The OCO‐2 and OCO‐3 ACOS Version 11 XCO$_2$ Data Products: A Benchmark Record for Space‐Based Global Carbon Cycle Research

NASA's Orbiting Carbon Observatory-2 (OCO-2) and OCO-3 have been observing Earth's atmosphere from space since September 2014 and August 2019, respectively. The version 11 (v11) Atmospheric Carbon Observations from Space (ACOS) Level 2 full physics (L2FP) retrieval algorithm has been used to provide estimates of the column-averaged dry-air mole fraction of carbon dioxide (XCO$_2$) from OCO measurements of surface reflected near-infrared solar radiation. In this work, updates to the ACOS v11 retrieval algorithm are discussed, and the quality filtering and bias correction of the XCO$_2$ products are described. XCO$_2$ estimates from OCO-2 and OCO-3 are compared to Total Carbon Column Observing Network (TCCON) observations and to atmospheric inversion models, which serve as truth-proxies for XCO$_2$. In addition, a direct comparison of collocated soundings between OCO-2 and OCO-3 is performed. Variations in v11 XCO$_2$ over small areas show improvement in agreement between actual and theoretical uncertainties, suggesting a reduction in systematic errors compared to v10, especially for OCO-3. There is reduced variance in the new v11 XCO$_2$ product relative to truth-proxies, including less divergence in the XCO$_2$ values with time against models. ... mehrThe collocations of XCO$_2$ observations between OCO-2 and OCO-3 indicate an overall mean difference of <0.2 ppm across the overlapping time range. Furthermore, OCO-2 and OCO-3 agree better with each other than they do with the validation sources, suggesting a well conditioned and unified set of XCO$_2$ concentrations that can be used concurrently for carbon cycle studies to provide a benchmark record spanning September 2014 through August 2026.