the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 01 Oct 2024
Cloud drop activation of insoluble aerosols aided by film forming surfactants
Abstract. Cloud droplet activation of insoluble aerosols covered by insoluble surfactant films has been studied theoretically by combining the FHH activation theory and an equation of state suitable for surfactant films that are in an expanded state. The key parameters governing the surfactant's ability to suppress critical supersaturations are its partial molecular area at the water surface, and the size of the molecule. For a fixed size, molecules with larger molecular area are more efficient, whereas with a fixed area-to-volume ratio, smaller molecules are more efficient. Calculations made for stearic acid films on black carbon and illite aerosols indicate that the critical supersaturations are significantly lower than with pure particles, especially when the dry particle sizes are several hundred nanometers and larger. Furthermore, the reductions of the critical supersaturation are similar when stearic acid is replaced by water-soluble organics with hygroscopicity parameter (κ) values up to 0.1. However, mixtures of surfactant and water-soluble organics are relatively weaker in reducing critical supersaturations than either of these compounds alone.
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RC1: 'Comment on ar-2024-24', Anonymous Referee #1, 11 Oct 2024
This study investigates the influence of surfactant films on the activation of insoluble aerosol particles with a theoretical modelling approach. The model is introduced and in the following applied to black carbon and illite particles coated with either stearic acid or a water soluble compound. Sensitivity of the results to surfactant properties is shown as well. The study presents two interesting findings: First, a coating with insoluble surfactants can facilitate CCN activation, and second, the activation enhancement of insoluble surfactants and water soluble organics seem not to add up linearly. The paper presents the results in a clear and concise way, however, it lacks some fundamental information about the (atmospheric) relevance of the general topic and the chosen cases. Furthermore, it is not clear what the novelty of the study is and therefore, it is not possible to judge if the study provides a substantial contribution to scientific progress. The paper would also benefit from placing the results in the context of the literature, from discussing the results in more detail, and from discussing uncertainties and limitations of the model. My specific comments to the individual sections are:
- The introduction describes why insoluble surfactant films can have a substantial impact on insoluble droplet activation (line 30-35), but it is not mentioned
- why a change in droplet activation is of scientific interest, especially the activation of insoluble aerosol particles?
- Where do insoluble particles with films of insoluble surfactants occur?
- How widespread are they?
- What are sources of insoluble surfactants in the atmosphere?
- Why are BC and illite chosen in this study?
- Why stearic acid?
- Why is not CaCO3 chosen, which would allow a comparison to the experimental results by Wang et al. 2018?
- What is the novelty of the study?
- Theory:
- Line 47: Help the reader by describing what A and B do qualitatively, like: “A low value of A refers to a strong interaction, i.e., a rather hydrophilic surface”, or vice versa. See also line 104: “Illite is somewhat more CCN active than BC” – can this be seen from the lower A parameter?
- Line 52: “amphiphilic organics […] are completely immiscible in water…”: I am wondering whether the acidic end of a fatty acid can interact with and partially dissolve in water. The presented model only takes into account the effect on the surface tension, but can the acidic group also change the water vapor pressure by some other (adsorption) effect? If this is an uncertainty of the model, it should briefly be discussed and estimated what effect it could have.
- Line 53-55: A reference should be given to the theory of a compressed, expanded and gaseous state and the two-dimensional ideal gas law.
- Line 58 and 61: What is the difference between “the surface area per surfactant molecule” (Ωs) and the “partial molecular surface area of […] the surfactant” (ωs)?
- How does equation 2 depend on the dilution/composition of the surface film or the adsorbed water mass or the wet diameter? Is it via Ωs and if so how? The specific equations should definitely be given, especially since the model code is not directly accessible.
- In the gaseous state following the 2D ideal gas law it is understandable that the water activity coefficient is approximated by unity. However, Line 64: “The film is assumed to be dilute enough …” seems to be in contrast with “that applies to expanded films.” in line 59. If the surface film is in an expanded film state, how can it be ensured that fw=1 is justified? It should be discussed what uncertainties this assumption introduces, at least qualitatively.
- Results:
- Line 72: “The two parameters that govern…”: How does S depend on vs? The mathematical relationship should be given or at least qualitatively be described.
- Before showing results of the critical supersaturation in Figure 1, it should be explained how the critical supersaturation is determined. Also it would be extremely useful to show in a plot the development of S over dwet (Köhler curve) or on a similar x-axis and how the surface tension, the film thickness evolve during water uptake. This would help to understand how S* was derived and help to illustrate other processes that are discussed in the following in the text.
- Line 75: Is a density of 850 kg/m3 realistic? Any reference? (Wikipedia gives a density for stearic acid of 941 kg/m3)
- Line 79-81: “However, if the ratio of surfactant molecular volume to surface area is kept constant, and both the volume and the surface area are increased, the result is decreased CCN activity (blue line)” – Why? Can this be explained on a molecular level?
- Line 91-92: “Water soluble organic with κ=0.09” – what molar mass does this approximately correspond to (assuming ideality and no dissociation)? Can an example be given for an organic compound with approx. κ=0.09?
- Line 97-98: “Interestingly, this mixture shows markedly reduced CCN activity compared with either of the 5% volume fraction mixtures.” – since this is a major new finding in this study, it should be elaborated more on it. Why could it be that it behaves like that? What was expected? How does it compare to a case with 2.5% water soluble & 0% stearic acid or to 2.5% stearic acid & 0% water soluble? Is there at least an enhanced activation compared to one of those cases?
