Using high frequency observations of δ¹³C-CH₄ and δ²H-CH₄ and uncertain regional isotopic signatures to estimate sources of UK methane emissions

Abstract. Methane is emitted from a range of anthropogenic and natural sources, and identifying these sources is important for emissions monitoring and mitigation. Different sources emit methane with different isotopic signatures; however these signatures are often uncertain or vary spatially or temporally. Top-down inverse models can be used with measurement of methane mole fractions to estimate total emissions of methane from all sources. We present an inverse model for estimating regional fossil-fuel (FF) and non-fossil-fuel (non-FF) emissions concurrently, using isotope ratio observations and uncertain isotopic signatures. This method is highly adaptable and could be used to estimate emissions from any number of sources. Synthetic data tests with this method show that this model can accurately estimate FF and non-FF methane emissions across the UK, when isotopic source signatures are fixed at known values. However, emissions estimation becomes less accurate when source signature uncertainties rise above approximately 50 % of their likely ranges. In a real-world test of this method, we estimated south-east UK FF and non-FF emissions using high-frequency δ¹³C-CH₄ and δ²H-CH₄ observations from one UK site, with source signature uncertainties reflecting our current understanding of these values. Results show a limited impact on emissions uncertainty or magnitude, when compared with output from an inversion using only mole fraction observations. This suggests that both the ongoing expansion of isotope ratio observations and an improved understanding of isotopic signatures is required for this method to be used to estimate UK FF and non-FF methane emissions, with reduced uncertainty compared to traditional inverse methods.