Rapid Estimation of Column-Averaged CO2 Concentration Using a Correlation Algorithm

Measurement of X-CO2, the column-averaged mole fraction of CO2, using reflected sunlight in the near-infrared bands of CO2, is strongly influenced by photons that are scattered in the atmosphere because scattering can either decrease or increase the mean pathlength compared with the direct path from the sun to the surface to the satellite. A very simple algorithm that can be used to compensate for the errors introduced by scattering is presented. The algorithm is based on the observation that the apparent optical path differences in selected pairs of channels in the weak CO2 band at 1.6 mu m and the O-2 A band at 0.76 mu m are tightly correlated for large ensembles of scattering atmospheres. The number of tightly correlated pairs of channels is many hundreds for the bands measured by NASA's Orbiting Carbon Observatory (000). The physical reasons for the correlation are that the mean photon pathlengths are comparable for the members of each pair of channels, and that the extinction profiles vary similarly with height. For atmospheres with modest scattering optical thickness (less than 0.3), the slope and the intercept of the linear correlation for any pair depends weakly on the surface reflectance, the surface pressure, and the viewing geometry. Through numerical simulations the slope and intercept may be parameterized simply in terms of these variables. Thereafter, the task of retrieving X-CO2 from measured spectra may be reduced to linear interpolation in precomputed tables of slopes and intercepts. Results with simulated data for NASA's OCO satellite are presented, and random errors and biases are investigated. Although OCO did not reach orbit, the method is applicable to any instrument that operates using similar principles [such as those on the Greenhouse Gases Observing Satellite (GOSAT) and the replacement satellite OCO-2].