Retrieval of atmospheric CO_2 with enhanced accuracy and precision from SCIAMACHY: Validation with FTS measurements and comparison with model results

Creators: Reuter, M.; Bovensmann, H.; Buchwitz, M.; Burrows, J. P.; Connor, B. J.; Deutscher, N. M.; Griffith, D. W. T.; Heymann, J.; Keppel-Aleks, G.; Messerschmidt, J.; Notholt, J.; Petri, C.; Robinson, J.; Schneising, O.; Sherlock, V.; Velazco, V.; Warneke, T.; Wennberg, P. O.; Wunch, D.

Abstract

The Bremen Optimal Estimation differential optical absorption spectroscopy (DOAS) (BESD) algorithm for satellite based retrievals of XCO_2 (the column-average dry-air mole fraction of atmospheric CO_2) has been applied to Scanning Imaging Absorption Spectrometer for Atmospheric Cartography (SCIAMACHY) data. It uses measurements in the O_2-A absorption band to correct for scattering of undetected clouds and aerosols. Comparisons with precise and accurate ground-based Fourier transform spectrometer (FTS) measurements at four Total Carbon Column Observing Network (TCCON) sites have been used to quantify the quality of the new SCIAMACHY XCO_2 data set. Additionally, the results have been compared to NOAA's assimilation system CarbonTracker. The comparisons show that the new retrieval meets the expectations from earlier theoretical studies. We find no statistically significant regional XCO_2 biases between SCIAMACHY and the FTS instruments. However, the standard error of the systematic differences is in the range of 0.2 ppm and 0.8 ppm. The XCO_2 single-measurement precision of 2.5 ppm is similar to theoretical estimates driven by instrumental noise. There are no significant differences found for the year-to-year increase as well as for the average seasonal amplitude between SCIAMACHY XCO_2 and the collocated FTS measurements. Comparison of the year-to-year increase and also of the seasonal amplitude of CarbonTracker exhibit significant differences with the corresponding FTS values at Darwin. Here the differences between SCIAMACHY and CarbonTracker are larger than the standard error of the SCIAMACHY values. The difference of the seasonal amplitude exceeds the significance level of 2 standard errors. Therefore, our results suggest that SCIAMACHY may provide valuable additional information about XCO_2, at least in regions with a low density of in situ measurements.

Additional Information

© 2011 American Geophysical Union. Received 13 September 2010; revised 16 November 2010; accepted 10 December 2010; published 23 February 2011. This work was in part funded by ESA/ ESRIN (ADVANSE, GHG‐CCI), EU FP7 (MACC, CityZen), DLR (SADOS), and the state and the University of Bremen. We thank NOAA for making available the CarbonTracker CO2 fields and ECMWF for providing the meteorological data. TCCON is funded by the NASA terrestrial carbon cycle program, grant NNX08AI86G. Lauder TCCON measurements are supported by New Zealand Foundation for Research Science and Technology contracts CO1X0204 and CO1X0406. We thank NASA for making available the CALIPSO data and the GEMS project for providing their aerosol product. We further thank our reviewers for their helpful and valuable comments to improve this work. Many thanks are given to S. Pfeifer and J. Reuter for proofreading the manuscript.

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