The surface tension and CCN activation of sea spray aerosol particles by Kleinheins et al. provides predictions of the critical supersaturation for sea spray aerosol. They explore a range of aerosol sizes and compositions reliant to sea spray in addition to droplets containing surfactants of varying strengths. This work uses the Eberhart-Monolayer model to describe the size- and composition-dependent surface tension during cloud droplet activation. The results are indeed interesting and agree with other model predictions. While the Eberhart model has been used to describe the surface tension of bulk solutions and the Monolayer model has been used with other surface tension parameterizations to predict the size- and composition-dependent surfactant partitioning, this work provides the first example of using them together. However, this new combination of modeling methods does not seem to have been validated against data of size-dependent aerosol surface tension or critical supersaturations. Additionally, while the online AIOMFAC model is widely used to predict the water activity of aqueous aerosol, no evidence has been provided to show its utility for the strong surfactants that are modeled in this work. While the results in this manuscript are interesting, some of the statements regarding the results may be too strong if these questions cannot be addressed. A list of specific comments are included below. 

1. First paragraph of section 2.1 – the first statement is too strong. Whether the surface tension is lowered depends on the size and composition of the particle as bulk depletion can bring the surface tension back up to that of pure water, even when quantities of surfactant present are sufficient to reduce the surface tension of bulk solutions. The second statement is also subject to the depletion strength.
2. Page 4 line 105 – how is the surface activity defined here?
3. Page 6 line 155 – Bain et al., 2023 provide an example of the Monolayer Model using a Szyszkowski–Langmuir type isotherm applied to quaternary aerosol droplets.
4. Page 6 line 180 – what evidence is there that this simple substitution of SDS with dodecanoic acid is a reasonable approximation? Furthermore, dodecanoic acid is not one of the molecules AIOMFAC is trained with, and it has a much longer hydrophobic tail than anything in the predefined list. Is there evidence that AIOMFAC predictions for something of this size and hydrophobicity are accurate? I have similar questions about oleic acid, which also has a much longer hydrophobic tail than anything in the predefined list.
5. Is there any evidence that the Eberhart-Monolayer model accurately predicts size-dependent partitioning in aerosol? By comparison to literature droplet surface tension data for example? The quality of the underlying expression for surface tension can play a large role in the size- and concentration-dependent predictions using the Monolayer partitioning model.
6. page 7 line 185  - The model predicts LLPS. Is there any experimental evidence that this is the case?
7. page 13 line 210 – SDS is a solid at room temperature, does this mean that this is the only solute that a solid phase density was used? How does using solid phase density impact the results?
8. Page 13 line 316 – can the authors provide some context as to why 50 nm dry diameter was selected?
9. Page 20 line 444 – “solutes in an aerosol particle are best described by three properties…” can the authors clarify this statement, it is not clear to me how this was determined to be the best. What was it compared to?
10. Page 20 line 446 – Can the authors quantify what SS_crit lowering to a moderate degree and substantially compared to classical Kohler theory are?
11. There are a number of places where the authors refer to surface tension data but then do not cite a source, or cite either El Haber et al., 2024 (which appears to be a review that compiles surface tension data from the literature) or Kleinheins et al., 2023 (a modeling paper again using data available in the literature). References to the primary sources where the data is found must be included.

Typographic corrections

1. Page 16 line 363 – please check van’t Hoff factor = 0. Typically, this is equal to 1 when something does not dissociate.
2. page 7 line 186 – CMC has not been defined
3. Can the authors please check the reference list. The comma after the doi seems to be part of the link, so the links do not work when they are followed. There are also some references with duplicated text.

Overview

Marine stratiform clouds are an important element of the Earth’s radiation budget due to their vast area coverage and dominant shortwave surface cloud radiative forcing over oceans. Sea spray aerosols (SSA) drive the formation of marine stratiform clouds, being the dominant source of cloud condensation nuclei (CCN) in pristine areas such as the Artic and Antartic regions. There is a general consensus on the dominant role of inorganic compounds in the droplet activation of accumulation and coarse aerosol modes. However, it is under debate if submicron SSA generated via bubble bursting can show enhanced cloud activation potential due to their enrichment in biosurfactants, or even allow droplet activation of freshly emitted ultrafine particles (sub-100 nm) in strong marine updrafts.

