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: final response (author comments only)
- RC1: 'Comment on egusphere-2026-3716', Anonymous Referee #1, 11 Sep 2026
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RC2: 'Comment on egusphere-2026-3716', Anonymous Referee #2, 18 Sep 2026
Manoharan et al.
This manuscript presents results of a laboratory investigation which used NEXAFS spectroscopy to determine that NaNO3 solutions did not crystalize in the presence of ice even when temperatures were 23 K below the eutectic point. When ice was not allowed to form, NaNO3 readily crystalized. These findings were attributed to persistence of liquid-like solvation or supercooled solution on the ice surface. The spectroscopy experiments were complemented by flow tube experiments using artificial snow prepared from NaNO3 solutions, at temperatures where either the salt would precipitate or brine would persist to allow "aqueous" processes to occur under irradiation. These flow tube experiments showed greater emissions of NO2 and HONO, and increased HONO/NO2, when nitrate "retains aqueous-like solvation at ice surfaces" compared to when it precipitates as NaNO3. Authors note that these findings indicate that "supercooled nitrate might be present over wide ranges of Arctic, Antarctic, and upper tropospheric temperatures". Photolysis of this nitrate is thus likely to significantly impact the nitrogen oxide chemistry of boundary layer air in both polar regions as well as near cirrus clouds in the upper atmosphere globally.
The manuscript is mostly very well written, and the arguments are presented clearly. One thing that caused me to make marginal notes the first two, maybe three, times I read through was "why are these people focusing on NaNO3". Sodium from sea salt may be important sink for nitrate in snow on sea-ice, and perhaps coastal regions of both Greenland and Antarctica, but in the high-altitude interior of the polar ice sheets the molar ratio NO3/Na (especially during sunlit summers) is greater than 20 (Greenland) and even higher around places like South Pole and Vostok. In the upper troposphere the dominance of NO3 compared to Na is even greater. My marginal notes also included, "what about snow dominated by HNO3, or NH4NO3 if it is essential to consider possible precipitation of salt" (in Greenland and in the upper troposphere during summer there is generally much more NH4 than Na). Given that the focus of discussion and conclusions is entirely on the "phase" of nitrate, with sodium essentially ignored, I eventually convinced myself that the study used NaNO3 solutions because Na K-edge NEXFAS spectra clearly discriminate between precipitated salt and brine (lines 182-183). In contrast, N K-edge NEXFAS spectra do not clearly make this distinction (lines 240-244). I urge the authors to consider making it clear early in the introduction that NaNO3 was chosen as the salt due to analytical considerations, rather than because it is central to nitrate photochemistry.
Comments below (referenced by line # in pdf) are mainly editorial suggestions.
198 "in" ---> "is": solution is an average
215 consider changing beginning of last sentence to "Clearly, sodium (and nitrate by inference) in the presence of ice stays....."
269 This first sentence of section 3.3 supports my suggestion above. You definitely convinced me that nitrate remained ~ionic, but only because you "established spectroscopically" that sodium did so (e.g. no sign of precipitated NaNO3).
271-285 and Fig 6. This section is very important, but not easy to follow. Adding legend with the different line colors to the Fig, and providing more detail in the caption would help. Text could also expanded to improve clarity.
302 "how" --> "that": results show that....
305 "reflection"--> "reflecting"
320 As an example of adding to caption for Fig 6, upper panels both show more than the 4 lines described in caption, nor is it clear why there are so many lines in lower panel of A
333-342 These are not conclusions drawn from this study. If it is important to place results from this study in context of earlier work that is better done in discussion section.
352-355 Would it be safe to assume in a system dominated by HNO3 (like summertime on plateaus of polar ice sheets) that nitrate would rarely precipitate as a salt? If yes, consider saying so. This relates to my opinion noted above that the important issue is behavior of nitrate in general, not NaNO3.
371 Should "Bartels" be "Bartels-Rausch". I note that in Thorsten's on-line CV the citation is Bartels-Rausch but on the paper at journal it is Bartels.
375 – 383 I note that Arkian et al., 2025 (JGR, doi.org/10.1029/2025JD044077) and Romer et al., 2018 (ES&T, DOI:10.1021/acs.est.8b03861) both argue that photolysis of nitrate in aerosol is unlikely to produce nearly so much NOx as Shah and Rowlinson assume it will.
411 "that are not be identified" should be "that are not identified", or "that cannot be identified", or "may not be identified"
Citation: https://doi.org/10.5194/egusphere-2026-3716-RC2 -
RC3: 'Comment on egusphere-2026-3716', Anonymous Referee #3, 27 Sep 2026
The study investigated the physical state of NaNO3 at lower temperatures in the presence and absence of ice. Liquid-like solvation persists at temperatures down to 23 K below the eutectic temperature at ice surfaces, and this state exhibits distinct photochemical behaviour relative to crystalline nitrate. The manuscript is logically coherent, but there are some issues regarding the physical state of nitrate. The following issues need to be addressed to improve the quality of the paper.
Major issues:
- The nitrate solution was adjusted to pH = 3.0 for sample preparation. Please explain the reason for selecting this pH value, and discuss how this acidity influences nitrate speciation, ice surface chemistry, and nitrate photolysis product partitioning in your experiments.
- The spectral range in this experiment starts at 290 nm (Figure 7). Our laboratory studies indicate that even very weak photon flux below 300 nm can drive substantially different photochemical pathways. Therefore, the slight mismatch between the lamp spectrum and the tropospheric solar spectrum may introduce inaccuracies when extrapolating the laboratory results to atmospheric environments. The authors need to verify the effect of the shortwave irradiation on the photolysis of nitrate.
- Figure 4: The sample (38 K>Teu) appears to exhibit bimodal features. This observation suggests that N K-edge NEXAFS may not be suitable for discriminating the physical state of nitrate. This raises a further, critical question: could the chemical environments of Na+ and NO3- be decoupled? That is, Na+ remains hydrated while NO3- crystallizes? This question is critical for the present experiments, because the physical state of NO3- is inferred solely from the state of Na+, rather than from direct measurements of NO3- Overall, this study relies on a limited set of analytical techniques, and the available evidence is not sufficiently robust.
- Samples frozen to 77 K and warmed to 253 K are intended to form crystalline nitrate, yet liquid-like solvation for nitrate cannot be ruled out. Please provide the characterization data to confirm the nitrate phase after this pretreatment.
Minor issues:
- Line 198-199: As the data are spatially averaged across the probing spot, standard deviations of the measurements should be supplied.
- Please explain the cause of the main peak shift between the two samples in Figure 3a.
Citation: https://doi.org/10.5194/egusphere-2026-3716-RC3
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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.