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

[Image: see text] Radiocarbon ((14)C) measurements offer a unique investigative tool to study methane emissions by identifying fossil-fuel methane in air. Fossil-fuel methane is devoid of (14)C and, when emitted to the atmosphere, causes a strong decrease in the ratio of radiocarbon to total carbon in methane (Δ(14)CH(4)). By observing the changes in Δ(14)CH(4), the fossil fraction of methane emissions can be quantified. Presently, there are very few published Δ(14)CH(4) 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 Δ(14)CH(4) measurements without having to transport large volumes of air. The system catalytically combusts CH(4) into CO(2) and adsorbs the combustion-derived CO(2) onto a molecular sieve trap, after first removing CO(2), CO, and H(2)O. Tests using reference air show a Δ(14)CH(4) 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 Δ(14)CH(4) measurements in central London and show that day-to-day differences in Δ(14)CH(4) in these samples can be attributed to fossil methane input. The new system could be deployed in a range of settings to investigate CH(4) sources.