Evaluating the consistency between OCO-2 and OCO-3 XCO₂ estimates derived from the NASA ACOS version 10 retrieval algorithm

<p><span id="page3174"/>The version 10 (v10) Atmospheric Carbon Observations from Space (ACOS) Level 2 full-physics (L2FP) retrieval algorithm has been applied to multiyear records of observations from NASA's Orbiting Carbon Observatory 2 and 3 sensors (OCO-2 and OCO-3, respectively) to provide estimates of the carbon dioxide (CO<span class="inline-formula">₂</span>) column-averaged dry-air mole fraction (XCO<span class="inline-formula">₂</span>). In this study, a number of improvements to the ACOS v10 L2FP algorithm are described. The post-processing quality filtering and bias correction of the XCO<span class="inline-formula">₂</span> estimates against multiple truth proxies are also discussed. The OCO v10 data volumes and XCO<span class="inline-formula">₂</span> estimates from the two sensors for the time period of August 2019 through February 2022 are compared, highlighting differences in spatiotemporal sampling but demonstrating broad agreement between the two sensors where they overlap in time and space. A number of evaluation sources applied to both sensors suggest they are broadly similar in data and error characteristics. Mean OCO-3 differences relative to collocated OCO-2 data are approximately 0.2 and <span class="inline-formula">−</span>0.3 ppm for land and ocean observations, respectively. Comparison of XCO<span class="inline-formula">₂</span> estimates to collocated Total Carbon Column Observing Network (TCCON) measurements shows root mean squared errors (RMSEs) of approximately 0.8 and 0.9 ppm for OCO-2 and OCO-3, respectively. An evaluation against XCO<span class="inline-formula">₂</span> fields derived from atmospheric inversion systems that assimilated only near-surface CO<span class="inline-formula">₂</span> observations, i.e., did not assimilate satellite CO<span class="inline-formula">₂</span> measurements, yielded RMSEs of 1.0 and 1.1 ppm for OCO-2 and OCO-3, respectively. Evaluation of uncertainties in XCO<span class="inline-formula">₂</span> over small areas, as well as XCO<span class="inline-formula">₂</span> biases across land–ocean crossings, also indicates similar behavior in the error characteristics of both sensors. Taken together, these results demonstrate a broad consistency of OCO-2 and OCO-3 XCO<span class="inline-formula">₂</span> measurements, suggesting they may be used together for scientific analyses.</p>