A method for evaluating bias in global measurements of CO_2 total columns from space

Creators: Wunch, D.; Wennberg, P. O.; Toon, G. C.; Frankenberg, C.; Keppel-Aleks, G.; Roehl, C. M.

Abstract

We describe a method of evaluating systematic errors in measurements of total column dry-air mole fractions of CO_2 (X_(CO_2)) from space, and we illustrate the method by applying it to the v2.8 Atmospheric CO_2 Observations from Space retrievals of the Greenhouse Gases Observing Satellite (ACOS-GOSAT) measurements over land. The approach exploits the lack of large gradients in X_(CO_2) 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 X_(CO_2) 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.

Additional Information

© 2011 Author(s). This work is distributed under the Creative Commons Attribution 3.0 License. Published by Copernicus Publications on behalf of the European Geosciences Union. Received: 28 June 2011 – Published in Atmos. Chem. Phys. Discuss.: 22 July 2011; Revised: 21 November 2011 – Accepted: 24 November 2011 – Published: 9 December 2011. The authors wish to thank Sergey Oshchepkov, Peter Rayner, editor Ilse Aben and an anonymous reviewer for insightful and constructive comments and suggestions. We had enlightening discussions with Hiroshi Suto (JAXA) about the apparent time-dependent drift in the ACOS-GOSAT data. GOSAT spectra were kindly provided to the California Institute of Technology through an RA agreement with JAXA, NIES and the MOE. US funding for TCCON comes from NASA's Terrestrial Ecology Program, grant number NNX11AG01G, the Orbiting Carbon Observatory Program, the Atmospheric CO2 Observations from Space (ACOS) Program and the DOE/ARM Program. The Darwin TCCON site was built at Caltech with funding from the OCO project, and is operated by the University of Wollongong, with travel funds for maintenance and equipment costs funded by the OCO-2 project. We acknowledge funding to support Darwin and Wollongong from the Australian Research Council, Projects LE0668470, DP0879468, DP110103118 and LP0562346. Lauder TCCON measurements are funded by New Zealand Foundation of Research Science and Technology contracts C01X0204 and CO1X0406. We acknowledge financial support of the Białystok and Orléans TCCON sites from the Senate of Bremen and EU projects IMECC and GEOmon as well as maintenance and logistical work provided by AeroMeteo Service (Białystok) and the RAMCES team at LSCE (Gif-sur-Yvette, France). The PEARL Bruker 125HR measurements at Eureka were made by the Canadian Network for the Detection of Atmospheric Change (CANDAC), led by James R. Drummond, and in part by the Canadian Arctic ACE Validation Campaigns, led by Kaley A. Walker. They were supported by the Atlantic Innovation Fund/Nova Scotia Research Innovation Trust, Canada Foundation for Innovation, Canadian Foundation for Climate and Atmospheric Sciences, Canadian Space Agency, Environment Canada, Government of Canada International Polar Year funding, Natural Sciences and Engineering Research Council, Northern Scientific Training Program, Ontario Innovation Trust, Polar Continental Shelf Program, and Ontario Research Fund. The authors wish to thank Rebecca Batchelor and Ashley Harrett for the near-infrared upgrade of the instrument, PEARL site manager Pierre Fogal, the staff at the Eureka weather station, and the CANDAC operators for the logistical and on-site support provided at Eureka. Part of this work was performed at the Jet Propulsion Laboratory, California Institute of Technology, under contract with NASA. NCEP Reanalysis data is provided by the NOAA/OAR/ESRL PSD, Boulder, Colorado, USA, from their Web site at http://www.cdc.noaa.gov/.

Attached Files

Published - acp-11-12317-2011.pdf

Files

acp-11-12317-2011.pdf

Files (4.6 MB)

Name	Size	Download all
acp-11-12317-2011.pdf md5:f70a03d3a0320fb940decc60773806f6	4.6 MB	Preview Download

Additional details

	All versions	This version
Views	0	0
Downloads	0	0
Data volume	0 Bytes	0 Bytes