The authors present a new multiyear record of ∆14CH4 and ∆14CO2 from the high-altitude Jungfraujoch, Switzerland observatory that spans from 2019 - 2024. The ∆14CO2 record shows the expected trend (gradual decline), attributable to continuing fossil CO2 emissions. In my review, I focus on ∆14CH4, as this is the main focus of the paper and also more within my expertise.

∆14C of atmospheric CH4 is useful for constraining the fossil fraction of methane emissions; this fraction is uncertain on both the regional and global scales. While the interpretation of ∆14CH4 is complicated by direct nuclear emissions (something the authors discuss in detail), ∆14CH4 can be used in conjunction with methane stable isotope measurements for assessing regional and global CH4 budgets with higher confidence. The new record from Laemmel et al. fills a void in published recent direct atmospheric ∆14CH4 measurements. It also captures a recent gradual increase in ∆14CH4 in the free troposphere over Europe. This is novel and valuable; the measurements appear to be of high-quality. I therefore consider this study to be a good fit for ACP, and recommend its publication after some minor revisions.

Detailed comments:

Impressive precision (as evaluated by repeated measurements of two ambient air standards) of 8 ‰ is achieved for such small samples – well done!

The passive sampling into a large volume steel tank, and nighttime time-averaged sampling are nice innovations well suited for the purpose of getting high-quality background measurements of ∆14CH4 at this location.

How stable is CH4 in the large volume Tecobags, and was this tested for time intervals representative of sample storage? A brief mention / description of this would be useful.

Has simple “memory” (residual carbon from prior sample affecting the next sample) been tested for the laboratory extraction system? This would be useful to describe briefly as well.

What are the typical % yields of the extraction (i.e., what % of the CH4 carbon is in the end recovered as CO2, as compared to the expected amount based on CH4 mole fraction and air volume processed)? This would be useful to describe briefly as well.

The form of Equation 1 is not familiar to me, and I think the equation needs to be better explained / supported. What is C_obs? What is 14C_n? I don’t see this equation derived in the reference that is given to support it (Graven et al., 2019). Considering this, I recommend deriving the equation starting with the most basic quantities (number of 14CH4 molecules, number of total CH4 molecules) in the manuscript (perhaps in the supplement).

Line 324. note that Hmiel et al (2020) did not report any Antarctic ice core 14CH4; for Antarctica they only reported firn air measurements

Figure 2 caption. Please clarify what exactly the uncertainty bars on panel B represent. Is this the statistical uncertainty from individual AMS measurements? Or combined uncertainty from measurements and other aspects of processing?

Figure 3 caption – same comment as above regarding uncertainty bars.

I am not sure that trying to correct the ∆14CH4 results for regional nuclear power plant influence makes sense, for two reasons. First, as the authors themselves note in another part of the manuscript, ∆14CH4 releases are episodic rather than constant, so model-based corrections for individual samples are unlikely to be accurate. Second, I think the uncorrected measurements are overall more valuable, as they represent well – mixed tropospheric air at this location; this is what a global transport model would need for interpretation. That said, since the authors are comparing with predictions from a 1-box model, I can see the value of trying to remove the more regional nuclear influence. Perhaps some caveats along above lines could be added when discussing this correction and the corrected results.

Figure 4 legend: Hmiel et al (2020), not 2019