Evaluation and attribution of OCO-2 XCO<sub>2</sub> uncertainties

Abstract

Abstract. Evaluating and attributing uncertainties in total column atmospheric CO 2 measurements (XCO 2 ) from the OCO-2 instrument is critical for testing hypotheses related to the underlying processes controlling XCO 2 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 XCO 2 measurements by examining variations of the XCO 2 measurements and their calculated uncertainties within small regions (∼ 100 km × 10.5 km) in which natural CO 2 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 has a precision and accuracy of 0.35 and 0.24 ppm respectively for calculated precisions larger than ∼ 0.25 ppm. These values are approximately consistent with the calculated errors of 0.33 and 0.14 ppm for the noise and interference error, assuming that the accuracy is bounded by the calculated interference 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 are not well understood. A typical land measurement, both nadir and glint, is found to have a precision and accuracy of approximately 0.75 and 0.65 ppm respectively as compared to the calculated precision and accuracy of approximately 0.36 and 0.2 ppm. The differences in accuracy between ocean and land suggests that the accuracy of XCO 2 data is likely related to interferences such as aerosols or surface albedo as they vary less over ocean than land. The accuracy as derived here is also likely a lower bound as it does not account for possible systematic biases between the regions used in this analysis.

Airborne measurements of CO<sub>2</sub> column concentrations made with a pulsed IPDA lidar using a multiple-wavelength-locked laser and HgCdTe APD detector

“…For GOSAT, standard deviations (SDs) of 1.7 ppm were found vs. TCCON measurements with 0.5-0.8 ppm of that error irreducible by averaging, implying a bias of that order (Kulawik et al, 2016). For OCO-2 typical land measurements are found to have a precision and accuracy of approximately 0.75 and 0.65 ppm, respectively, based on the small region consistency assumption, which may well underestimate the bias between regions (Worden et al, 2017). A substantial portion of this error is likely related to interferences such as aerosols or surface albedo.…”

Emissions and topographic effects on column CO2 () variations, with a focus on the Southern California Megacity

“…It is in a Sun‐synchronous orbit and has an equatorial crossing time of around 1 P.M. local solar time. Worden et al [] found typical land measurement precision (1 σ ) and accuracy to be 0.75 ppm and 0.65 ppm with the caveat that the precision estimate includes effects of synoptic variability. We describe the filtering of OCO‐2 data and “background” selection in Appendix .…”

Contrasting carbon cycle responses of the tropical continents to the 2015–2016 El Niño

“…The standard deviation of the observations is used to approximate the model representation errors. The mean of the observation error statistics within the 1° × 1° box are used to represent biases, assuming biases are dominant relative to random errors (88).…”