G01_07

Atmospheric ¹⁴CH₄ measurements over Switzerland: first data and modeling results

Laemmel T^1,2, Geissbühler D^1,2, Espic C^1,2, Bantle M¹, Leuenberger M^2,3, Henne S⁴, Brunner D⁴, Szidat S^1,2

¹Department of Chemistry, Biochemistry and Pharmaceutical Sciences, University of Bern, Bern, Switzerland, ²Oeschger Centre for Climate Change Research, University of Bern, Bern, Switzerland, ³Climate and Environmental Physics, Physics Institute, University of Bern, Bern, Switzerland, ⁴Empa, Laboratory for Air Pollution/Environmental Technology, Dübendorf, Switzerland

Methane (CH₄) is the second most important anthropogenic greenhouse gas after carbon dioxide (CO₂). While biogenic emissions of CH₄ contain present-day radiocarbon (¹⁴C) levels, CH₄ derived from fossil sources is ¹⁴C-free so that ¹⁴CH₄ measurements can be used as a source apportionment proxy to distinguish anthropogenic from biogenic CH₄ sources. Recently, a new methane preconcentration and purification setup (MPPS) was developed at the Laboratory for the Analysis of Radiocarbon with AMS, University of Bern (Espic et al., 2019). Typical samples are 60L of atmospheric air collected in bags, resulting after extraction in about 60 µg carbon in CH₄-derived CO₂ form, enough for a ¹⁴C gas measurement on a MICADAS (Mini Carbon Dating System) accelerator mass spectrometer.

This contribution presents the new MPPS, its performance and ¹⁴CH₄ measurements of biweekly air samplings at four sites in Switzerland, for most of them continuously since 2019: the high altitude research station Jungfraujoch considered as a European continental background station, two tall towers in Beromünster and Sottens and an urban site in Bern. The CH₄ source apportionment at these sites is challenged by sporadic transport of ¹⁴CH₄ emitted from pressurized water reactors of nuclear power plants in Switzerland and neighboring countries. To identify and filter out these situations, forward simulations of atmospheric ¹⁴CH₄ transport using the model FLEXPART-COSMO, including nuclear power plants emissions, are applied.

This sampling strategy is part of the ongoing project RICH (Radiocarbon Inventories of Switzerland) aiming to develop a national radiocarbon inventory in the atmospheric, terrestrial and aquatic carbon pools.