Summary:
Based upon the feedback of the reviewers and the authors’ response, the authors have done a much better job of explaining their data and interpretation. The re-organization of the paper was necessary to make this something that is publishable in the peer-reviewed literature. There are some suggestions below for streamlining the paper – it is still overly long and repetitive. The authors have also added a good deal of new interpretation/discussion since the previous version, which brings in new caveats. Overall, the dataset is novel and interesting and expands our understanding of the use of the isotopic composition of nitrate for interpreting snow and ice results in Antarctica.
General Comments:
I think the authors should consider combining results and discussion and streamlining the manuscript. The results and discussion are still very repetitive and at times statements come up in the results that are not then fully discussed for 8 more pages!
The authors should also consider synthesizing a bit more regarding the Zatko et al 2016 results. The GEOS-Chem model is taking into account the loss, transport and recycling of nitrate/NOx. So it seems relevant to see whether the features predicted by the model are represented in the Dome C and DML comparison.
Specific comments:
Abstract:
Rephrase the first sentence. This study is specific to low accumulation sites in Antarctica. The sensitivity to UV and TCO is not universal as this has not been shown to be a strong predictor anywhere else but the East Antarctic ice sheet (and not at all at the North Pole).
Line 25-28: Delete the sentence starting with “Secondly,….” And instead begin the next sentence with Based on the TRANSITS model, we find that NO3- is…”
Also this sentence should report the 2 times recycling as the average for the skin layer.
Introduction:
Lines 53-56: The phrasing here regarding nitrate (NOx) sources is a bit strange. Later (much later) in the manuscript, marine air masses are deemed important, yet couched here as if they are minor. Further, the model results of Lee et al. are not really explained. Rephrase here to indicate that all of these sources are shown to be important. In fact, Lee et al can explain Antarctic nitrate concentration at all times of the year except the seasonal increase due to PSC sedimentation b/c this is not included in the model. So the tropospheric transport COULD be important and could be more important at other times of the year than spring.
Lines 95-96: It does not at all follow the previous text that evaporation of nitrate is negligible. This is further explained, at too much length, in the discussion section. Here the loss of nitrate from the surface and its impact on the isotopic composition of nitrate is being discussed. The evaporation part needs to be rephrased to make clear that isotopically evaporation is negligible b/c of the very large fractionation associated with photolytic loss. And/or introduce the fact that based on the Shi et al study conditions at DML would warrant that evaporation should have a negligible effect b/c the isotopes effects reported in Shi et al are negligible.
Line 118: The nitrate is not archived necessarily in ice. The e-folding depth shown in this study is very shallow and clearly in the firn layer. Rephrease here.
Lines 120-123: This completely disregards the previous paragraph on nitrate sources at lines 53-56. Here only the stratosphere is being considered as a source.
Line 128-132: This is repetitive. In the Introduction and in the Discussion section the authors flip flop between the importance of source and the importance of post-depositional processing and re-explain post-depositional processing multiple times. The primary deposition signal is only erased if there is post-depositional recycling. If there is only loss, then based upon the modeling results presented in this paper, the loss can be accounted for and a primary signal could be detected. But with lots of loss and recycling the original signal is clearly too overprinted.
Line 131: The “Therefore, photolysis induced NO3- loss and….” Does not follow from the previous sentence.
Throughout the next two paragraphs I still found the text repetitive and much of the material is really more important for the discussion and is all repeated there.
Methods:
The beginning of section 2 is not Methods, it’s really an overview of the project’s purpose and also discusses some results. It might make more sense to move this into introduction and reduce the last two paragraphs as suggested as above. Further, it is strange that Figure 2 is introduced before Figure 1. Figure 1 is really results and should be introduced in that section.
Similarly, lines 230-234 and lines 330-335 are results not methods.
Line 337: This needs to be reworded to make clear that recycling occurs before the nitrate is archived.
