XCO2 Retrieval Based on Correlation Between Weak (Strong) CO2 and O2 Channels

OCO will measure spectra of reflected radiance at the top of the atmosphere in two CO2 bands and the O2 A-band. Because the radiance depends strongly upon fluctuations in solar radiance and surface albedo, we work with the apparent optical path difference (AOPD), defined by [ Lν = - m-1 log Iν/Iν0 ] where m is the geometric air mass factor, in order to reduce the sensitivity to both. The AOPDdepends strongly on the difference between the optical thicknesses at frequencies ν and ν0, and for atmospheres with low scattering optical thickness is a weak function of the surface reflection properties, atmospheric composition and meteorology (temperature, humidity and pressure profiles). Linear correlations exist between selected CO2 and O2 spectral points, with the slope and offset of each correlation depending on the surface albedo, surface pressure and average temperature of the atmospheric column. The dependence was obtained from analysis of spectra simulated for an ensemble of~450 atmospheric models, based on a library of atmospheric profiles from ECMWF. To assess the accuracy of the technique, we used an independent data base of spectra simulated for atmospheres (typically 125 layers) with aerosol and cloud properties derived from Calipso and Cloudsat observations, and meteorology from simultaneous and collocated ECMWF analyses. The accuracy of a single retrieval is ≈ 2 ppmv, with the average bias over a simulated OCO orbit with 488 frames being ≈ 0.5 ppmv. Only 7% of the retrievals had to be rejected because they failed to meet the convergence criteria.