OCO-2/3 v10/11 10-second average XCO2 data used in a recent v11-v10 comparison

NASA's Atmospheric Carbon Observations from Space (ACOS) Level 2 full physics (L2FP) retrieval algorithm has been used to estimate of the total column dry-air mole fraction of carbon dioxide (XCO2) from satellite measurements of reflected solar radiation. A recent update of the L2FP code (Version 11, or 'v11'), has been used to retrieve XCO2 from data from the Orbiting Carbon Observatory 2 (OCO-2) satellite, as well as from the near-identical Orbiting Carbon Observatory 3 (OCO-3) instrument mounted on the International Space Station (ISS). The performance of this v11 update, along with the associated screening-out of bad data and the post hoc bias correction of the remaining data, has been assessed by Taylor et al. (2026), in part by comparing to the previous versions ('v10') of the bias-corrected XCO2 from OCO-2 and OCO-3. Part of this analysis used a reduced-volume version of the data, achieved by averaging the original XCO2 retrievals across 10-second spans. Four files containing the 10-second average XCO2 retrievals used in the Taylor et al. analysis are presented here: OCO-2 v11 (20140906-20240731): OCO2_b11.2_10sec_GOOD_r3.nc4 OCO-2 v10 (20140906-20220228): OCO2_b10c_10sec_GOOD_r7.nc4 OCO-3 v11 (20190806-20250331): OCO3_b11_10sec_GOOD_r4.nc4 OCO-3 v10 (20190806-20231112): OCO3_b10.4-lite_10sec_GOOD_190806-231112.h5 Below, we give some details on the OCO-2 and OCO-3 viewing approach, the CO2 and XCO2 values retrieved from their data, and the weighted average used to compute the 10-second averages presented here. The Orbiting Carbon Observatory 2 (OCO-2) satellite, as well as the near-identical Orbiting Carbon Observatory 3 (OCO-3) instrument mounted aboard the ISS with a pointing mechanism that allows it to mimic OCO-2's thin-swath scan, measure solar radiance reflected from the Earth's surface in two CO2 absorption bands (1.6 and 2.0 um), as well as in the O2 A-band. By comparing the CO2 / O2 absorption ratio, the dry air mixing ratio of CO2 may be estimated along the observed path, i.e. averaged across the full atmospheric column, though with sensitivity peaking near the Earth's surface. A radiative transfer model that accounts for the effects of thin clouds and aerosols, water vapor, and surface albedo variations, is used, and the vertical profile of CO2 mixing ratio is estimated from the radiance data. This CO2 profile is then collapsed to a scalar vertical average (XCO2), which is then bias corrected post hoc against Earth-based Fourier spectrometer data from the Total Carbon Column Observation Network (TCCON), which itself is tied to CO2 measurement standards using in situ aircraft CO2 profile measurements. OCO-2 & OCO-3 take measurements in a thin swath (nominally 10-km wide) underneath the satellite/ISS, with a 3 Hz cross-scan rate. Each cross-scan is divided into 8 individual fields of view (FOVs) of approximate size 2.3km x 1.3km. These small FOVs increase the chances of seeing through clouds and can reveal details of point-source CO2 emissions, but are generally much finer-scale than can be modeled by the atmospheric transport models used in global CO2 flux inversions, which typically use grid boxes 100s of km on a side. Rather than assimilating each fine-scale FOV XCO2 value individually and comparing them to modeled XCO2 values that change much more slowly, it is convenient to average the original OCO-2/3 data to coarser scales beforehand, then assimilate these averaged values in the flux inversions. This averaging has the beneficial side-effect of reducing the OCO-2/3 data volume considerably. In the files presented here, the data have been averaged across 10-second spans, equivalent to an along-track distance of ~67.5 km for OCO-2 (or ~73 km for OCO-3) on the Earth's surface. The bias-corrected XCO2 retrievals at the original fine-scale resolution are available at NASA's Goddard Earth Sciences (GES) Data and Information