General comments

This paper aims at studying the optical characteristics and sources of CDOM in the seasonal snow. The paper is generally well written. Please find below my general comments.

  • A large proportion of the introduction is devoted to present various usages of spectrometry indices. Rather, authors should use this space to better present the problems they are trying to address.

  • In the methods section, authors said they frozen water samples before optic measurements. Freezing DOM samples are problematic because of sedimentation/precipitation processes that are further causing scattering (Thieme et al. (2016); Fellman, D’Amore, and Hood (2008)). Authors need to carefully address this issue. To cite Fellman, D’Amore, and Hood (2008):

We further show that when surface water samples were frozen, there was a decrease in the specific ultraviolet absorbance (SUVA) of DOC that is particularly evident with high concentrations of DOC.

  • Many figures in the manuscript are used to present the relationships among the calculated optical indices. These do not contribute to increasing our knowledge about DOM in the snowpack. Actually, there are many studies that compared optical indices. Hence, these figures are not interesting in the context of the current study. Authors should carefully review the objectives of the paper and use appropriate figures.

  • In relation with my previous comment, I found the ratio between the length of the paper and new knowledge to be rather high. I believe that the results/discussion section could be shortened by at least 50%.

Specific comments

Introduction

Page 4, line 4: Helms et al. (2008) did not show that. It was already known in the 1960’s. This is the same for the next citation.

Page 5, line 16: Do not start a paragraph with however.

Page 6, lines 10-14: Please review the sentences (2 times however). I do not understand the sentence at line 10. I thought you were talking about CDOM in the snow, not in the atmosphere or in water bodies.

Page 6, line 20: Why 280 nm? This is rather unusual in the literature. Most people use either 275 nm or 254 nm.

Page 6, lines 17: This paragraph on the use UV-vis spectroscopy could be shortened.

Page 7, lines 9-14: These advantages are also valid for absorption measurements.

Page 7, lines 15-16: Authors should be a clear distinction between CDOM and FDOM (EEMs). FDOM is a sub-fraction of the CDOM.

Methods and materials

Page 9, lines 10-13: Freezing DOM samples is problematic. See my general comments. Also, at line 15, it is said that samples were analyzed within 24h. It is not clear how samples were processed.

Page 9, line 19: determined measured

Page 9, line 21:8 What are the 8 pixels? I never heard about that term.

Page 10: Equations are not written correctly. For example, \(I_{370}^{450}\) is the integrated fluorescence between these two wavelengths. Please use the appropriate notation.

Page 11, lines 16-21: How the exponential fit was performed? Was a background coefficient \(K\) used? If so it is problematic to fit a non-expoential function of such narrow spectral range with a background coefficient.

Page 12, equation 5: More details should be given about this metric since it is not widely used by the community.

Page 13, lines 3-8: It is not clear how the clustering was performed. What are the multiple correlation coefficients?

Page 13, lines 9-17: This is the first time authors talk about fires. How is it related to the current study? This is an example where authors should better use the introduction to present the problem and what they did to address it.

Results and discussion

Figure 3: Why results from Qinghai region are not presented?

Page 14, lines 1-2: In Fig. 3, a280 varies between 0 and 4.5, not between 0.15 and 10.57 as said in the text.

Page 14, lines 13-15: Why comparing S measured in snow and S measured in oceans? This sentence is detached from the rest of the text.

Figure 4: There is a relation (which is already known in the literature) between S275-350 and a280. What does it mean in the context of this study? As I said, this relation is already known, so I am not sure that this figure is needed.

Page 14, line 22: What is HULIS?

Page 15, lines 2-6: Why AAE values not presented in a map like for S and a?

Pages 15-17: These results are cite specific and can not be generalized. The Editor should check if this is in line with the scope of the journal. Since authors are interested in presenting differences among regions, I suggest using boxplots instead of Fig. 3 and Fig. 7. Then, ANOVA or t-test could be used.

Section 3.3: These results are not related to paper objectives that aim to study DOM in the snow.

Table S1 same results as in Fig. S5.

Fig. 5: This should be in the appendix.

Section 3.2.1: Three pages are dedicated to present PARAFAC components. Once again, what kind of information this brings in the context of the paper?

Fig. 7: This figure is very difficult to interpret. It is rarely a good idea to present pie chart because human eyes are very bad at judging angles. I suggest a figure like this:

With such figure, regional variations will be better visualized.

Figure 8: The clustering should be done using all optical indices (S, AAE, a280).

Figure 9: Any reasons to present sites in that specific order? This can be confusing if there is no link among regions. What are the a and b letters under stations 51 and 52?

Figure 10: See my other comments about showing how optical indices compare and the aim of the study.

Tables 3, 4 and 5: This data could be presented using boxplots are better than tables for visualization. Raw data should be given in the appendix.

References

Fellman, Jason B., David V. D’Amore, and Eran Hood. 2008. “An evaluation of freezing as a preservation technique for analyzing dissolved organic C, N and P in surface water samples.” Science of the Total Environment 392 (2-3):305–12. https://doi.org/10.1016/j.scitotenv.2007.11.027.

Helms, John R., Aron Stubbins, Jason D. Ritchie, Elizabeth C. Minor, David J. Kieber, and Kenneth Mopper. 2008. “Absorption spectral slopes and slope ratios as indicators of molecular weight, source, and photobleaching of chromophoric dissolved organic matter.” Limnology and Oceanography 53 (3):955–69. https://doi.org/10.4319/lo.2008.53.3.0955.

Thieme, Lisa, Daniel Graeber, Martin Kaupenjohann, and Jan Siemens. 2016. “Fast-freezing with liquid nitrogen preserves bulk dissolved organic matter concentrations, but not its composition.” Biogeosciences 13 (16):4697–4705. https://doi.org/10.5194/bg-13-4697-2016.