Results:
Line 377: This is an overly broad statement that the annual cycle is similar “across Antarctica”. It would be useful here to state explicitly what locations are being compared to… this does not appear true for the coastal work nor for site on the West Antarctic Ice sheet. So this should be more specific rather than an overreach. Furthermore, this is introduced here as a fact and then re-discussed much later in the discussion in more detail.
Sections 3.5, 3.6 and part of 3.8 are really methods. Here you are introducing the method of calculation then doing the calculation and stating a result. But most of this belongs in methods as an explanation of the approach taken and then the results and discussion and discuss the results of these calculations referring back to the equations in the methods section. And section 3.7 is really discussion not results since you cannot explain the result and instead state a hypothesis that should be part of discussion rather than then referring to a much later section.
Most of section 3.8 is focused on results for Dome C and this is confusing since the purpose of the paper is to really understand air to snow transfer of nitrate at DML and it’s potential for ice core nitrate interpretation at DML. The simple calculated flux is a huge overestimate; then the TRANSIT model is a large underestimate. Why are we spending so much time here on this when there actually is no real constraint on this in the region?
Discussion:
The title of 4.1 is confusing. How is this a validation of results? Throughout the results section the DML results and additional Dome C results are “confirmed” by comparison with past results at Dome C. Here it is really the TRANSITS model that is being evaluated.
Lines 565-568: Notably, have you considered a volume/mass effect here? Presumably the deposition as diamond dust or hoar frost is occurring in a much more limited volume of water equivalent then the collections of the surface layer. Were the masses of each sample collected recorded? Could the nitrate concentrations be mass-weighted and compared to explain the difference in results? In fact at line 585 it is suggested that possible wet deposition of nitrate can explain the lower mass fractions in the skin layer? Why isn’t this discussion tied together?
Section 4.2.1 is repetitive – this has all already been covered at least twice in the manuscript so far.
Line 577: How is station contamination completely ruled out? State clearly that the station was ALWAYS downwind if that is the case.
Line 605: But how does this compare to Zatko et al 2016 model results which do account for transport of snow-sourced emissions and re-deposition?
Lines 637-650: This section on sources is oddly worded and does not reflect the most recent literature. It also is really not necessary. The paper has already established the importance and large isotopic imprint of post-depositional loss and recycling. It could easily be stated here what the range in measured sources are, or make a table in the supplement of the reported ranges and how they do not at all cover the observed range at DML. Fibiger et al. (ES&T, 2016) contains positive values for some types of biomass burning so stating that it is a negative d15N source is incorrect. And the large ranges reported here do not reflect the fact that the variability in those ranges can be explained. For instance, the d15N-NOx from biomass burning is dependent upon the d15N-biomass. The more recent vehicle measurements suggest that older measurements are outdated and the range for plumes (versus tailpipes) is much more narrowly defined. Overall, this paragraph is the same arguments made in other papers and could be made more simply earlier on in the manuscript.
The formatting in section 4.3.2 differs from all of the other sections (i.e. why is there 1., 2., etc. sections?).
Section 5 here is another opportunity to consider the Zatko et al. 2016 model predictions.
Section 4.4 – why and what is the Weller approach as opposed to what is already been done using TRANSITS?
Section 4.5 – is is unclear to me why McCabe et al’s work at South Pole is not be discussed here? This is the only work I am aware of that makes a quantitative link between TCO and d15N of nitrate.
Section 4.5.1 and 4.5.2 are totally repetitive of earlier discussion and results. I suggest you combine results and discussion and eliminate the repeats.
Line 813-814: Variation in snow accumulation is clearly important, and you model this and the sensitivity of d15N to it. But you suggest that TRANSITS can explain and capture this so the statement here that this needs to be carefully accounted for seems counter to the fact that the model can reproduce the d15N so well (as suggested by the authors).
Line 839-841: Why is Domine’s work on nitrate diffusion not considered here? (Domine et al., Atmos. Chem. Phys., 8, 171–208, 2008 www.atmos-chem-phys.net/8/171/2008/). Based upon the temperature and accumulation rate at DML, diffusion should be relatively straightforward to constrain rather than raise as a big open question. |
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