We have analyzed the effect of aerosols on the retrieval of the dry air mixing ratio of carbon dioxide (XCO₂) in the Earth's atmosphere from instruments like OCO-2 and GOSAT. High-fidelity simulations of multi-angle spectropolarimetric observations in the O₂ A-band and the weak and strong CO₂ bands are used to evaluate the information contained in different measurement subsets/synergies for the retrieval of aerosol, surface, and molecular parameters. We contrast the biases and uncertainties in the retrieved XCO₂ resulting from the assumption of free or fixed aerosol microphysical parameters in the retrieval algorithm. It is very difficult to achieve the required retrieval accuracy of 0.2% for XCO₂ using intensity-only Nadir mode measurements. The uncertainty in the retrieved XCO₂ can be minimized by introducing multiangle and polarimetric measurement synergies. While the retrieval bias on XCO₂ is practically eliminated by the addition of measurement synergies for free aerosol microphysical parameters, fixed aerosol retrievals can lead to an increase in XCO₂ bias. In both cases, our full multi-angle polarimetric dataset produces a maximum uncertainty of ~1.6% in the retrieved XCO₂ at low aerosol optical thicknesses and over dark surfaces. The XCO₂ retrieval uncertainty improves to better than 0.2% at greater aerosol optical thickness and brighter surfaces. The problematic low aerosol optical thickness and low surface brightness regime produces an XCO₂ bias of 1 − 8 % for fixed aerosol microphysics, while the free parameters produce a maximum bias well under 10⁻³% for all retrieval scenes considered
