Abstract
Abstract. Retrievals of methane isotopologues have the potential to differentiate between natural and anthropogenic methane sources types, which can provide much needed information about the current global methane budget. We investigate the feasibility of retrieving the second most abundant isotopologue of atmospheric methane (13CH4, roughly 1.1 % of total atmospheric methane) from the shortwave infrared (SWIR) channels of the future Sentinel-5/ultra-violet, visible, near-infrared, shortwave infrared (UVNS) and current Copernicus Sentinel-5 Precursor TROPOspheric Monitoring Instrument (TROPOMI) instruments. With the intended goal of calculating the δ13C value, we assume that a δ13C uncertainty of better than 1 ‰ is sufficient to differentiate between source types, which corresponds to a 13CH4 uncertainty of <0.02 ppb. Using the well-established information content analysis techniques and assuming clear-sky, non-scattering conditions, we find that the SWIR3 (2305–2385 nm) channel on the TROPOMI instrument can achieve a mean uncertainty of <1 ppb, while the SWIR1 channel (1590–1675 nm) on the Sentinel-5 UVNS instrument can achieve <0.68 ppb or <0.2 ppb in high signal-to-noise ratio (SNR) cases. These uncertainties combined with significant spatial and/or temporal averaging techniques can reduce δ13C uncertainty to the target magnitude or better. However, we find that 13CH4 retrievals are highly sensitive to errors in a priori knowledge of temperature and pressure, and accurate knowledge of these profiles is required before 13CH4 retrievals can be performed on TROPOMI and future Sentinel-5/UVNS data. In addition, we assess the assumption that scattering-induced light path errors are cancelled out by comparing the δ13C values calculated for non-scattering and scattering scenarios. We find that there is a minor bias in δ13C values from scattering and non-scattering retrievals, but this is unrelated to scattering-induced errors.
Highlights
With the recent launch of the TROPOspheric Monitoring Instrument (TROPOMI) aboard the Copernicus Sentinel-5 Precursor (S5P) satellite, global monitoring of methane concentrations and fluxes has been put firmly at the forefront of the efforts towards understanding global greenhouse gas (GHG) emissions and climate change
The global spread of Degrees of freedom of signal (DFS) values generated by combining the SWIR1 and SWIR3 channels is shown in Fig. 13 below
This study used the well-established information content analysis techniques to determine the potential for 13CH4 retrievals from the shortwave infrared (SWIR) channels of the current S5P/TROPOMI instrument (2305– 2385 nm) and the future S5/UVNS instrument (1590–1675 and 2305–2385 nm) assuming clear-sky, non-scattering conditions
Summary
With the recent launch of the TROPOspheric Monitoring Instrument (TROPOMI) aboard the Copernicus Sentinel-5 Precursor (S5P) satellite, global monitoring of methane concentrations and fluxes has been put firmly at the forefront of the efforts towards understanding global greenhouse gas (GHG) emissions and climate change. This disagreement is likely due to currently limited observations, incorrect atmospheric transport assumptions, or uncertainties associated with bottom-up inventories and uncertainties in modelling CH4 chemical losses (Kirschke et al, 2013) This is best shown through the current multiple, sometimes contradicting theories as to the reasons for the pause in atmospheric methane growth at the start of the last decade and its subsequent rise several years later (Kai et al, 2011; Aydin et al, 2011; Nisbet et al, 2014, 2016; Mcnorton et al, 2016; Rigby et al, 2017).
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