Abstract
Abstract. Atmospheric methane is comprised of multiple isotopic molecules, with the most abundant being 12CH4 and 13CH4, making up 98 and 1.1 % of atmospheric methane respectively. It has been shown that is it possible to distinguish between sources of methane (biogenic methane, e.g. marshland, or abiogenic methane, e.g. fracking) via a ratio of these main methane isotopologues, otherwise known as the δ13C value. δ13C values typically range between −10 and −80 ‰, with abiogenic sources closer to zero and biogenic sources showing more negative values. Initially, we suggest that a δ13C difference of 10 ‰ is sufficient, in order to differentiate between methane source types, based on this we derive that a precision of 0.2 ppbv on 13CH4 retrievals may achieve the target δ13C variance. Using an application of the well-established information content analysis (ICA) technique for assumed clear-sky conditions, this paper shows that using a combination of the shortwave infrared (SWIR) bands on the planned Greenhouse gases Observing SATellite (GOSAT-2) mission, 13CH4 can be measured with sufficient information content to a precision of between 0.7 and 1.2 ppbv from a single sounding (assuming a total column average value of 19.14 ppbv), which can then be reduced to the target precision through spatial and temporal averaging techniques. We therefore suggest that GOSAT-2 can be used to differentiate between methane source types. We find that large unconstrained covariance matrices are required in order to achieve sufficient information content, while the solar zenith angle has limited impact on the information content.
Highlights
Of the major greenhouse gases (GHGs) currently considered to have a major impact on atmospheric chemistry, methane is amongst the most important
The exact technical details of GOSAT-2 are not yet available, but, due to the similarity of the instruments, we assume that the signal-tonoise ratio (SNR) and instrument line shape function (ILSF) on GOSAT-2/TANSO-FTS-2 and GOSAT/TANSO-FTS are similar; they are explained in more detail below (GOSAT-2 Project Team, 2017)
This work investigates the possibility of whether 13CH4 can be retrieved with a sufficient level of accuracy by bands 2 and 3 of the GOSAT-2/TANSO-FTS2 instrument via the use of the δ13C ratio in order to make a judgement on the nature of a methane source type
Summary
Of the major greenhouse gases (GHGs) currently considered to have a major impact on atmospheric chemistry, methane is amongst the most important. We apply the well-established information content analysis (ICA) techniques originally proposed by Rodgers (2000) to determine the potential benefit of retrieving total column methane isotopologue concentrations using bands 2 and 3 of the GOSAT-2/TANSO-FTS-2 instrument. The value of such studies has been proven on multiple occasions (Frankenberg et al, 2012; Herbin et al, 2013; Kuai et al, 2010; Yoshida et al, 2011), providing guidance on appropriate potential retrieval set-ups in order to maximise information received from trace gas retrievals. We describe the requirements for detecting methane isotopologues as well as the tools and assumed instruments employed during the course of this research
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