Radiocarbon in atmospheric CH 4 and CO 2 at Jungfraujoch in 2019–2024: influence of regional nuclear emissions and current global atmospheric 14 CH 4 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.