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<p>Characterization of errors and sensitivity in remotely sensed observations of greenhouse gases is necessary for their use in estimating regional-scale fluxes. We analyze 15 orbits of the simulated Orbiting Carbon Observatory-2 (OCO-2) with the Atmospheric Carbon Observations from Space (ACOS) retrieval, which utilizes an optimal estimation approach, to compare predicted versus actual errors in the retrieved <span class="inline-formula">CO₂</span> state. We find that the nonlinearity in the retrieval system results in <span class="inline-formula">XCO₂</span> errors of <span class="inline-formula">∼0.9</span>&thinsp;ppm. The predicted measurement error (resulting from radiance measurement error), about 0.2&thinsp;ppm, is accurate, and an upper bound on the smoothing error (resulting from imperfect sensitivity) is not more than 0.3&thinsp;ppm greater than predicted. However, the predicted <span class="inline-formula">XCO₂</span> interferent error (resulting from jointly retrieved parameters) is a factor of 4 larger than predicted. This results from some interferent parameter errors that are larger than predicted, as well as some interferent parameter errors that are more strongly correlated with <span class="inline-formula">XCO₂</span> error than predicted by linear error estimation. Variations in the magnitude of <span class="inline-formula">CO₂</span> Jacobians at different retrieved states, which vary similarly for the upper and lower partial columns, could explain the higher interferent errors. A related finding is that the error correlation within the <span class="inline-formula">CO₂</span> profiles is less negative than predicted and that reducing the magnitude of the negative correlation between the upper and lower partial columns from <span class="inline-formula">−0.9</span> to <span class="inline-formula">−0.5</span> results in agreement between the predicted and actual <span class="inline-formula">XCO₂</span> error. We additionally study how the postprocessing bias correction affects errors. The bias-corrected results found in the operational OCO-2 Lite product consist of linear modification of <span class="inline-formula">XCO₂</span> based on specific retrieved values, such as the <span class="inline-formula">CO₂</span> grad del (<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M13" display="inline" overflow="scroll" dspmath="mathml"><mrow><mi mathvariant="italic">δ</mi><msub><mi mathvariant="normal">∇</mi><mrow class="chem"><msub><mi mathvariant="normal">CO</mi><mn mathvariant="normal">2</mn></msub></mrow></msub></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="32pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="91b8b4dce3a20b884ea9d8cee8996633"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="amt-12-5317-2019-ie00001.svg" width="32pt" height="14pt" src="amt-12-5317-2019-ie00001.png"/></svg:svg></span></span>), (“grad del” is a measure of the change in the profile shape versus the prior) and dP (the retrieved surface pressure minus the prior). We find similar linear relationships between <span class="inline-formula">XCO₂</span> error and dP or <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M15" display="inline" overflow="scroll" dspmath="mathml"><mrow><mi mathvariant="italic">δ</mi><msub><mi mathvariant="normal">∇</mi><mrow class="chem"><msub><mi mathvariant="normal">CO</mi><mn mathvariant="normal">2</mn></msub></mrow></msub></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="32pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="3555b62de2a83d078e101df1114d1291"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="amt-12-5317-2019-ie00002.svg" width="32pt" height="14pt" src="amt-12-5317-2019-ie00002.png"/></svg:svg></span></span> but see a very complex pattern of errors throughout the entire state vector. Possibilities for mitigating biases are proposed, though additional study is needed.</p>