This manuscript (egusphere-2025-2170) reports measurements made on bulk solutions and aerosol droplets containing a range of organosulfates of different chain lengths. Organosulfates are an important component of atmospheric aerosol that can also be surface active. The surface tension and hygroscopicity of these organosulfates was measured using macroscopic approaches and aerosol-based approaches, with particular focus on droplet measurements made under supersaturated solute conditions using an electrodeformation method coupled to Raman spectroscopy. Ternary mixtures of the organosulfates with salt or organic acid were also studied to explore how organosulfates can alter surface tension and hygroscopic response in aerosols containing multiple solutes.

This manuscript reports on a broadly interesting and timely topic, namely the properties of organosulfates and how they can influence aerosol surface tension and hygroscopicity. The experimental approach is unique, providing a significant new data in a domain space inaccessible to other approaches and therefore benefitting the community. The manuscript is well written and the figures are clear and interpretable to the reader. This manuscript will be suitable for publication in Atmospheric Chemistry and Physics once the below comments are fully addressed.

Comments:

1. Page 4, line 111: The authors should clarify why their electrodeformation approach is only applicable at lower relative humidities and is apparently not applicable at higher relative humidities (and therefore pendant droplet measurements are required). The authors should also provide some commentary regarding the different volume scales associated with these two measurement approaches. Given the pendant drop measurements are effectively macroscopic (1-2 mm droplet) whereas the electrodeformation measurements are microscopic (5-10 µm droplet), do the authors need to concern themselves with potential bulk depletion effects in the microscopic measurements that, if not accounted for, could lead to discontinuities between the two approaches? Looking at the concentration ranges explored and comparing the two data sets for sodium methyl and sodium ethyl sulfates, depletion seems unlikely to be important for these systems. However, for the longer chain organosulfates, bulk depletion could be very significant.
2. Figure 2a: sodium methyl sulfate is misspelled in the figure legend.
3. Figure 3 and related discussion: This reviewer infers from the experimental section that surface tension and refractive index are simultaneously retrieved in the electrodeformation measurement. The authors should therefore explain why, in Fig. 3b, refractive index measurements extend to lower RH (~20%) than the surface tension measurements (~30%).
4. Page 9, line 204: It is not clear to this reviewer why the stepwise increase in voltage limits measurements to only highly viscous droplets. What prevents one from setting the relative humidity to 80%, for instance? The authors should more clearly define the underlying factors that limit the range of their electrodeformation approach.
5. Figure 4 and related discussion: For sodium ethyl sulfate, the authors note in the discussion of Fig. 3b that surface tension for this system continues to decrease as RH decreases. This also seems to be the case when sodium ethyl sulfate is mixed with NaCl (Fig. 4a). However, when mixed with citric acid (Fig. 4b) the surface tension plateaus at low RH. Do they authors have any explanation for why sodium ethyl sulfate on its own or with NaCl continues to reduce surface tension at lower RH values but plateaus at a value around 60 mN/m when mixed with citric acid?
6. Page 10, line 225 (mixing ratios for different surfactants): The authors should provide some additional context describing the motivation for their choices of the various solute:surfactant mole ratios explored in this manuscript. The ratios span 10:1 to 1000:1. Presumably this is due to the relative differences in surface activity for the organosulfates and a desire to study a ratio where clear changes occur, but do the chosen ratios relate in any way to expected compositions in atmospheric aerosol?
7. Page 12, line 258: The authors can go a bit farther and say explicitly that given the hygroscopic response observed, the surfactant content is insufficient to alter the droplet’s water activity.

The manuscript entitled “Surface tension and hygroscopicity analysis of aerosols containing organosulfate surfactants” by Shababadi et al. addresses properties of organosulfates relevant for aerosol water uptake in the atmosphere with a particular focus on surface tension of metastable supersaturated aqueous solution droplets.

Organo sulfates are important constituents of atmospheric aerosols and can influence their properties although little is known in this direction. 

