Retrieval of Atmospheric Optical Depth Profiles from Downward-Looking High-Resolution O2A-Band Measurements: Optically Thin Conditions

Abstract

A quasi-linear retrieval was developed to profile moderately thin atmospheres using a high-resolution O2 A-band spectrometer. The retrieval is explicitly linear with respect to single scattering; the multiple-scattering contribution is treated as a perturbation. The properties of the linear inversion, examined using singular value decomposition of the kernel function, demonstrate the impacts of instrument specifications, such as resolution, out-of-band rejection, and signal-to-noise ratio, on information content. A system with 0.5 cm−1 resolution, signal-to-noise ratio of 100:1, and out-of-band floor of 10−3 has four independent pieces of information. A fast radiative transfer model was developed to compute the multiple-scattering perturbation, in which multiple scattering is calculated at 16 different O2 absorption depths to synthesize the O2 A band. The linear system is then solved using Tikhonov's regularization with inequality constraints. Tests with synthetic data, including noise, of O2 A-band retrievals illustrate that this algorithm is accurate and fast for retrieving aerosol profiles. The errors are less than 10% for the integrated total optical depth for the cases tested. It is shown that instruments with the needed performance are practical.