Accounting for Aerosols Effect in GHGSat Methane Retrieval
Abstract. GHGSat comprises a constellation of high spatial and spectral resolution satellites, specializing in monitoring methane emissions at 1.65 μm. This study investigates the ability to accurately retrieve both the methane mixing ratio enhancement (ΔXCH4) and aerosol optical depth (XAOD) simultaneously from simulated GHGSat observations that incorporate angle-dependent scattering information. Results indicate that the polarity of ΔXCH4 when neglecting aerosols changes from negative to positive as surface albedo increases, which is consistent with previous studies. Biases in ΔXCH4 are most pronounced when XAOD is not simultaneously retrieved, ranging from −3.0 % to 6.3 % with a 0.1 AOD, a 60° solar zenith angle, and a 0.2 surface albedo for the nadir-only retrieval. Using multiple satellite viewing angles during the GHGSat observation sequence with a scattering angle ranging from 100° to 140°, the study shows that the mean bias and standard deviation of XCH4 are within 0.2 % and 2.7 % relative to the background. The correlation between simultaneously retrieved ΔXCH4 and XAOD shifts from positive to negative as surface albedo increases and aerosol asymmetry factor decreases, signifying a transition of the dominating aerosol effect from aerosol-only scattering to aerosol-surface multiple scattering. The variety of scattering angle ranges has little impact on the performance of the multi-angle viewing method. This study improves the understanding of the aerosol impact on the GHGSat ΔXCH4 retrieval and provides guidance for improving future GHGSat-like point-source imagers.
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