- Line 98-99: “This is especially important to notice, as it may be difficult to produce particles coated with pure insoluble surfactant via vapor deposition mechanism” - Here also a reference to the atmospheric situation could be made since it is rather unlikely to have pure surfactant films of one single compound on atmospheric aerosol particles, but rather organics appear in complex mixtures. This is also something that should be discussed. Are the pure surfactant films in this study even realistic? What could be different for more complex compositions?
- Line 114: I assume “the surface tension drops rapidly” should be “the surface tension ‘at activation’ drops rapidly”
- Since the model is not validated by experimental data, the uncertainties of the model approach should be discussed and quantified.
- Finally, how do the results compare to previous work (if there is some)?
- Conclusions
- First sentence (lines 118-119): Has this been done before? Is this a novel approach? If it has been done before, what is new here? How does it compare to other work?
- The section “conclusions” reads more like a “summary”, not a conclusion. A conclusion, should ideally go beyond repeating what was observed but give more of an interpretation on a larger scale. What impact do the results have (on clouds, on lab experiments…)? Also an outlook to future work could be given.
Technical corrections:
- Theory: Before eq1, give again a reference (Sormakaa and Laaksonen 2007)
- Eq1: S is not introduced in the text above
- Before Figure 2 is mentioned, it is not explained what values were chosen for A and B for the calculation of Figure 1.
- Line 47: Add “A and B” into the sentence: “Here, the FFH parameters A and B describe …”
- The legend of Figure 1 uses mo (I guess molecular mass) that has not been introduced and lacks a unit. Also why now here the subscripts “o” instead of “s”?
- Figure 1: S* as a symbol for the critical supersaturation is not introduced in the text or the Figure caption
- Caption of Figure 2: instead of using (1), (2), (3), and (4) that can’t be found in the Figure, use (black), (blue), (cyan), and (red) and the corresponding descriptions or (solid line), (dashed line), etc.
- Line 108: an “s” is missing. “Figure 4 show[s]…”
- Text in figures (axes labels and legend text) is a bit too small
Citation: https://doi.org/10.5194/ar-2024-24-RC1 - The introduction describes why insoluble surfactant films can have a substantial impact on insoluble droplet activation (line 30-35), but it is not mentioned
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RC2: 'Comment on ar-2024-24', Anonymous Referee #2, 29 Oct 2024
The author presents a framework for estimating the cloud droplet activation properties for an insoluble (but water-adsorbing) particle that is coated with a film-forming surfactant. It is pointed out that, because water uptake by such particles is so limited, the particle remains small at its critical diameter and the surfactant can be effective in lowering surface tension. In contrast, strongly hygroscopic particles grow significantly and prior work has shown that the impact of the surfactant can be minimal under high dilution. The conditions under which the surfactant plays a significant role in reducing critical supersaturation for the insoluble particles considered here are mapped out and contrasted with the impacts of instead adding small amounts of hygroscopic material to the particle.
Major Comments: Overall, the manuscript is a nice contribution to the literature on modifications to Kohler theory that consider a broader range of particle types than classic “soluble” and purely “insoluble but wettable” particles. However, it is somewhat difficult to gauge the originality of the contribution based on the prior literature cited. Gohil et al. (2022), not cited in the manuscript, presented the “Hybrid Activity Model” that included the FHH isotherm along with solute water uptake, but they did not include effects on surface tension, as they explicitly acknowledge in their Summary. Therefore, by adding in the equation of state for surfactant films, this manuscript extends their framework and findings. I suggest the author include some additional description along these lines (and compare the two set s derived equations) to better motivate this paper. [Note, There may be other papers since Gohil et al. that have also addressed this topic; I did not check.]
The non-additivity of effects (lines 136 to end) is an important result. This is discussed here as a specific result for certain parameter choices. It would be very interesting to have more discussion of this point and if possible, to generalize the findings.
Minor Comments:
- The FHH approach is useful but in practice, are the fit parameters known for atmospherically-relevant particle types? A brief discussion of its practical utility might be helpful.
- The partial molecular volume and partial molecular area are identified (lines 72-73) as the relevant parameters for film formation. However, the subsequent discussion and figures sometimes refer to these differently, causing confusion; please make the terminology consistent throughout. Also, on line 76, it’s noted that the choices used for illustrative calculations “are not necessarily very realistic”. This is unsatisfying; it would be better to provide some estimates of what might be realistic for atmospheric components and use specific examples or a reasonable range for the illustrative calculations.
- Re Figure 1and similar figures: I wondered if presenting as a classic ln S-ln D_dry plot might be helpful? They should appear more linear, and lines of constant kappa can be shown for the solute-only comparisons. However, this is just a suggestion.
Gohil, K., Mao, C.-N., Rastogi, D., Peng, C., Tang, M., and Asa-Awuku, A.: Hybrid water adsorption and solubility partitioning for aerosol hygroscopicity and droplet growth, Atmos. Chem. Phys., 22, 12769–12787, https://doi.org/10.5194/acp-22-12769-2022, 2022.
Citation: https://doi.org/10.5194/ar-2024-24-RC2
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