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c Author(s) 2016. CC-BY 3.0 License. 1 Title: Evaluation And Attribution Of...

Worden, J., G. Doran, S. Kulawik, A. Eldering, D. Crisp, C. Frankenberg, C. O’Dell, and K. Bowman (2016), c Author(s) 2016. CC-BY 3.0 License. 1 Title: Evaluation And Attribution Of OCO-2 XCO2 Uncertainties 2 3 Authors: John Worden1, Gary Doran1, Susan Kulawik2, Annmarie Eldering1, David 4 Crisp1, Christian Frankenberg3,1, Chris O’Dell4, and Kevin Bowman1 5, Atmos. Meas. Tech., doi:10.5194/amt-2016-175.

Evaluating and attributing uncertainties in total column atmospheric CO2 measurements (XCO2) from the OCO-2 instrument is critical for testing hypotheses related to the underlying processes controlling XCO2 and for developing quality flags needed to choose those measurements that are usable for carbon cycle science.
Here we test the reported uncertainties of Version 7 OCO-2 XCO2 measurements by examining variations of the XCO2 measurements and their calculated uncertainties within small regions (~100 km x 10.5 km) across the globe in which CO2 variability is expected to be small relative to variations imparted by noise or interferences. Over 39,000 of these “small neighborhoods” comprised of approximately 190 observations per neighborhood are used for this analysis. We find that a typical ocean measurement should have a precision and accuracy of 0.35 and 0.2 ppm respectively for calculated precisions larger than ~0.25 ppm. These values are consistent with the calculated errors (assuming that the accuracy is bounded by the calculated interference error and does not include smoothing error). The actual precision for ocean data becomes worse as the signal-to-noise increases or the calculated precision decreases below 0.25 ppm for reasons that not well understood. A typical land measurement (both nadir and glint) is found to have a precision and accuracy of approximately 0.75 ppm and 0.6 ppm respectively with the accuracy likely being a lower bound; these values can be compared to the calculated precision and accuracy of approximately 0.36 ppm and 0.2 ppm. This bound on the accuracy is related to an observed, slowly varying XCO2 across a typical neighborhood that is about three times larger than expected from models and independent data, and is correlated with interferences such as aerosol optical depth, albedo, and surface pressure suggesting that these effects are possibly responsible for this error.

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Orbiting Carbon Observatory-2 (OCO-2)