The apportionment of emission sources of atmospheric methane using 14C: capabilities and challenges

Prof. Soenke Szidat1,2, Dr. Thomas Laemmel1,2, Mr. Christophe Espic1,2, Mr. Matthias Bantle1, Dr. Stephan Henne3, Prof. Dominik Brunner3, Prof. Markus Leuenberger2,4

1University of Bern, Dept. of Chemistry, Biochemistry and Pharmaceutical Sciences, Bern, Switzerland, 2University of Bern, Oeschger Center for Climate Change Research, Bern, Switzerland, 3Empa, Laboratory for Air Pollution / Environmental Technology, Duebendorf, Switzerland, 4University of Bern, Climate and Environmental Physics, Bern, Switzerland

Introduction
Methane (CH4) is the second most important greenhouse gas after carbon dioxide (CO2). Radiocarbon (14C) analysis of atmospheric CO2 is a powerful tool for the apportionment of its emission sources and especially for the quantification of fossil CO2 emissions. This story of success may also be expanded to the source apportionment of CH4. However, several circumstances hamper such an approach, e.g. the ~200-times lower ambient CH4 concentrations compared to CO2, the requirement of a bias-free extraction of CH4 from ambient air, missing reference datasets from monitoring stations or interfering 14CH4 emissions of nuclear power plants (NPPs).

Methods
Short-term air samples have been collected biweekly since 2018 at three Swiss sites: the high-alpine research station Jungfraujoch, the tall tower Blosenbergturm in the rural area of Beromünster and an urban site in Bern. We developed a methane preconcentration and purification setup that enables the isolation of CH4 from atmospheric samples for 14C measurements (Espic et al., 2019). With this, we extract pure methane from 50-100 L of ambient air. After oxidation to CO2, samples of ~50 µgC are measured with the gas ion source of our accelerator mass spectrometer MIni CArbon DAting System (MICADAS).

Results
The methane preconcentration and purification setup guarantees a bias-free isolation of CH4 from air, which particularly prevents an interference of CO2. This is mainly achieved by a multi-trap preconcentration, followed by purification using gas chromatography (GC), in which the retention times of both gases differ comfortably by ~4 minutes. The GC also allows the surveillance of this separation for each sample, which is not possible for simplified methane extraction systems that exclude the GC step.
Most of the short-term air samples from Jungfraujoch showed a fraction modern (F14C) for CH4 of ~1.35, which agrees well with few observations at other background sites. Results from Beromünster and Bern revealed larger scattering, however, which indicates an influence from other sources. These may include emissions from fossil and biogenic sources as well as from nearby NPPs. Whereas the first two of which are related to the aims of this project, the last of which unfortunately complicates the 14C source apportionment of CH4 for Switzerland so that further filtering of the data is needed.

Conclusions
– The combination of our methane preconcentration and purification setup with the the MICADAS provides the technical basis for the apportionment of emission sources of atmospheric methane using 14C.
– The high-alpine research station Jungfraujoch is well suited as a reference station of 14CH4 for central Europe.
– Due to the impact from nuclear power plants, the 14C source apportionment of CH4 requires additional filtering of data, for example by analyzing atmospheric transport, to be effective for Switzerland.
– In a recently started project, we aim at a broadened sampling strategy combined with atmospheric inversion models in order to improve the capabilities of 14CH4 measurements.

Reference
Espic, C., Liechti, M., Battaglia, M., Paul, D., Röckmann, T., Szidat, S. (2019). Compound-Specific Radiocarbon Analysis of Atmospheric Methane: A New Preconcentration and Purification Setup. Radiocarbon, 61(5), 1461-1476, doi:10.1017/RDC.2019.76.


Biography:

Soenke Szidat studied Chemistry and graduated at the University of Hannover, Germany. He is now Associated Professor and head of the LARA (the Laboratory for the Analysis of Radiocarbon with AMS) at the University of Bern, Switzerland. He was awarded the Atmospheric Chemistry and Physics (ACP) Award and the Fritz-Strassmann-Preis. His research targets the apportionment of sources of atmospheric components such as carbonaceous aerosols, carbon dioxide and methane using radiocarbon and the development of hyphenated analytical systems for radiocarbon analysis.

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Date

Nov 17 2021

Time

WEDNESDAY
9:45 am - 10:40 am