A novel sampling system for radiocarbon measurements of atmospheric methane

Radiocarbon in atmospheric methane (Δ14CH4) is a powerful tracer of fossil methane emissions and can be used to attribute methane emissions to fossil or biogenic sources. However, few Δ14CH4 measurements are reported since 20001,2, due to challenges in sampling enough carbon for 14C measurements and in assessing the influence of 14C emissions from nuclear power plants on the 14C observations.At Imperial College London we addressed the sampling limitation by developing a unique sampling system that separates carbon at the point of sampling and uses small traps of molecular sieves. 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 UCI. This novel system has been used for collection of samples in central London and has been made portable for collection of samples in different settings. Here we describe the system and report the evaluation of the measurement uncertainty and the processing blank. We achieved a measurement precision of 6 ‰, which is similar to or better than the reported precision of the most recent observations1,3.1 Townsend‐Small et al JGR 117(D7) 20122 Sparrow et al Sci. Adv 4(1) 20183 Espic et al Radiocarbon 61( 5) 2019