the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 27 Jul 2026
Supercooled nitrate solution on ice at 232 K: Enhanced HONO yields
Abstract. Snowpack nitrate photolysis is a major source of nitrogen oxides and nitrous acid that control atmospheric oxidants in polar environments. It remains unclear at what temperature nitrate salts crystalize in snow and how this impacts the photolysis product yields. Here we show, using near‑edge X‑ray absorption fine structure (NEXAFS) spectroscopy, that sodium nitrate does not precipitate at temperatures down to 23 K below the eutectic temperature at ice surfaces, but does so in absence of ice. This indicates that strong interfacial supercooling or liquid‑like solvation persists. Complementary snow photolysis experiments demonstrate that such liquid-like nitrate shows a higher HONO to NO2 emission ratio than precipitated nitrate, which is consistent with the enhanced role of secondary chemistry in the aqueous phase. These findings show that supercooled nitrate might be present over wide ranges of Arctic, Antarctic, and upper tropospheric temperatures with a significant impact on the HONO and NO2 fluxes and thus the composition and chemistry in the air above snow or in contact with ice clouds.
Competing interests: At least one of the (co-)authors is a member of the editorial board of Atmospheric Chemistry and Physics.
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this paper. While Copernicus Publications makes every effort to include appropriate place names, the final responsibility lies with the authors. Views expressed in the text are those of the authors and do not necessarily reflect the views of the publisher.- Preprint
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Status: open (until 28 Sep 2026)
- RC1: 'Comment on egusphere-2026-3716', Anonymous Referee #1, 11 Sep 2026 reply
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- 1
This manuscript presents a laboratory study, using NEXAFS spectroscopy, of temperature and ice-dependent nitrate salt crystallization, as well as nitrate photolysis to produce HONO(g) and NO2(g). It is an elegant study that is well-written. The authors note that this chemistry is expected to be relevant to the Arctic, Antarctic, and upper troposphere. My comments and suggestions mainly focus on minor clarifications, particularly to the figures and Section 3.3.
In addition to the relevant locations globally, I would think that snow-covered regions more broadly would be relevant, given the pertinent temperature range. For example, snowpack HONO production has previously been observed to be important in wintertime urban locations: Beine et al 2008 (Environ Res Lett), Michoud et al 2015 (Atmos Env), Chen et al. 2019 (ACS Earth Space Chem), Cheng et al. 2025 (GRL). If so, then adding this additional “location” would further broaden the impact of the current work.
L10: If this sentence is read too quickly, it sounds like sodium nitrate doesn’t precipitate down to 23K. I recommend listing the actual precipitation temperature, as well as the eutectic temperature, to clarify this critical, key statement so that it is not accidentally misunderstood.
L24-25: Cheng et al. (2025) is mistakenly referenced here because the fieldwork was completed in China, not Antarctica as implied in the sentence.
L160: What was used to achieve the pH of 3, and why was this pH chosen?
L166: It would be useful to add reference to Domine et al 2008 (ACP) for authentic snow density agreement.
L174: Please provide instrument models.
L193-195: Is this repeating the text on L181-182?
Figure 2 caption: As written it sounds like the red line corresponds to crystalline sodium nitrate in ice at 260 K, as well as aqueous sodium nitrate in ice at 260 K. However, from the text, I don’t believe that’s correct. Please clarify the phrasing in the caption.
Figure 4: Please improve the clarity of the legend and caption. Perhaps label the traces a-d and refer to them in the caption or add these labels next to each trace with the description currently in the legend. Also, I suggest changing the colors to be colorblind friendly.
Figure 5: The images are difficult to differentiate visually. I also suggest removing the references to color lines in the caption, as it can accidentally lead the reader to look for these lines in this figure if they read too quickly.
L275 & Figure 6: Please note that emissions are a rate, not a concentration, as reported here.
L276: I am confused about which sample/experiment has precipitated nitrate. The prior sentences discuss ice at 253 K and 258K, which are both above the temperature observed in the earlier sections for nitrate crystallization. Is this a separate sample? If so, please clarify, as the prior sentences in this paragraph appear to be talking about snow experiments.
L278: I am confused by this statement of “a slower increase and decline”. Please clarify what this is referring to in Figure 6. It’s possible that this confusion is related to the confusion in the above comment.
L282: This is confusing because the snow density reported in the methods is a pretty small range. As written, it sounds like you are discussing the snow experiments, but perhaps you are commenting more generally? Please clarify.
Page 13: Please provide your ratios in this paragraph so that the reader can make quantitative comparisons to the literature values discussed.
Figure 6: Please add legends to these graphs. It is difficult to interpret when going back and forth between the figure and caption.
Figure 6: Similar to L276, I am confused by “precipitated salt in snow” for 258 K and 253 K, temperatures at which nitrate should not have precipitated according to the prior sections. Please clarify here and in the text.
Figure 7: Please add a legend to make the figure more easily interpretable.
L332-333: Please directly state the results (what the “different outcomes” actually are).
L333-342: This is a literature summary that is not connected to the results of the current work. The literature should be directly connected to the results, with the results interwoven and preferably stated first before the connection to the literature.
L396: Similar to the above comments, the reference to “potentially precipitated nitrate” in snow is confusing here.