The role of sampling height in interpreting methane isotope ratios for source attribution and inversion modelling

Methane (CH₄) has a global warming potential 28-36 times that of carbon dioxide over a 100-year period [1]. Different sources of CH₄ have distinct isotopic signatures, with CH₄ from biological sources having a lighter signature than those from fossil sources [2]. Greenhouse gas (GHG) emissions are typically reported using bottom-up methods (based on data such as emission factors) that are verified using top-down methods (based on atmospheric transport models (ATMs) and observations) which infer fluxes, often through Bayesian methods [3]. Isotope ratio data are generally used in atmospheric models to understand individual contributions of various CH­₄ sources, globally and regionally. However, there is uncertainty regarding isotopic signatures due to large temporal variabilities and regional specificities [4].

Methane isotope ratio source signature information is typically gained through discrete mobile measurement campaigns, with the aim of capturing the emissions directly from the sources, through downwind transection of plumes as closely as possible to the source [5]. These measurements fill databases that are used for atmospheric modelling [2,6,7].

Continuous measurements of CH₄ isotope ratios are also carried out at from varying sampling heights [7,8,9] (ranging from tens to hundreds of meters) with the lower heights, closer to emissions sources, capturing more local influences and higher heights capturing more regional emissions. While they offer the advantage of being continuous, they are further away from the emission sources, therefore have larger uncertainties.

Understanding the information that can be gained from continuous CH₄ isotope ratio measurements at different sampling heights and locations will be an important factor to consider when using observational data in inversion frameworks, in terms of accurately quantifying source signatures. We present results of mean isotopic signatures from continuous measurements, resolved using the Keeling approach and compare to modelled data to understand the inferred source contributions.

Continuous measurements of CH₄ isotope ratios have been carried out at 10 European atmospheric GHG monitoring stations. This study focuses on two sites: Heathfield (an inland, 100 m a.g.l tall tower) and Krakow (an urban, 35 m a.g.l site). We present CH₄ isotope ratio datasets from these sites and aim to use them to interpret isotopic signatures in the surrounding areas.

[1] IPCC 2021. Cambridge University Press.

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[5] Bakkaloglu et al. 2022. Atmos. Environ. 276, 119021.

[6] Menoud et al. 2020. Tellus B. 72, 1823733.

[7] Menoud et al. 2022. Earth Syst. Sci. Data. 14, 4365-4386.

[8] Röckmann et al. 2016, Atmos. Chem. Phys. 16, 10469-10487.

[9] Rennick et al. 2021. Anal. Chem. 93, 10141-10141.