General Comments

This paper presents a comprehensive analysis of a substantial and unique dataset. The result is of significant interest, and the method can be applied to additional ice core data to extend the record. My comments are of a peripheral nature and I recommend the paper be published after minor revision.

Specific Comments

In the introduction, I suggest adding a paragraph to give perspective. What are the main research questions that this work seeks to address? In the conclusion, how well have they been addressed?

The Transfer of Atmospheric Nitrate Stable Isotopes To the Snow (TRANSITS) model is used and more attention should be given to model validation. What is the model good at and what further improvements would be useful?

One of the main results is summarized in the Abstract, '..the SE Dome ice core NO₃⁻ record offers a precise reconstruction of NOx emissions and atmospheric oxidation chemistry during transport, preserving records from both North America and Western Europe, thereby providing reliable insight into atmospheric nitrogen cycling.' I question the use of the distinction between N American and W European NOx as useful/distinguishable categories. For example, it could be that variations are due to changes in atmospheric circulation and whether precipitation originates from within the circumpolar circulation or outside. To what extent are the observed changes due to (climate change driven) changes in atmospheric circulation?

NOx emissions are well studied with numerous reports from local, regional and international organizations. Please comment on this dataset within that context. How well does it compare to these other records? What unique insights are derived from the new information presented here? One useful result may be that there is something useful to go after, and that the post-depositional processes do not destroy the record. Therefore it is important to validate the veracity of the method by comparison to other data sets.

Line 38, 'Owing to increasing fossil fuel and chemical fertilizer use since the beginning of the Industrial Revolution, NOx levels in the atmosphere have increased, which is reflected in the elevated NO₃⁻ concentrations in ice cores, including those collected from Greenland (Neftel et al., 1985; Mayewski et al., 1986).' Are there no publications on this after 1986? What impact did the widespread introduction of the three way catalyst have, and the campaign to reduce NOx emissions to combat acid rain? How have road traffic and the vehicle fleet evolved with time?

Line 83, 'The SSA for the SE-Dome site was determined to be 46.95 m² kg⁻¹using the relationship between the SSA and ρ_snow' Given the complexity of snow (sintering, seasonal changes, etc etc), is this value really known to four significant figures and is there an error, a range?

Technical Corrections

Be sure to define m w e a^-1 at first use.



Line 194, Chemical names should not be italicized e.g. '[BC]' see also e.g. Table 1.



Paragraph starting at Line 254, we see both '100%' and '+0.9 ‰' one has a space between number and unit the other does not. Please be consistent, and please use recommended style - a space.

General comments

This manuscript presents valuable data as few NO₃^- records from ice cores are little or not impacted by post-depositional processes. The demonstration that the differences observed with the Summit record are due to reduced post-depositional processes is clear and relevant. The study therefore provides good quality atmospheric data that can be used to investigate NO_x chemistry and atmospheric oxidizing capacity in the recent past. The comments below serve only to improve the overall understanding of the study, and I recommend publication of this manuscript after minor revisions.

Specific comments

The last paragraph of the introduction is very specific, but I would advise first stating more clearly the objective of the study, by explaining that SE-Dome and Summit are compared in order to discuss the extent to which the concentration and isotopic composition of NO₃^- are altered by post-depositional processes.

In the Methods section, the choice of sampling – for some periods with a 2-season resolution and for others with a 4-season resolution – should be justified. A 4-season resolution is indeed interesting for comparison with the TRANSITS model data (fig.4), but these two different resolutions can be confusing, particularly when looking at figure 2.

I suggest including an explanation of the inverse influence of f_exp on the deviations in δ¹⁵N and Δ¹⁷O to facilitate understanding of Figure 3 and the discussion (fractionation and export of NO₃^- depleted in ¹⁵N without recycling vs export of NO₃^- affected by the exchange of oxygen atoms).

Technical corrections

l.57: “Since NO₃^- deposited to ice core are preserved in ice cores, past atmospheric chemistry has been discussed by measuring Δ¹⁷O(NO₃^-) in ice cores (Geng et al., 2017).” This is unclear: in many cases, NO₃^- is not preserved in ice cores precisely because it is subject to post-depositional processes.

l.66: Photoproducts of NO₃^- are NO₂, NO₂^- or ONOO^-. “NO₃^-” is confusing.

Figure 1: It would be good to display the location of SE-Dome and Summit sites more clearly on the map or on a dedicated panel.

l.113: “Changes in the isotopic compositions of NO₃^- during ion separation were negligible, as determined previously (Noro et al., 2018)” is a repetition of the preceding sentence.

l.115: I suggest mentioning that the bacterial method is used to convert NO₃^- to N₂O.

l.137: Typing error: “ace” instead of “ice”.

l.213: “variations in f_exp between initial deposition and ice core analysis.” This is unclear: f_exp no longer has any influence after NO₃^- is archived in the ice. Do you mean between initial deposition and archiving?

Figure 5: Please mention in the caption what the shaded area represents for SE-Dome data.

l.299: “Fig. S3a”: the text actually refers to Fig. S3b.

l.300: “Fig. S3b”: the text actually refers to Fig. S3c.

l.375: “the SE-Dome ice core-recorded”: typing error, no hyphen.