A new portable sampler of atmospheric methane for radiocarbon measurements

Abstract. Radiocarbon (¹⁴C) is an optimal tracer of methane emissions, as ¹⁴C measurements enable distinguishing fossil from biogenic methane (CH₄). However, ¹⁴C measurements in atmospheric methane are still rare, mainly because of the technical challenge of collecting enough carbon for ¹⁴C analysis from ambient air samples. In this study we address this challenge by advancing the system in Zazzeri et al. (2021) into a much more compact and portable sampler, and by coupling the sampler with the MICADAS AMS system at ETH, Zurich, using a gas interface. Here we present the new sampler setup, the assessment of the system contamination and a first inter-laboratory comparison with the LARA AMS laboratory at the University of Bern. With our sampling line we achieved a very low blank, 0.7 µgC compared to 5.5 µgC in Zazzeri et al. (2021), and a sample precision of 0.9 %, comparable with other measurements techniques for ¹⁴CH₄, while reducing the sample size to 60 liters of air. We show that this technique, with further improvements, will enable routine ¹⁴CH₄ measurements in the field for an improved understanding of CH₄ sources.