XCO2 retrieval error over deserts near critical surface albedo

Large retrieval errors in column-weighted CO2 mixing ratio (X-CO2) over deserts are evident in the Orbiting Carbon Observatory 2 version 7 L2 products. We argue that these errors are caused by the surface albedo being close to a critical surface albedo ((c)). Over a surface with albedo close to (c), increasing the aerosol optical depth (AOD) does not change the continuum radiance. The spectral signature caused by changing the AOD is identical to that caused by changing the absorbing gas column. The degeneracy in the retrievals of AOD and X-CO2 results in a loss of degrees of freedom and information content. We employ a two-stream-exact single scattering radiative transfer model to study the physical mechanism of X-CO2 retrieval error over a surface with albedo close to (c). Based on retrieval tests over surfaces with different albedos, we conclude that over a surface with albedo close to (c), the X-CO2 retrieval suffers from a significant loss of accuracy. We recommend a bias correction approach that has significantly improved the X-CO2 retrieval from the California Laboratory for Atmospheric Remote Sensing data in the presence of aerosol loading.