Comment on acp-2021-843

This study investigates the influence of emission sources on CH4 and d13C-CH4 seasonal cycles between 2000 and 2012, comparing results from two different EDGAR emission inventories. A number of model simulations using the TM5 model have been performed, analysed independently and then the results compared to a small subset of atmospheric observations available. The main conclusions outlined in the abstract are that the seasonal cycle of emissions from enteric fermentation and manure management are better represented in EDGAR v5.0 (where they have no seasonal cycle) than in v4.3.2, that the modelled d13C-CH4 seasonal cycle at Alert was about half that indicated by the observations, and that results from Niwot Ridge indicate that the proportion of biogenic to fossil emissions should be revised.

This study investigates the influence of emission sources on CH4 and d13C-CH4 seasonal cycles between 2000 and 2012, comparing results from two different EDGAR emission inventories. A number of model simulations using the TM5 model have been performed, analysed independently and then the results compared to a small subset of atmospheric observations available. The main conclusions outlined in the abstract are that the seasonal cycle of emissions from enteric fermentation and manure management are better represented in EDGAR v5.0 (where they have no seasonal cycle) than in v4.3.2, that the modelled d13C-CH4 seasonal cycle at Alert was about half that indicated by the observations, and that results from Niwot Ridge indicate that the proportion of biogenic to fossil emissions should be revised.
Whilst I believe this study does contain new useful information concerning the methane budget and our understanding of its seasonal cycles, I cannot recommend it for publishing in its current form. The manuscript as it stands contains a lot of detail in the text which does not add to scientific understanding and is hard to follow in places, which could be made more concise. In addition, the main aims of the simulations and conclusions could be better defined. For example, what are the major uncertainties in the methane budget and how/where can examining its seasonal cycles help reduce these uncertainites? What is new here compared to previous studies analysing the seasonal cycles of CH4 and d13C-CH4? There are also a variety of typos/grammar which require correcting.

Specific Comments:
L3: change to 'These include emissions' etc.
L8: The text says that the phase ellipses do not form straight lines due to emissions. However, later in the text one of the conclusions is that even with constant emissions, the phase ellipses are still not straight lines due to the influence of sinks other than OH.
L36: Do the authors mean 'Reported inventories for anthropogenic based thermogenic and biogenic CH4 emission seasonal cycles mainly depend on political decisions' rather than the emissions themselves?
L55: A literature range for the magnitude of the soil sink would be useful here.
L58: Rephrase the way it is written, it sounds like the seasonality of the sinks is due to the KIE which is incorrect.  Table 2)? Table 2 appears in Section 2.3, but is not mentioned in the text until Section 2.5. L245: Section 3.1.1: There is a lot of detail here that I'm not sure contributes to the science understanding gained from this paper, and most of which can be deduced from looking at Figure 2. I think it would be easier to follow if the amount of detail regarding e.g. exact lag periods, percentage change in seasonal cycle magnitudes between certain simulations, could be reduced, so that the text highlights the most important differences between the simulations that can be seen in Figure 2, and how this adds to our understanding. L321: "The effect of the soil sink is small at these latitudes, so probably the seasonal cycle of d13C is preliminarily driven by the atmospheric sinks at these latitudes". I think a bit more information would be useful here. The ellipse produced for SIM_NS differs in gradient at all latitudes from the theoretical KIE line when only OH is considered. Why do the authors think this is if not the soil sink? Section 3.1.2: Again there is a lot of detail here, but it's not clear what the take away message is to the reader. As for Section 3.1.1, this section should be made more concise.
L429-432: Could seasonal changes in transport patterns (horizontal or vertical from the stratosphere) also influence the seasonal cycle at Alert? Given the high latitude of Alert, the CH4 sink there is likely to be relative small all year compared to the tropics.
L476: typo -'Delta13' L487: The Discussion section does not discuss the results of the paper, but is instead a mostly a summary of literature available regarding seasonal variations in emissions.
Line 557: reference missing? L611-613: I disagree that results from this study support the conclusions of Gromov et al. (2018) who concluded a negligible tropospheric Cl sink for CH4, as the simulations are able to capture much of the seasonal variation in CH4 and d13C-CH4 at the South Pole. I think a further simulation including a tropospheric Cl sink and an assessment of its impact on the seasonal cycle at SPO would be required to back this statement up. As the authors point out, the highest Cl concentrations are anticipated to be in the tropics, and may not have a strong influence at SPO. Also, if the d13C-Ch4 seasonal cycle at SPO is mainly controlled by the atmospheric sinks, could the choice of KIE for 13CH4+OH used in TM5 influence results here? As the authors point out, there are 2 differing values in the literature.
L631-632: To conclude that the comparison between observed and modelled d13C-CH4 at SPO suggests the emission seasonality in the model is at the right level, I think that Figure 4 needs to show an influence of emission seasonality on the d13C-CH4 seasonal cycle, which is not clear from the current plot. Perhaps if SIM_NS could also be plotted in Figure 4, this would show more of an impact? L633: typo, 'moel' should be 'model' Powered by TCPDF (www.tcpdf.org)