The Role of Emission Sources and Atmospheric Sink in the Seasonal Cycle of CH4 and δ13-CH4: Analysis Based on the Atmospheric Chemistry Transport Model TM5
, Aki Tsuruta 1, Leif Backman 1, Pyry Mäkinen 2, Sander Houweling 3,4, Arjo Segers 5, Maarten Krol 6,7, Edward J. Dlugokencky 8, Sylvia Michel 9, James W. C. White 9 and Tuula Aalto 1Round 1
Reviewer 1 Report
This study has the potential to be a useful paper. It describes useful modelling of the seasonal cycle of delta-13C and shows how this could be used to understand the seasonality of methane sources.
This paper was a struggle to read. This is a multi-author paper, so it is essential that all authors contribute to the writing and that all authors are happy with the draft before it is submitted. Many of the sentence were poorly constructed and difficult to understand. This was often because the sentences were too long. These need to be broken down into two or more shorter sentences. Sometimes verbs were missing. Sometimes the subject and verb didn’t agree. I have commented on some of these for the first couple of pages, but the corrections are in no way exhaustive and should be considered merely as a sample. The authors themselves need to read through the text and come up with their own corrections.
Often explanations are missing and too much is assumed of the reader. The text needs to be thoroughly revised and read through from the viewpoint of a reader not intimately familiar with your analysis.
The scientific outcomes of the study were not clear. Numerous explanations were given for the discrepancy between model and observations such that made it was difficult to follow the overall arguments. What can the authors say about the modelled emissions?
There needed to be more discussion of the uncertainties in the observations in the modelling before any conclusions can be drawn about the accuracy of the emissions data.
Line 8: “for other sources a globally uniform value” – what does this mean?
Line 11: “Due to surface fluxes …”, explain why surface fluxes cause this.
Line 13-14: This long sentences discusses the tropics but references 60-90N.
Line 21: How can modelled amplitudes be underestimated?
Line 29: What does “severely perturbed” mean?
Line 30: Presumably this is GWP100. It needs a reference.
Line 58: “has” -> “has been”.
Line 62: “have” -> “has”.
Line 68: “…the fractionation factor by OH...” This needs much more explanation.
Line 70: “of”-> “of the”; “is” -> “it is”
Line 74: Need to explain kinetic isotopic effect
Line 76: “in” -> “in the”. Explain “mirroring”
Line 78: Emitting something can’t deplete it.
Line 85: Who has decided “It is time”
Line 93: “are”->”is”
Lines 96-99: Split into several sentences.
Lines 101: What do you mean by “flaws”?
Line 107: “This also initiates …” I don’t understand this sentence.
Line 111: Please provide a diagram of the zoom regions. Why are you using zoom regions?
Line 120: “off-line” – but presumably the methane reactions are done within the model? Or do you mean just offline oxidants?
Line 121: Explain briefly what is entailed in “based on Howeling et al.”
Line 122: Expand on “scaled by 0.92 based on an evaluation …”
Line 128: In what sense are they “small”, who often assumes this?
Line 146: Had the stratospheric delta-13C stabilised by the end of the spin up or was it still increasing. Did you check that the south pole delta-13C had stabilised?
Line 147: Explain what “converting CH4 fields using isotopic signature means”.
Line 250: I didn’t understand what “filled value (grids with no initial value assigned)” means.
Line 257: What does “up to decimal level” mean?
Lines 260-267: These values would be better in a table.
Line 268: I couldn’t see which figures and tables in the supplement correspond here as they don’t seem to have been labelled.
Line 277: Maybe easier just to say 2004-2012 used.
Line 279-281: Give more detail on what is meant by “short-term curve” and “trend curve”
Line 323: It is not explained anywhere which simulation modifies emission north of 30N.
Line 331: But 30-60N and 60-90N look very similar in the figure?
Line 367: Explain why this would be a straight line
Line 375: It seems the OH KIE is a poor representation of the data. This would be better including other sinks.
Line 376: Explain where these equations come from.
Lines 408: Presumably it would have been much better to include the other sinks in the line.
Figure 3: It is clear that the KIE line disagrees with the data.
Section 4.1, 4.2, 4.3: These were long discussions that were difficult to follow. Consider shortening and focussing. Are they any key points coming from this?
Author Response
Please see the attachment.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
This paper considers influence of methane emissions and sinks on stable isotope d13-CH4 seasonal variations in the atmosphere. This topic is very relevant because of current discussions about drivers of atmospheric methane growth. Stable isotope d13C is important marker of methane origin. The paper is well written and contains important results. I believe that it can be published after some minor additions/corrections.
