Generalized Canonical Transform method for radio occultation sounding with improved retrieval in the presence of horizontal gradients.

As a greenhouse gas with strong global warming potential, atmospheric methane (CH4) emissions have attracted a great deal of attention. Remote sensing measurements can provide information about CH4 sources and emissions. However, accurate assessment of CH4 emissions is challenging due to the influence of aerosol scattering in the atmosphere. In this study, imaging spectroscopic measurements from the Airborne Visible/Infrared Imaging Spectrometer-Next Generation (AVIRIS-NG) in the short-wave infrared are used to analyze the impact of aerosol scattering on CH4 retrievals. Using a numerically efficient two-stream-exact-single-scattering radiative transfer model, we also simulate AVIRIS-NG measurements for different scenarios and quantify the impact of aerosol scattering using two retrieval techniques - the traditional Matched Filter (MF) method and the Optimal Estimation (OE) method, which is a popular approach for trace gas retrievals. The results show that the MF method exhibits up to 50% lower fractional retrieval bias compared to the OE method at high CH4 concentrations (>100% enhancement over typical background values) and is suitable for detecting strong CH4 emissions, while the OE method is an optimal technique for diffuse sources (<50% enhancement), showing up to five times smaller fractional retrieval bias than the MF method. In addition, the impacts of aerosol scattering as a function of different parameters, such as surface albedo, CH4 concentration, aerosol optical depth, single scattering albedo and asymmetry parameter, are also discussed. [ABSTRACT FROM AUTHOR]