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 2000^1,2, 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 observations^1,3.

¹ Townsend‐Small et al JGR 117(D7) 2012

² Sparrow et al Sci. Adv 4(1) 2018

³ Espic et al Radiocarbon 61( 5) 2019