Carbon isotope measurements of methane for UK sources: spatial and temporal changes and implications for inventories and model inputs

The carbon isotopic signature (ð¹³C) of CH₄ fugitive emissions to atmosphere has been measured in downwind plumes from nearly 400 sources in the United Kingdom by the RHUL group since first measurements of the natural gas supply in 1998. The isotopic measurements have the ability to distinguish the main source categories, separating combustion, fossil fuels, waste and agriculture. Further isotopic subdivisions are possible between solid and gaseous fuels, and between different types and methods of waste processing, such that signatures can be assigned to the categories used for the UK reporting to UNFCCC and in production of mapped inventory products.

For many source categories the current spread of ð¹³C signatures is small, such as onshore natural gas distribution at -39.3 ±2.6‰ (171), or landfill sites at -57.1 ±2.5‰ (54). This has allowed production of isotopic prediction maps and an assessment of the changing source mix isotopic signature emitted by the UK, which contributes to long-range transport and eventually the changing signatures recorded at background measurement sites. The predicted ð¹³C for averaged UK emission of CH₄ changed from -50.5‰ in 1990 to -59.3‰ in 2021 based on the most recent NAEI inventory. Approximately 75% of this ¹³C depletion is explained by changing proportions of the different sources, particularly emissions reduction from coal mining, landfill sites and gas leaks, but 25% is the result of changing signatures within source categories, particularly in gas distribution with the switch from southern North Sea fields to sources further north, and from changes in landfill practice. This signature is depleted by 3‰ compared to the lowest signature calculated by Miller-Tans analysis of multi-year UK background records, suggesting discrepancies between actual emissions and the inventory source mix. Isotopic mass balance of sources to match observations would require additional fossil fuel emissions, unless there are enrichments rather than depletions occurring in other source sectors.

Changes to sources in the last decade, particularly within the agricultural sector and agricultural waste, has created added complexities to the sector isotopic separations and potential isotopic enrichments. These include C4 plant supplements to diets of animals such as maize that enriches the eructation in ¹³C, whereas the grass-fed and sustainable cattle have a tightly constrained eructation ð¹³C signature of -70.5 ±1.7‰ (14). Even more extreme is the range of signatures generated by the burgeoning ‘green’ biogas industry fed by energy crops such as maize at one end of the spectrum with ð¹³C up to -35‰ to NH₃-mediated CH₄ production from chicken waste feedstock with signatures around -80‰. Surveys in 2023 highlighted emissions underestimation and the preponderance of maize use in this expanding sector. New isotopic data changed the assigned sector source signature by +1.5‰.