Quantification of the fossil fraction of methane emissions in London using radiocarbon measurements of atmospheric methane

Abstract:

The fossil fraction of methane emissions remains very uncertain, especially at local scales and in urban environments, where methane sources are often juxtaposed and gas leaks from the natural gas distribution network can be difficult to monitor. Several studies show that fossil-derived methane emissions in urban areas including London are greatly underestimated by national inventories ^1,2,3.

In this study we use radiocarbon measurements in atmospheric methane (Δ¹⁴CH₄) to quantify fossil-derived methane emissions in central London. Methane samples have been collected at Imperial College London using a unique sampling system we developed. The system uses small traps containing molecular sieves, which are particularly selective for CO₂. Collection of a sample is made by three main steps: 1) removal of CO₂ and CO from air, 2) combustion of CH₄ into CO₂ and 3) adsorption of the combustion-derived CO₂ onto the molecular sieve trap. ¹⁴C analysis of our samples was carried out at the accelerator mass spectrometry facility at University of California, Irvine. Sampling days were chosen to avoid interference of the ¹⁴C emissions from European nuclear power plants on our measurements.

Following the mass balance approach in Graven et al. (2019)⁴, we found that the fossil fraction of added methane in central London ranges from 80 to 100 %, significantly higher than the 30 % fossil fraction of emissions reported in the national emission inventories.

¹Zazzeri et al. 2017, Scientific Reports-UK 7: 4854

²Helfter et al. 2016, Atmospheric Chemistry Physics, no. 16: 10543-10557

³Plant et al. 2019, Geophysical research letters, 46(14), 8500-8507

⁴Graven et al. 2019, Earth's Future 7 (3), 283-299