of total column dry-air mole fractions of CO2 (XCO2) from space, and we illustrate the method by applying it to the v2.8 Atmospheric CO2 Observations from Space retrievals of the Greenhouse Gases Observing Satellite (ACOS-GOSAT) measurements over land. The approach exploits the lack of large gradients in XCO2 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 XCO2 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.

Final-revised paper