X_(CO2) retrieval error over deserts near critical surface albedo

Abstract

Large retrieval errors in column‐weighted CO_2 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 (α_c). Over a surface with albedo close to α_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 α_c. Based on retrieval tests over surfaces with different albedos, we conclude that over a surface with albedo close to α_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.