Efficient Sampling of Atmospheric Methane for Radiocarbon Analysis and Quantification of Fossil Methane

Radiocarbon (¹⁴C) measurements offer a unique investigative tool to study methane emissions by identifying fossil-fuel methane in air. Fossil-fuel methane is devoid of ¹⁴C and, when emitted to the atmosphere, causes a strong decrease in the ratio of radiocarbon to total carbon in methane (Δ¹⁴CH₄). By observing the changes in Δ¹⁴CH₄, the fossil fraction of methane emissions can be quantified. Presently, there are very few published Δ¹⁴CH₄ measurements, mainly because it is challenging to collect and process the large volumes of air needed for radiocarbon measurements. We present a new sampling system that collects enough methane carbon for high precision Δ¹⁴CH₄ measurements without having to transport large volumes of air. The system catalytically combusts CH₄ into CO₂ and adsorbs the combustion-derived CO₂ onto a molecular sieve trap, after first removing CO₂, CO, and H₂O. Tests using reference air show a Δ¹⁴CH₄ measurement repeatability of 5.4‰, similar or better than the precision in the most recent reported measurements. We use the system to produce the first Δ¹⁴CH₄ measurements in central London and show that day-to-day differences in Δ¹⁴CH₄ in these samples can be attributed to fossil methane input. The new system could be deployed in a range of settings to investigate CH₄ sources.