Scattering by atmospheric aerosols is the largest source of systematic errors in XCO2 retrievals, for which accuracy is a key requirement. Aerosols impart a high degree of anisotropy to the radiation field of the atmosphere, whose form depends on a number of parameters like the number density of the aerosol, microphysical properties like its chemical composition, size distribution and shape, and its vertical profile, in addition to surface reflectance. These properties vary in space and time, and are hence difficult to constrain. The multi-angle viewing capability of OCO-3 measurements provide the angular information needed to better constrain the properties of atmospheric aerosols, thus promising a physically justified means of constraining biases in the retrieved XCO2.

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-accuracy vSmartMOM simulations of multi-angle spectropolarimetric observations in the O2 A-band and the weak and strong CO2 bands with their Jacobian matrices were used to evaluate the information contained in different measurement subsets/synergies for the retrieval of aerosol, surface, and gaseous parameters. We compared the biases and uncertainties in the retrieved XCO2 when aerosol microphysics are treated as free and as fixed parameters, respectively.

We found that it is difficult to achieve the required retrieval accuracy of 0.2% for XCO2 using intensity-only Nadir mode measurements. The addition of polarimetric and multi-angle information can achieve orders-of-magnitude improvements in most retrieved state parameters, although the response of the uncertainty in XCO2 is weak. The bias induced in the retrieved XCO2 due to uncertainties in other parameters can be eliminated by expanding measurement synergies for retrievals with free aerosol microphysical parameters, while the sensitivity of XCO2 to smoothing biases tends to increase when they are fixed.

In this talk, I will discuss the early development of aerosol radiative transfer modeling and remote sensing in parallel with that of trace gases. Most of the science I present will show the information content, uncertainty, and bias response of XCO2 retrievals when aerosols (including microphysical parameters) are co-retrieved with XCO2 for different measurement strategies. I will conclude with the recent application of the core formalisms we have developed in Earth science to independent but related fields like exoplanet and substellar atmospheric research.