Aerosols in OCO-2/GOSAT retrievals of XCO2: An information content and error analysis

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.