These biogenic organic surface active species can modify the cloud forming potential of SSA via surface tension lowering and bulk-surface partitioning, also giving them excellent ice nucleating abilities. However, it is difficult and computationally expensive to add a realistic picture of these surfactant-induced effects into cloud modelling studies. Resolving how freshly emitted particles grow until CCN activation require iterative calculations to account for surface adsorption and partitioning due to the cooperative effects between inorganic ions and organic species.

This study proposes a modelling framework to assess these surfactant-induced effects on the critical supersaturation needed for droplet activation of sub-micron SSA. It uses a systematic approach to treat a very complex problem. It starts with the definition of a quaternary model droplet solution containing a strong surfactant, a water soluble organic compound and and inorganic salt. Droplets are formed from dry aerosol particles with size-dependent composition. The droplet is treated as a two-compartment system comprising a surface layer of monomolecular thickness in equilibrium with a bulk solution. Surface tension lowering and partitioning are resolved in tandem combining the multicomponent Eberhart model with the monolayer model. The approach allows to account for organic-inorganic interactions leading to non-idealities in the droplet surface and bulk solutions using the salting-out factors and the AIOMFAC model for activity coefficients. The critical point for droplet activation is estimated via Köhler curve using Raoult and Kelvin terms that reflect surfactant-induced changes in the water activity and surface tension of the droplet bulk solution.

Authors carried out a sensitivity analysis using their modelling approach to estimate the critical supersaturation of dry particles with different size and composition. They kept glucose and sodium chloride as representative species found in freshly emitted submicron SSA, and varied the surfactant type between five organic acids of atmospheric relevance, and the well-studied sodium dodecyl sulphate (SDS) as a surrogate of a strong surfactant. In general, critical points were negatively biased from those obtained with the classical Köhler theory, and positively biased from those obtained after assuming no bulk depletion. Biases increased with increasing surfactant content for a fixed particle size and with decreasing size for a fixed composition.

Finally, authors used critical supersaturation values for SSA model particles (SDS, glucose and NaCl) to estimate the smallest particle in an aerosol population of uniform composition that can activate at a supersaturation level of 0.5 %, and with that concluded that surfactant-induced effects can allow the activation of dry particles with diameter as low as 50 nm. They highlight that the use of a classical Köhler approach would lead to an underestimation of CCN activation in this scenario since the minimum predicted size would be 70 nm.

General comment:

Authors presented a very-well thought approach to deal with a very complex problem, the CCN activation of surfactant-enriched aerosol particles. The model is presented in a very fluent and clear way, easy to understand for possible users. The sensitivity analysis is performed carefully to assess dependencies to particle size and composition. However, the discussion in section 5 could benefit from a comparison of model outputs to laboratory measurements of critical supersaturation for similar particle-systems (e.g. [8-11]).

Even when I understand that this task is challenging due to the scarce data along the size-composition range of atmospheric relevant systems, the statements related to underestimation of CCN activity of ultrafine SSA particles in climate models are too strong without a proper validation of model results. Even if particles with 50 nm-diameter activate at 0.5% supersaturation level in updrafts, they could also deactivate in downdrafts leading to non significant changes in cloud droplet number concentrations at cloud base.

Nonetheless, I recommend the manuscript for publication after addressing the comments due to the completeness of the modelling approach.

Its future implementation in cloud models could bring valuable information about the formation of marine stratocumulus in pristine areas where sea spray emissions from leads can be richer in organic compared to those from open oceans. If statements in this study become proven, it would be necessary to reformulate how SSA emissions are depicted in the marine boundary layer. Even if statements do not hold, being nonactivated, surfactant-enriched SSA could be transported vertically promoting the formation of mixed-phase clouds at higher altitudes in pristine atmospheres. This is particularly important to improve our understanding of the Artic amplification phenomenon.

Minor comments :

1. Line 86 : It is important to include here more information about the pure component surface tension required to perform calculations with the model. This parameter is crucial for the model implementation. Although this is explained in detail in Kleinheins et al. (2024), the model user would benefit from a short summary of the different assumptions related to this variable. It could be useful to explore correlations based on a hypothetical supercooled liquid state for substances that are solid at atmospheric temperatures (e.g.[1-3])

2. Line 142 and 305: The molecular volume of each substance is also crucial in the estimation of the monolayer thickness. As before, the model user would benefit from a short summary of the possible data sources and assumptions related to this variable, especially in the case of surfactants (e.g. [4])

3. Line 300 : Salting-out effects modify the CMC in SDS-NaCl aqueous solutions leading to minimum surface tension values below the levels observed in aqueous SDS solutions (e.g. [5-7]). This effect should be explored more if SSA model particles are going to be represented in the same way in future studies. The Eberhart model assumes that the surface tension is the linear combination of the pure compound surface tension, and even using the salting-out parameters can fail representing cloud droplet solutions along the Köhler curve, especially at low relative humidity values (e.g. RH = 95 %).

