the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 15 Jan 2025
Homogeneous ice nucleation in adsorbed water films: A theoretical approach
Abstract. Ice nucleation plays a critical role in cloud formation and atmospheric processes, influencing precipitation and climate. In this study, we present a theoretical approach for describing homogeneous ice nucleation within adsorbed water films on insoluble substrates, and suggest that it may be a mechanism for deposition ice nucleation with non-porous ice nuclei that induce ice premelting. Our theory is based on the Frenkel-Halsey-Hill (FHH) adsorption model, which characterizes the substrate-adsorbate interaction, and the classical nucleation theory of homogeneous freezing, which describes the probability of ice formation. We use the theory to model the melting point, critical ice nucleus size, and nucleation rates as functions of adsorbed water film thickness and substrate properties. Our results indicate that the melting point depression can be as much as 5 K on hydrophilic substrates when the thickness of the water film is 1 nm. The onset temperature for homogeneous ice nucleation (235 K for cloud droplets) can shift 1–2 K lower in adsorbed films. At temperatures below 235 K, the humidity at which ice nucleation occurs is determined by the condition that the adsorbed water film must be thick enough to accommodate the critical ice nucleus. Comparisons of calculated relative humidity conditions with experimental ice nucleation data for silica particles show promising agreement, validating the FHH model as a framework for describing deposition ice nucleation in the atmosphere.
Competing interests: Ari Laaksonen 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 preprint. The responsibility to include appropriate place names lies with the authors.- Preprint
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RC1: 'Comment on egusphere-2024-4095', Anonymous Referee #1, 05 Feb 2025
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This is the review of the manuscript entitled “Homogeneous ice nucleation in adsorbed water films: A theoretical approach” by Laaksonen et al.
This study presents a theoretical approach based on the Frenkel-Halsey-Hill (FHH) adsorption model to describe deposition ice nucleation in thin films of adsorbed water in absence of pores. This is used to formulate equations to derive the homogeneous ice nucleation rate coefficients within adsorbed water films on insoluble substrates. This approach is then applied to derive the ice melting point, critical ice nucleus size, and nucleation rates as functions of adsorbed water film thickness and substrate properties. The theoretically derived thermodynamic conditions that result in ice nucleation are compared to experimental results.
The topic of this study fits well in Atmospheric Chemistry and Physics considering previously published theoretical and experimental ice nucleation articles in this journal.
Deposition ice nucleation is an understudied topic and still eludes complete understanding of the underlying physical processes that result in ice formation. How ice nucleates from the supersaturated vapor phase on non-porous substrates is understood little. Approaching this by invoking homogeneous ice nucleation in thin water films, typically not detected in ice nucleation experiments, provides a new conceptual model that can be further tested. The application of the FHH adsorption model to derive homogeneous ice nucleation rate coefficients is a neat way to think about this.
The manuscript is well written, and I enjoyed reading it. My comments mostly address clarifications to make reading this manuscript a bit easier for the reader. Although I feel that proposed model has validity and advances our understanding, I have a minor comment regarding the apparent agreement of the experimental data with the model.
Comments:
Line 28: Here and throughout the manuscript: You use monolayers, films and multilayer films. This may need more careful definition. One would assume that a water film consists of several monolayers of water. But what is meant by a multilayer water film?
Line 44 and following: I feel the expression of “film-wise” is unfortunate. I would try to find a better wording what is meant here.
Lines 65 -70: Here, we have A(T) and A’. The latter is then switched to A. This juggling of parameter definitions is also done on line 202 (there you use A for A_w…). Frankly speaking, it took extra effort to keep track of which parameters are temperature dependent and which not. I suggest not substitute but stick with a fix set of parameters, like A(T) and A^298 K, etc. This would facilitate reading of the manuscript. On line 68, I assume you meant the A’ parameter?
Lines 94-97: Maybe provide references for studies that show single and/or multiple monolayer water adsorption on substrates below 100% relative humidity.
Line 98-99: What is the difference between bulk ice and hexagonal ice? Can hexagonal ice form in a thin water film? As you outlined below, it needs some extra layers. If the film has the same number of water molecules, the ice film is larger in surface area? I assume this does not matter when treating it as truly 2D? So, is it a postulate or an assumption to make things work?
Line 128: Could you provide references for typical LJ values?
Lines 150, Eq. 23: Please explain further why you can just replace the exponent “3” with “B”. Above you argue that B=3 for liquid and solid phase. Why can you substitute and allow for its variation in this case? Is it for the sake of having a free parameter?
Line 158, 160: Explain how you derived those two equations. Just looking at the above equations, it is difficult to follow.
Line 164 and following: We would assume that Jhom and ice properties are not decoupled from the water saturation vapor pressures. Can you use different sets of parameters for ice nucleation and properties of water and ice? Espinosa et al. is somehow only chosen to yield better Jhom? In this regard, there might be an even better description of Jhom (Knopf and Alpert, 2023).
Line 202: See comment above on parameter naming.
Line 260: What do you mean by “where V_A denotes the volume of adsorbed water on a single ice nucleus”? How is this volume defined? Does it include additional layers of water?
Figure 8: The discussion of Fig. 8 does not mention much the difference in results in response of using Murphy and Koop vs. Wagner and Pruss saturation vapor pressures. In the former case, the critical humidity drops to 110% for some cases but not when applying the latter. Is this just because the water saturation line by Wagner and Pruss is a bit steeper at lower temperatures? This is surprising. So, it is very sensitive to the saturation vapor pressure? Would you expect this?
Line 287-289: Above you mentioned to ignore the Kelvin effect for this study. Did you account for the Kelvin effect when modeling 100 nm particles (Fig. 8)?
Figure 9 discussion: Your model sensitivity to R is stated as quite weak: an order of magnitude change in R for 3% RHice variation. However, the measurement points have more than 10% RH uncertainty. In other words, uncertainties in R cannot explain the trend in the data? It “has to” be due to variation in A and B parameters and amount of water present? Maybe this could be clearer stated.
Line 327: I am not sure, if “promising agreement” is the right wording. I appreciate that the authors clearly state the caveat that this apparent agreement can only be achieved by using the vapor pressure equation derived by Wagner and Pruss (2002), though the vapor pressure equation of Murphy and Koop has held up well for ice nucleation studies across many disciplines. Would it be worthwhile to consider uncertainties in those vapor pressure formulations and perform some sensitivity tests? What does it mean for the theory, if somehow greater vapor pressures are needed below the homogeneous freezing limit to achieve apparent agreement? Which parameters are affected by this? This discussion could point to further research needs.
References
Knopf, D. A. and Alpert, P. A.: Atmospheric ice nucleation, Nat. Rev. Phys., 10.1038/s42254-023-00570-7, 2023.
Citation: https://doi.org/10.5194/egusphere-2024-4095-RC1
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