Compound-Specific Radiocarbon Analysis of Atmospheric Methane: A New Preconcentration and Purification Setup

C Espic,M Battaglia,T Röckmann,S Szidat,D Paul,M Liechti

doi:10.1017/rdc.2019.76

Abstract

ABSTRACTMethane contributes substantially to global warming as the second most important anthropogenic greenhouse gas. Radiocarbon (14C) measurements of atmospheric methane can be used as a source apportionment tool, as they allow distinction between thermogenic and biogenic methane sources. However, these measurements remain scarce due to labor-intensive methods required. A new setup for the preparation of atmospheric methane samples for radiocarbon analysis is presented. The system combines a methane preconcentration line with a preparative gas chromatography technique to isolate pure methane samples for a compound-specific radiocarbon analysis. In order to minimize sample preparation time, we designed a simplified preconcentration line for the extraction of methane from 50 L atmospheric air, which corresponds to 50 µg C as required for a reliable 14C analysis of methane-derived CO2 gas measurement with accelerator mass spectrometry (AMS). The system guarantees a quantitative extraction of methane from atmospheric air samples for 14C analysis, with a good repeatability and a low processing blank. The setup was originally designed for the measurement of samples with low methane concentrations, but it can also be adapted to apportion sources from environmental compartments with high methane levels such as freshwaters or wetlands.

Highlights

Since the beginning of the industrial revolution, the concentration of methane (CH4) in the atmosphere has increased by a factor of 2.5, which is mainly due to anthropogenic emissions (Dlugokencky et al 2011)
50–100 L of atmospheric air is collected in an aluminum bag which is brought to the lab, where it is connected to a methane preconcentration line (PRECON), to dramatically decrease the sample size by removing CO2 and most of the bulk air (i.e. N2, O2 and Ar)
The preconcentrated samples were transferred to the GC and the thermal conductivity detector (TCD) was used to assess the performance of the preconcentration

Summary

Introduction

Since the beginning of the industrial revolution, the concentration of methane (CH4) in the atmosphere has increased by a factor of 2.5, which is mainly due to anthropogenic emissions (Dlugokencky et al 2011). The sources of atmospheric CH4 are evaluated by top-down measurements and bottom-up inventories (e.g. Hiller et al 2014; Jacob et al 2016), but as a result of the natural variability of CH4 emissions, these approaches usually do not agree well (Nisbet and Weiss 2010). To improve this approach, stable and clumped isotopes of methane are widely studied, because many CH4 sources have a specific isotopic signature (Quay et al 1999; Stolper et al 2015; Sapart et al 2017)

Methods

Results

Conclusion