Vertical Distribution of Arctic Methane in 2009–2018 Using Ground-Based Remote Sensing

Round 1

Reviewer 1 Report

The authors used existing method for assessing the methane concentration profile using ground-based sensor and the results are comparable with the results assessed by the airborne sensors. The proposed method is not really novel, however, it is interesting to archive the goal with easier setup.

The most limitation on the manuscript is on the presentation. The paper is quite clear but not flawless. 1- The authors should improve the language because there are grammatical mistakes, clumsy sentences, and the use of comma, such as:

- Abstract: “compared compared”

- Page 2: “Inside troposphere also oxidation in soils and by  photochemistry in the marine boundary layer play some role.” Is not understandable.

- Page 3:

 +  Swirlab is not an algorithm, it is a toolbox that supports MCMC method for radiative transport.

 + “(in this work four)”, should be something like (i.e., four in this work)

- Page 4:

 + FTS site is quite confusing. FTS is methodology then it is strange to use to identity a place.

 

2- The authors should also use citations for the references such as:

- A547N, IFS125HR, HITRAN2012, HITRAN2016

 

3- Unit for the metrics in figures, such as

- Horizontal axis of the right of Figure 4,

- Horizontal axes of the upper graphs in Figure 5.

- Unit of color bar in Figure 7.

- Figure 10

 

4- Although the proposed method is comparable with other references, the authors should discuss about which method is more accurate, ground-based sensing method or airborne sensing methods. Which one is economical? Advantages and disadvantages of these methods.

Author Response

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Reviewer 2 Report

The authors report on a novel method for retrieving CH4 vertical profiles using a dimensionality reduction method that was fitted to ground based remote sensing. The methods and results are sound, albeit the manuscript can be hard to follow for readers outside the discipline. 

 

Specific comments:

The introduction should make reference to the ill-posed problem and how the authors approach of dimensionality reduction is a valid approach and counter to Tikhonov regularization.

It is not always clear which of the listed data sets are used for retrievals and which for validation. 

Author Response

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Reviewer 3 Report

This manuscript presents results on the vertical distribution of methane over Northern Finland in the European Arctic using a ground based FTS instrument. The measurements were further evaluated against the retrieved profiles of ACE-FTS and balloon borne profile measurements. The results showed good agreement (within 10%) for altitudes below 20 km and within 30% for altitudes between 20 and 40 km. The time series of the CH4 profile data sets were further analyzed to estimate the trend using a dynamic linear model. Results from this study indicate a recent sudden increase of CH4 in the Arctic stratosphere could point to enhanced circulation from the tropics or to a decrease in stratospheric methane sink. There are very few vertical distribution measurements of methane over the Arctic region and these are very important from the Climate perspective.

Author Response

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Reviewer 4 Report

This paper presents 10 years of methane vertical profiles retrieved from ground based FTS measurements in northern Finland using a dimension reduction retrieval method. This retrieval method allows more information on the vertical profile shape to be obtained from the measurements, resulting in generally more accurate retrievals than the standard TCCON prior scaling retrieval approach. Comparisons to reference datasets are also generally favorable for this retrieval method.

The retrieval method is well documented in this paper, and the analysis throughout seems sound and robust. It does an excellent job demonstrating the dependencies of these types of retrievals on a priori assumptions and solar geometry. My main critique of this paper is that it should do a better job emphasizing the main advantages of this retrieval method, and the new information on methane transport and sources that we can learn from it. The discussion hints at this at the end, but these points should be made much clearer throughout to avoid underselling this new approach. Additionally, this paper would benefit from English editing, as some of the grammar is presently clunky, particularly in the introduction. After these concerns are addressed, I would recommend publication.

 

Specific comments:

Lines 90-96: Here the authors should emphasize why Swirlab is advantageous over existing retrieval algorithms. Later they show that it provides better sensitivity to stratospheric variability, results in less SZA dependence, and is generally more accurate compared to the AirCore measurements. Are there other benefits? Is it significantly less computationally expensive?

Lines 194-201: These advantages are very important to know. They should be included earlier in the paper.

Line 343-344: I’m not sure I understand the gap filling of the ACE-FTS profiles… does this mean there are levels at which no methane is retrieved? Does this occur at the top or bottom edges of the profile, or somewhere in the middle? If it’s the latter, why not just interpolate rather than fill the gap with the Swirlab prior?

Line 378-380: Here is another place where the authors can use more convincing language to highlight the advantages of the dimension reduction method, rather than just simply describing the differences between it and the TCCON prior scaling approach.

Figure 9: The top panel can be omitted from this figure, as it’s too hard to see any differences on that scale (the symbol color is also too similar).

Figure 10: axes should be labeled and the dates should be placed at the top of each panel rather than the bottom

Section 4.6: This section needs work. Currently the discussion is an overly detailed description of the trend differences between Swirlab and the mast or ACE-FTS results at different levels, with a lot of emphasis on uncertainties associated with missing data. It should be distilled down to the main points. Also, I’m not sure I’m convinced that one can say how well the annual trends agree between Swirlab and the other measurements given the large variability in the measured trends. It seems that the Swirlab trend generally lies pretty close to the prior, whereas the other measurements are much more variable, particularly for the 50m mast. What makes the authors say these agree “remarkably well” versus the ACE-FTS comparisons? It seems that they are all still within the envelope of uncertainty?

Section 5: The first 4-5 paragraphs just contain summarized information that could be further shortened. The authors should focus more attention on the interpretation of their results, such as the potential information they provide on enhanced circulation from the tropics, etc., as noted in the last paragraph.

 

Technical comments:

Line 8: the word “compared” is repeated twice

Lines 32-33: this is not a complete sentence.

Lines 93-94: PROFFIT and SFIT4TCCON acronyms should be defined at first use here.

Line 203: delete the word “the” before “a brief”

Line 433: delete the word “the” before “a dimension”

Author Response

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Round 2

Reviewer 1 Report

The revision is acceptable and the manuscript is solid for publication.