the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 11 Apr 2025
Mechanistic insights into nitric acid-enhanced iodic acid particle nucleation in the upper troposphere and lower stratosphere
Abstract. In the upper troposphere and lower stratosphere (UTLS), new particles frequently form to seed cloud condensation nuclei (CCN), thereby affecting radiative forcing and global climate. Iodic acid (IA) particles have been widely detected in the UTLS; however, how they form is still largely unknown. Given the abundance of nitric acid (NA) and ammonia (NH3) in the UTLS and their nucleation potential, we explore the influence of NA and NH3 on IA nucleation by quantum chemical calculations and cluster dynamics simulations. The structural analysis indicates that NA and NH3 can cluster with IA via hydrogen bonds, halogen bonds, and electrostatic attractions between ions. The small-sized IA–NA–NH3 clusters have lower free energies than typical sulfuric acid (SA)–NA–NH3 clusters in the upper troposphere, exhibiting greater stability and higher nucleation efficiency. Moreover, the NA-enhanced effect on the established efficient IA–NH3 nucleation is more evident at lower temperatures, especially with richer NA and NH3. In the extremely low-temperature UTLS, the proposed IA–NA–NH3 ternary pathway dominates nucleation, while in the mid troposphere with higher temperatures, the role of NA is minor due to its rapid evaporation. These findings underscore the important role of NA in iodine particle formation in the UTLS, offering mechanistic insights into the missing secondary particle sources.
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Status: open (until 23 May 2025)
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RC1: 'Comment on egusphere-2025-1194', Anonymous Referee #1, 28 Apr 2025
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Jing Li and co-authors have performed computational simulations on HIO3-HNO3-NH3 clustering relevant to UTLS (upper troposphere - lower stratosphere) new-particle formation, including also some comparisons to H2SO4-HNO3-NH3 clusters. This is a highly relevant and timely topic for atmospheric chemists and physicists. For example, there is an ongoing discussion, even debate, on the relative roles of iodine oxyacids (HIO3, HIO2) versus iodine oxides (e.g. I2O5) in promoting particle formation. Engsvand and Elm recently showed, using similar methods as the authors, that in combination with bases such as amines, the latter are more efficient (https://chemrxiv.org/engage/chemrxiv/article-details/67b4894bfa469535b9db7fa5). However, the present manuscript suggests that this comparison should probably be broadened to include also species like HNO3, which might possibly interact more favourably with the oxyacids (or not, this remains to be seen). Overall, the article is generally well written and easy to follow, the simulation methods are broadly appropriate (I especially commend the authors for both thinking carefully about appropriate temperature-dependent boundary settings, and also reporting these so clearly), and I’m happy to recommend publication of the study in ACP. I only have some minor questions and comments that I’d like the authors to briefly address.
Scientific (minor) issues
1)Concerning the discussion of temperature on page 2, any gas-to-particle nucleation mechanism will be more efficient at low temperatures if the concentrations of participating vapors are kept constant. This follows straight from the lesser role of entropy at lower temperatures. The catch is that most potentially nucleating vapor concentrations (in the real world, if not always in simulations or laboratory experiments) also tend to decrease (often very strongly) with temperature. Is the IA-driven mechanism somehow especially efficient in this regard? For example, is it known that IA concentrations in the air decrease less with temperature compared to other potentially nucleating vapours? Or does the IA-related nucleation rate at constant concentrations increase much more steeply with decreasing temperature than most competing nucleation rates? Just saying that it is shows “remarkable efficiency” at low temperatures doesn’t really say that much, the same is arguably true for almost any mechanism. Please elaborate on this.
2)The authors use a aug-cc-pVTZ basis set (and the associated pseudopotential) for I atoms, and a 6-311++G(3df,3pd) basis set for other atoms (C, O, N, H). Previously, the use of imbalanced basis sets (specifically, large basis sets on I atoms and small basis sets on other atoms) has been shown to lead to catastrophically large biases in favour of forming bonds with iodine, see e.g. Finkenzeller et al, https://www.nature.com/articles/s41557-022-01067-z, for a discussion on this. Now, the difference in size between 6-311++G(3df,3pd) and aug-cc-pVTZ is not that dramatic, for example for C atoms its 39 vs 46 basis functions. So I don’t expect the present results to be qualitatively incorrect because of this issue - especially as the final energies are then corrected by coupled-cluster calculations, which do consistently use the same basis set for all atoms. However, a few test calculations comparing e.g. the structures and binding energies (both pure DFT energies and coupled-cluster corrected energies on top of structures optimised with different basis sets) of the smallest HIO3 - containing clusters obtained with the authors’ approach, and with aug-cc-pVTZ for all atoms (with the PP for iodine of course) also at the DFT stage, might be warranted, to check whether the bias in the present results is negligible or not.
3)Just to confirm: when the collision rates are multiplied by 2.3 to account for long-range attractions, also the evaporation rates go up by the same fact, right? (They should, by detailed balance, equation S2 in the authors own supplement. I.e. I just want the authors to confirm that the multiplication by 2.3 is applied to both the collision and the evaporation rates.)
4)Concerning the discussion in section 3.2, of course the nucleation rate goes up with the concentration of participating species. This is inevitable and obvious. Thus it is not an actual discussion-worthy result that J goes up (“exhibits a positive correlation”) with [IA], or that the J rate with NA present is higher than the rate of the otherwise identical system with NA absent. Now, the numerical values themselves are of course interesting, e.g. the fact that even 1E9 per cm3 of NA substantially increases the rate is a valid results. But please reformulate this so that mathematically inevitable consequences of how ACDC works are not reported as novel or “notable” results.
Technical or language issues:
-Figure 5b. How can the two pie charts corresponding to NA=1E9 and IA=1E6 (2nd pie chart from the left in both rows) be different? NH3, T, CS are the same in these runs, as are NA and IA - why are the branching ratios different? Is this a typo, or a bug, or what?
-The last sentence on page 1 (ending with “undisclosed”) seems to be missing some words, should this be “has led to… REMAINING undisclosed”? Also, the word choice is odd: “undisclosed” implies purposeful keeping of secrets (by a sentient actor, typically a human), while what the authors presumably mean is that this facet of the natural world has simply not yet been discovered or understood.
-Page 6, it’s trivially true that SA-NA-NH3 clusters do not form halogen bonds - they do not contain halogen atoms! The first sentence on page 6 thus sounds a bit odd, and could use some rephrasing. (The general point that XBs make the IA-containing clusters substantially stronger is of course valid and worth making, I’m just commenting on the formulation here.)
Citation: https://doi.org/10.5194/egusphere-2025-1194-RC1
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