General remarks
1. It’s not clear why considered time interval is limited by 2012 (10 years ago).
2. If this time interval for some reasons can not be extended toward the present day I would suggest to pay more attention to interannual variability of d13-CH4 seasonal variations. The chosen range includes periods of CH4 stabilization till 2006 and its renewed growth since 2007. It would be very interesting to check if model runs catch this change apparently caused by transformation of sources/sinks ratio that should be reflected on d13-CH4 magnitude and variability. I think it worth to add this topic to rationale of work, and to extend main part of the paper and, probably, conclusions. Corresponding figures are ought to be added either. Moreover, there are not so many of them in the article.
Other remarks
line 5 - it’s better to replace “Those” by “Emissions”
lines 227-228 - reference numbers are missed
Author Response
Please see the attachment.
Author Response File: Author Response.pdf
Reviewer 3 Report
The manuscript presents a detailed analysis on the seasonal cycle of methane concentrations and stable carbon isotope ratio calculated over 2000-2012, divided into 30 degree latitude bands. There is a lot of interesting understanding past methane trends, including which source types are driving growth. The results are compared to TM5 model simulations driven by different methane emissions fields, providing insight into the accuracy of those assumptions. Anthropogenic emissions are used from EDGAR 5.0 and EDGAR 4.3.2. Comparison is also made primarily at three stations: Alert, Niwot Ridge and South Pole. The work appears sound, and I have only small comments:
Supplementary figures and tables are references, but it was unclear looking into the zip file which these correspond to, therefore the SI was not reviewed
Figure 1 – I think I understand the legend now, but it was confusing to have EDGAR 4.3.2 shown as a solid thick line and EDGAR 5.0 as dashed. Initially I thought those were trying to represent total numbers. Secondly, the natural sources show as solid lines but aren’t all from EDGAR..
Table 3 caption. Suggest mentioning that the data sources for the natural emissions are shown in Table 1, since these are also important to the simulation
L545-L554. It is a little confusing with these two paragraphs talking about ‘source signatures’ not having seasonality represented. Suggest mentioning the word ‘isotopic’ here for context
L574 suggest not abbreviating approx.
L629-631 The reference to 23% of variance in OH being due to seasonality does not seem relevant here, since daily averages are used in the manuscript and the cited paper attributes the rest to diurnal variability. Also, that diurnal variability is controlled by many of these same factors (UV radiation, temperature, etc.)
Author Response
Please see the attachment.
Author Response File: Author Response.pdf
Reviewer 4 Report
I regret that for several reasons, both family illness and also prior commitments, I am unable to give a proper review to this paper in the short time frame allowed.
It looks like a fine paper although I have not yet been able to look in detail at it. All I have been able to do is skim it quickly. Many Apologies.
One small point is that there are some new source signature references they should look at in the recent double volume of Phil. Trans R Soc London that has many papers on this topic. In particular they could consider:
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MOYA/ZWAMPS Team, 2022. Isotopic signatures of methane emissions from tropical fires, agriculture and wetlands: the MOYA and ZWAMPS flights. Philosophical Transactions of the Royal Society A, 380(2215), p.20210112.atmospheric methane a worthwhile option? Phil. Trans. R. Soc. A 20210108. https://doi.org/10.1098/rsta.2021.0108 |
Lan, X.,et al . (2021). What do we know about the global methane budget? Results from four decades of atmospheric CH4 observations and the way forward. Phil. Trans. Royal Soc. A, 379 (2210), 20200440.
France J.L., et al., (2021) δ13C methane source signatures from tropical wetland and rice field emissions. Phil. Trans. R. Soc. A 20200449. 10.1098/rsta.2020.0449
Author Response
Please see the attachment.
Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
The specific review comments have been taken on board by the authors.
However it is still really difficult to understand what the takeaway messages are from this study. The paper is more likely to attract readership if the Abstract and Conclusions were framed more in terms of the key scientific outcomes rather than the methods and technical results..
Author Response
Authors' response to anonymous referee
We thank the referee for this feedback. Below we provide point-by-point answers for the feedback. The referee comments appear in normal text and our answers in bold italic.
The specific review comments have been taken on board by the authors.
However it is still really difficult to understand what the takeaway messages are from this study. The paper is more likely to attract readership if the Abstract and Conclusions were framed more in terms of the key scientific outcomes rather than the methods and technical results..
We thank the referee for this feedback. We have now rewritten the Abstract and Conclusion section based on your feedback.
