Abstract
Abstract. As a greenhouse gas with strong global warming potential, atmospheric methane (CH4) emissions have attracted a great deal of attention. Although remote sensing measurements can provide information about CH4 sources and emissions, accurate retrieval is challenging due to the influence of atmospheric aerosol scattering. In this study, imaging spectroscopic measurements from the Airborne Visible/Infrared Imaging Spectrometer – Next Generation (AVIRIS-NG) in the shortwave infrared are used to compare two retrieval techniques – the traditional matched filter (MF) method and the optimal estimation (OE) method, which is a popular approach for trace gas retrievals. Using a numerically efficient radiative transfer model with an exact single-scattering component and a two-stream multiple-scattering component, we also simulate AVIRIS-NG measurements for different scenarios and quantify the impact of aerosol scattering in the two retrieval schemes by including aerosols in the simulations but not in the retrievals. The presence of aerosols causes an underestimation of CH4 in both the MF and OE retrievals; the biases increase with increasing surface albedo and aerosol optical depth (AOD). Aerosol types with high single-scattering albedo and low asymmetry parameter (such as water-soluble aerosols) induce large biases in the retrieval. When scattering effects are neglected, the MF method exhibits lower fractional retrieval bias compared to the OE method at high CH4 concentrations (2–5 times typical background values) and is suitable for detecting strong CH4 emissions. For an AOD value of 0.3, the fractional biases of the MF retrievals are between 1.3 % and 4.5 %, while the corresponding values for OE retrievals are in the 2.8 %–5.6 % range. On the other hand, the OE method is an optimal technique for diffuse sources (<1.5 times typical background values), showing up to 5 times smaller fractional retrieval bias (8.6 %) than the MF method (42.6 %) for the same AOD scenario. However, when aerosol scattering is significant, the OE method is superior since it provides a means to reduce biases by simultaneously retrieving AOD, surface albedo, and CH4. The results indicate that, while the MF method is good for plume detection, the OE method should be employed to quantify CH4 concentrations, especially in the presence of aerosol scattering.
Highlights
Atmospheric methane (CH4) is about 85 times more potent per unit mass at warming the Earth than carbon dioxide (CO2) on a 20-year timescale (Myhre et al, 2013), implying that reduction in CH4 emissions could be very efficient to slow down global warming in the near term
The traditional matched filter (MF) and the optimal estimation (OE) methods are used to quantify the effects of aerosol scattering on CH4 retrievals based on simulations of AVIRIS-NG measurements
The results show that the retrieval biases increase with increasing aerosol optical depth (AOD) and surface albedo for both techniques
Summary
Atmospheric methane (CH4) is about 85 times more potent per unit mass at warming the Earth than carbon dioxide (CO2) on a 20-year timescale (Myhre et al, 2013), implying that reduction in CH4 emissions could be very efficient to slow down global warming in the near term. Y. Huang et al.: Aerosol scattering impacts on methane retrievals from airborne measurements in greater CH4 concentrations (Holmes et al, 2013). The sources and sinks of methane are reasonably well known, there are large uncertainties in their relative amounts and in the partitioning between natural and anthropogenic contributions (Nisbet et al, 2014, 2016). This uncertainty is exemplified by the CH4 “hiatus”, which refers to the observed stabilization of atmospheric CH4 concentrations from 1999–2006, and the renewed rise thereafter (Kirschke et al, 2013).
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