4. Figure S2 : About the mass closure. The tolerance value for the function makes use of the absolute value. I am sure that you check this, but just in case I kindly ask... have you check the sign of the individual number of moles at the droplet bulk and surface compartments? Would it be possible to have cases when the numerator is negative but the condition is still satisfied? . At very small droplet sizes for pure surfactant dry particles, it is impossible to accommodate surfactant molecules in a monomolecular layer at the droplet surface compartment. The model, being just a model, tends to give negative solutions.

References

[1] Cachadiña, I., & Mulero, Á. (2020). A New Corresponding-States Model for the Correlation and Prediction of the Surface Tension of Organic Acids. Industrial & Engineering Chemistry Research, 59(17), 8496–8505. https://doi.org/10.1021/acs.iecr.0c00832

[2] Zhang, C., Tian, J., Zheng, M., Yi, H., Zhang, L., & Liu, S. (2017). A new corresponding state-based correlation for the surface tension of organic fatty acids. Modern Physics Letters B, 32(01), 1750361. https://doi.org/10.1142/S0217984917503614

[3] di Nicola, G., Coccia, G., & Pierantozzi, M. (2016). A new equation for the surface tension of carboxylic acids. Fluid Phase Equilibria, 417, 229–236. https://doi.org/10.1016/J.FLUID.2016.03.001

[4] Calderón, S. M., & Prisle, N. L. (2021). Composition dependent density of ternary aqueous solutions of ionic surfactants and salts: Capturing the effect of surfactant micellization in atmospheric droplet model solutions. Journal of Atmospheric Chemistry. https://doi.org/10.1007/s10874-020-09411-8

[5] Persson, C. M., Jonsson, A. P., Bergström, M., & Eriksson, J. C. (2003). Testing the Gouy–Chapman theory by means of surface tension measurements for SDS–NaCl–H2O mixtures. Journal of Colloid and Interface Science, 267(1), 151–154. https://doi.org/10.1016/S0021-9797(03)00761-6

[6] Johnson, C. M., & Tyrode, E. (2005). Study of the adsorption of sodium dodecyl sulfate (SDS) at the air/water interface: targeting the sulfate headgroup using vibrational sum frequency spectroscopy. Physical Chemistry Chemical Physics, 7(13), 2635. https://doi.org/10.1039/b505219j

[7] Corrin, M. L., & Harkins, W. D. (1947). The Effect of Salts on the Critical Concentration for the Formation of Micelles in Colloidal Electrolytes. Journal of the American Chemical Society, 69(3), 683–688. https://doi.org/10.1021/ja01195a065

[8] Prisle, N. L., Raatikainen, T., Laaksonen, A., & Bilde, M. (2010). Surfactants in cloud droplet activation: mixed organic-inorganic particles. Atmospheric Chemistry and Physics, 10(12), 5663–5683. https://doi.org/10.5194/acp-10-5663-2010

[9] Bilde, M., & Svenningsson, B. (2004). CCN Activation of Slightly Soluble Organics: Importance of Small Amounts of Inorganic Salt and Particle Phase. Tellus, Series B: Chemical and Physical Meteorology, 56(2), 128–134. https://doi.org/10.3402/tellusb.v56i2.16406

[10] Svenningsson, B., Rissler, J., Swietlicki, E., Mircea, M., Bilde, M., Facchini, M. C., Decesari, S., Fuzzi, S., Zhou, J., Mønster, J., & Rosenørn, T. (2006). Hygroscopic growth and critical supersaturations for mixed aerosol particles of inorganic and organic compounds of atmospheric relevance. Atmospheric Chemistry and Physics, 6(7), 1937–1952. https://doi.org/10.5194/acp-6-1937-2006

[11] Henning, S., Rosenørn, T., D’Anna, B., Gola, A. A., Svenningsson, B., & Bilde, M. (2005). Cloud droplet activation and surface tension of mixtures of slightly soluble organics and inorganic salt. Atmospheric Chemistry and Physics, 5(2), 575–582. https://doi.org/10.5194/acp-5-575-2005