the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Comparison of six approaches to predicting droplet activation of surface active aerosol – Part 2: strong surfactants
Sampo Vepsäläinen
Silvia M. Calderón
Abstract. Surfactants have been a focus of investigation in atmospheric sciences for decades due to their ability to modify the water uptake and cloud formation potential of aerosols. Surfactants adsorb to the air–solution interface and can decrease the surface tension, while in microscopic aqueous droplets simultaneously depleting the droplet bulk. While this mechanism is now broadly accepted, the representation in atmospheric and cloud droplet models is still not well constrained. We compare the predictions of five bulk–surface partitioning models and a general bulk solution model documented in the literature to represent aerosol surface activity in Köhler calculations of cloud droplet activation. The models are applied to a suite of common aerosol particle systems, consisting of strong surfactants (sodium myristate or myristic acid) and sodium chloride in a wide range of relative mixing ratios. The partitioning models predict comparable critical droplet properties at small surfactant mass fractions, but differences between the model predictions for identical particles increase significantly with the surfactant mass fraction in the particles. For the same particles and simulation conditions, the partitioning models also predict significantly different surface compositions and surface tensions for growing droplets along the Köhler curves. The inter-model variation is furthermore different for these particles comprising strongly surface active organics, than for moderately surface active atmospheric aerosol components. Our results show that experimental validation across a range of atmospherically relevant aerosol compositions, surface active properties, and droplet states is necessary before a given model can be generally applied in atmospheric predictions.
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Sampo Vepsäläinen et al.
Status: final response (author comments only)
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RC1: 'Comment on egusphere-2022-1188', Anonymous Referee #2, 21 Mar 2023
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
Citation: https://doi.org/10.5194/egusphere-2022-1188-RC1 -
RC2: 'Comment on egusphere-2022-1188', Anonymous Referee #3, 24 Mar 2023
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.
Citation: https://doi.org/10.5194/egusphere-2022-1188-RC2 - AC1: 'Author response to reviewers’ comments: egusphere-2022-1188', Sampo Vepsäläinen, 30 May 2023
Sampo Vepsäläinen et al.
Sampo Vepsäläinen et al.
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