Radiocarbon in atmospheric CH₄ and CO₂ at Jungfraujoch in 2019–2024: influence of regional nuclear emissions and current global atmospheric ¹⁴CH₄ signal

Abstract. Radiocarbon (¹⁴C) is a valuable tracer to determine the relative fossil fractions of emitted carbonaceous greenhouse gases, such as CO₂ and CH₄. While atmospheric ∆¹⁴CO₂ measurements have been conducted at multiple sites for several decades, ∆¹⁴CH₄ measurements remain more limited, mainly due to measurement challenges. In addition, nuclear power plant ¹⁴CH₄ emissions can complicate data interpretation. In this study, biweekly ∆¹⁴CH₄ and ∆¹⁴CO₂ measurements at the Swiss High-Altitude Research Station Jungfraujoch (JFJ, about 3500 m a.s.l.) between 2019 and 2024 are presented. Over this period, ∆¹⁴CH₄ values showed an increase from 350 ± 19 ‰ to 381 ± 13 ‰, while ∆¹⁴CO₂ values decreased from -2.0 ± 3.8 ‰ to -12.7 ± 2.0 ‰, respectively. The former is related to the slight increase of ¹⁴CH₄ emissions from the nuclear industry over the last years, while the latter is linked to the dilution of the ¹⁴CO₂ signal due to the release of ¹⁴C-devoid CO₂ from combustion of fossil fuels. Despite its high elevation, JFJ is still influenced by nuclear power plants (NPPs) operating in Europe. Considering a European-scale atmospheric dispersion model and ¹⁴CH₄ and ¹⁴CO₂ emissions from European NPPs, the mean nuclear ¹⁴C contribution to our individual measurements was estimated to be 7 ± 9 ‰ for ∆¹⁴CH₄ and 0.2 ± 0.4 ‰ for ∆¹⁴CO₂. Furthermore, our ∆¹⁴CH₄ measurements reasonably agree with simulated atmospheric values of ∆¹⁴CH₄ estimated by a global atmospheric one-box model and an estimation of global nuclear ¹⁴CH₄ emissions.