Information Content Analysis: The Potential for Methane Isotopologue Retrieval from GOSAT-2

Abstract

Abstract. Atmospheric methane is comprised of multiple isotopic molecules, with the most abundant being 12 CH 4 and 13 CH 4 10 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 δ 13 C value. δ 13 C values typically range between -10 and -80 per mil, with abiogenic sources closer to zero, and biogenic sources showing more negative values. Initially, we suggest that a δ 13 C difference of 10 per mil is sufficient, in order to differentiate between methane source types, based on this we derive that a precision of 15 0.25ppbv on 13 CH 4 retrievals may achieve the target δ 13 C variance. Using an application of the well established Information Content Analysis (ICA) technique for assumed clear sky conditions, this manuscript shows that using a combination of the Shortwave Infrared (SWIR) bands on the planned Greenhouse gases Observing SATellite (GOSAT)-2 mission, 13 CH 4 can be measured with sufficient Information Content (IC) to a precision of between 0.7 and 1.2ppbv from a single sounding (assuming a total column average value of 19.14ppbv), which can then be reduced to the target precision through spatial and 20 temporal averaging techniques. We, therefore, suggest that GOSAT-2 can be used to differentiate between methane source types. Large unconstrained 'a priori' covariance matrices are required in order to achieve sufficient information content and that varying the solar inclination angle has limited impact on information content or retrieval errors.

On the consistency of methane retrievals using the Total Carbon Column Observing Network (TCCON) and multiple spectroscopic databases

“…Based on these factors, we assume a rough total column δ 13 C variability of 1 ‰, which equates to a total uncertainty of < 0.02 ppb in 13 CH 4 retrievals, or roughly 0.1 % of the total column, calculated using Eq. ( 1) (Malina et al, 2018). This is clearly an unrealistic target for individual retrievals given the uncertainty requirements for 12 CH 4 described above.…”

“…Finally, although the quality of the 13 CH 4 fit metrics in this study is not covered in detail, we calculate δ 13 C in order to understand the plausibility of and variation in retrieving 13 CH 4 from TCCON. The final aim of retrieving 13 CH 4 is to calculate δ 13 C. How much δ 13 C varies in the total column is a complex issue (Weidmann et al, 2017;Malina et al, 2018Malina et al, , 2019. In situ studies (Nisbet et al, 2016;Rigby et al, 2017;Fisher et al, 2017) all show that an uncertainty of 1 ‰ in δ 13 C is required in order to determine natural annual variability at the surface.…”

“…Almost all measurements of this value are limited to in situ studies or airborne flask measurements, which although highly accurate, by their nature are spatially limited. Some effort has gone into satellite-based retrievals of this value (Buzan et al, 2016;Weidmann et al, 2017;Malina et al, 2018Malina et al, , 2019, but the results of these studies show this to be a challenging task. Therefore the calculation of the δ 13 C value is a target of secondary importance in this study.…”