Services Center (DISC) as 'Lite' files. The file collections corresponding to the XCO2 retrievals that are used as input to the four 10-second average files given above are (for reference): "OCO-2 Level 2 bias-corrected XCO2 and other select fields from the full-physics retrieval aggregated as daily files, Retrospective processing V11.2r (OCO2_L2_Lite_FP)", https://disc.gsfc.nasa.gov/datasets/OCO2_L2_Lite_FP_11.2r/summary?keywords=OCO2_L2_Lite_FP, "OCO-2 Level 2 bias-corrected XCO2 and other select fields from the full-physics retrieval aggregated as daily files, Retrospective processing V10r (OCO2_L2_Lite_FP)", https://disc.gsfc.nasa.gov/datasets/OCO2_L2_Lite_FP_10r/summary?keywords=OCO2_L2_Lite_FP, "OCO-3 Level 2 bias-corrected XCO2 and other select fields from the full-physics retrieval aggregated as daily files, Retrospective processing V11r (OCO3_L2_Lite_FP)", https://disc.gsfc.nasa.gov/datasets/OCO3_L2_Lite_FP_11r/summary?keywords=OCO3_L2_Lite_FP. "OCO-3 Level 2 bias-corrected XCO2 and other select fields from the full-physics retrieval aggregated as daily files, Retrospective processing v10.4r (OCO3_L2_Lite_FP)", https://disc.gsfc.nasa.gov/datasets/OCO3_L2_Lite_FP_10.4r/summary?keywords=OCO3_L2_Lite_FP. The original XCO2 values contained in these data collections, along with other auxiliary variables provided for analysis purposes, have been averaged in the same manner across each 10-second span, with the approach used with the Version 10 release of the OCO-2 XCO2 data, as described in Section 3.2.1 of Byrne et al. (2023) and Section 3.1.1 of Baker et al. (2022): each value in the span is weighted with the inverse square of its retrieved XCO2 uncertainty value, taken from variable 'xco2_uncertainty' from the 'Lite" file. Data inside each 10-second span are averaged separately based on viewing mode and surface type, as indicated by variable 'data_type'. Only data with a 'good' retrieval quality flag (variable 'xco2_quality_flag' in the 'Lite' file equal to zero) are included in the average; the number of such 'good' data values (1-240) included in each average value is indicated in variable 'N_total_shots'. It has been found that 10-second spans with fewer 'good' retrievals tend to be affected more by the unwanted effects of aerosols and undetected clouds: these may be mitigated somewhat by not using 10-sec averages for spans with low 'N_total_shots' values. Variable 'assimilate_flag' separates those data that are not taken in glint viewing mode over either land or water, or in nadir mode over land, from data taken in other modes (target mode, or in transition to/from target mode, or nadir mode over water, or mixed land/water scenes) that are generally not assimilated in flux inversions; for OCO-3, it indicates which scenes are part of a Snapshot Area Map (SAM); and it also flags 5% of the assimilable data by orbit for use as withheld evaluation data, for use in inversion studies. The uncertainty on the 10-sec average XCO2 value is given in variable 'xco2_uncertainty'; this accounts for correlations in error between individual scenes (+0.3 over land, +0.6 over ocean) as described in Section 3.2.1 of Baker et al (2022), as well as variability in the averaged XCO2 values not captured by the retrieval uncertainties. References Baker, DF, E Bell, KJ Davis, JF Campbell, B Lin & J Dobler, A new exponentially-decaying error correlation model for assimilating OCO-2 column-average CO2 data, using a length scale computed from airborne lidar measurements, Geosci. Model Dev., 15, 649–668, https://doi.org/10.5194/gmd-15-649-2022, 2022. Byrne, B, DF Baker, et al., National CO2 budgets (2015-2020) inferred from atmospheric CO2 observations in support of the Global Stocktake, Earth System Sci. Data, https://doi.org/10.5194/essd-2022-213, 2023. Taylor, TE, CW O'Dell, et al., The OCO-2 and OCO-3 ACOS version 11 XCO2 data products: a benchmark record for space-based global carbon cycle research, submitted to Earth and Space Science