Review of egusphere-2022-1188, “Comparison of six approaches to predicting droplet activation of

surface active aerosol – Part 2: strong surfactants” by Sampo Vepsäläinen, Silvia M. Calderón, and Nønne L. Prisle.

Vepsäläinen and co-authors report the results of a modeling study in which six models of surfactant action in cloud condensation nucleation are explored for strong surfactants. The models are based on surfactant properties measured for bulk, and each model accounts for the aerosol phase in a different way (including a model in which the bulk properties are used as-is). Each model has a different effect on the activation of CCN, as demonstrated by Kohler theory. The models are compared, and differences are discussed. Though the manuscript is well-written and well-organized, I find that there are some shortcomings of the paper that need to be addressed before publication. In its current form, the manuscript would be better suited as a technical note. For publication as a research article, I recommend adding a discussion of the how these model differences propagate into uncertainty in cloud droplet number or a similar impact on the aerosol-cloud-climate system. Further, I find that there are a few assumptions in the model that should be discussed a little more. There is no Discussion section – perhaps there should be. If these concerns can be addressed, I think the work would be suitable for prompt publication and would be of interest to the community.

Comments

Line 123-125. how confident are the authors in the assumption of volume additivity? Granted, this has always been the assumption for Kohler theory. Many mixtures do not mix with volume additivity in the bulk – how might this affect the Kohler curves shown here? Is density of particles a function of size?

Line 262. it seems like the CMC will depend rather strongly on the NaCl content. Please comment on this – can the uncertainty here be constrained?

Line 272-273. even if these acids were to partition very strongly, would the surface tension be suppressed? Does the mixture’s surface tension depend on surface concentration necessarily? For example, pure liquids also can have a suppressed surface tension relative to water.

Line 445. can the authors make an estimate of the change in uncertainty in, e.g., cloud droplet number concentration, with the change in surface tension model? This is missing from the discussion.

Minor corrections

Line 3. depleting the droplet bulk? This is a little ambiguous

Line 83. the notation of NaC14 is very confusing, as it seems to imply NaCl_4. Maybe the “14” could be a subscript?

Line 90. extra period after “Table 1”

Line 210. Extra period between sentence and citation

Line 261-262. this does not seem like a numerical artifact – I think the sentence needs to be clarified

Review of: Comparison of six approaches to predicting droplet activation of surface active aerosol – Part 2: strong surfactants

Authors: S. Vepsäläinen et al.

This paper is written largely as a repetition of Part 1: S. Vepsäläinen et al.: "Comparison of six approaches to predicting droplet activation of surface active aerosol – Part 1: moderately surface active organics". The only differences I see are (1) that the six approaches are more completely described in Part 1, and (2) Parts 1 and the submission treat different aerosol: NaCl/Sodium myristate (NaC14) in the submission versus (NH4)2 SO4/Malonic acid in Part 1. For example Part 1 provides a conceptual figure of the different models  that is not present in the submission. To get such detail the reader has to go back and forth between the two papers (Part 1 and submission ) anyway – so why not, in the submitted paper, save journal space and simply refer to Part 1 for the six models and the theory already described there throughout?

The authors refer frequently to Part 1, but I find these comparisons to be more descriptive than insightful. It would be useful to provide the reader with a broader understanding/overview of the effects of moderate versus strong surfactants learned from the two papers as a whole. Part 1 succeeds better in this respect as well, with a conclusions section that makes broad connections of that work to cloud microphysics, fluctuations in supersaturation, and other large-scale effects. The submitted manuscript gives neither discussion nor conclusions, which is suprising in that there are now two papers in the series to draw from.

In summary, the authors should shorten their paper given the repetitive overlap with Part 1 noted in the first paragraph of this review. Moreover, the authors should end their paper by providing general discussion along the lines described in the second paragraph of this review – if for no other reason than to contextualize their point-by-point and figure-by-figure comparisons made throughout the paper. Finally, the authors should consider, perhaps in a separate figure, expanding the range of particle size, now limited to the single size of Dp = 50nm. The most important location along any Kohler curve is the critical point, or maximum, marking the threshold for cloud droplet activation. Showing a locus of these points as particle size is varied for one or more of the six models would add this new dimension.