The manuscript targets five model sodium salts of organosulfates (sodium methyl sulfate (SMS), sodium ethyl sulfate (SES), sodium octyl sulfate (SOS), sodium decyl sulfate (SDeS), and sodium dodecyl sulfate (SDS) and provides density, refractive index and water activity for aqueous solutions of these organo-sulfates using known methods. 

In relation to surface tension, in addition to the pendant drop method, the authors apply a new and novel method (electrodeformation of trapped droplets) to determine surface tension of metastable supersaturated droplets which allow them to determine surface tension in a concentration regime not approached before.  This method was applied for the two short chain organosulfates in binary and ternary systems. 

The authors also measured hygroscopic growth factors for pure citric acid droplets and citric acid mixed with organosulfates.  Finally, experiments were conducted to probe both efflorescence and deliquescence of optically trapped droplets containing SES and NaCl. 

The manuscript is timely and presents interesting new results from laboratory experiments using state-of-the-art methods. I have some comments and suggestions for improvements which I find should be addressed before publication. The main ones are: 

1) In several places there is room to better use and cite existing literature. Some examples are given below. Please check throughout.

2) the purity of chemicals: some of the chemicals had up to 8% impurity. What could be potential implications of the impurities? This should be discussed.

3) the last part of the manuscript on hygroscopic growth and efflorescence and deliquescence dynamics is very interesting and provides a good basis for further studies, but it is not as well described as the first part of the manuscript, see further details/suggestions below.  I think the manuscript could benefit from including some more details and show more of the available data in 3.3 and 3.4.

Major comments

Abstract

Use of the word “supersaturated conditions”.   I suggest clarifying already in the abstract how the authors use the word supersaturation, since I believe this can cause some confusion. When talking about cloud droplets it refers to the saturation ratio of water vapor, however here it refers to the droplet concentration of solute. Perhaps write “in supersaturated aqueous solution droplets” instead of “supersaturated conditions”. 

The last part of the abstract is difficult to follow, for example “the coating takes up water” – but a coating has not been mentioned in the abstract before, and it is unclear at this point what the coating is. 

Introduction

In some cases, I think more original references could be used/included. E.g. Line 31 in relation to partitioning of surfactants between bulk and surface e.g. [1, 2]. 

Line 44:  The two references to Wang et al.  seem to be in the wrong place. They do not address SDS as a proxy for marine aerosols? one is on terpene derived nitroxy organosulfates (2021a) the other (2019) on mono-terpene and sesquiterpene derived organosulfates.  There are several studies that have used SDS as proxy for marine aerosol e.g. [3, 4]. 

Methods: 

The article by Bain et al. 2023[5]  provides the same parameters (surface tension, density, refractive index and water activity) for sodium methyl sulfate and sodium ethyl sulfate as in this work.  This could be made clearer. Why are slightly different equations used for parameterizations of e.g. density? It would be easier for the reader to compare if the same parameterizations were used.

It would be helpful if it was explained how droplets were generated for the electrodedeformation approach and in what order RH was varied – was is increased or decreased. Also, it could be explained how it was known that the droplets were supersaturated. Were the droplets injected at high RH and then the RH was decreased targeting the efflorescence branch? 

Regarding surface tension measurements with the Pendent drop tensiometer How was a “stable” drop defined?

Results

3.1: “density measurements reveal that short-chain OS exhibit higher densities at increased concentrations.”  - this was also reported in Bain et al. and Koda and Namura 1985[6], I suggest rephrasing to acknowledge this. 

Line 182: why does it say, “in contrast” ? the longer chain ones also show such behavior.  In the caption to figure 2 it should say what the lines are – I assume they are the fitted lines? I suggest showing previous data in figure 2a for comparison. In Bain et al. the concentration unit was solute mass fraction. The authors could write or include in supporting material what the solute mass fraction was in these experiments to aid comparison. 

Line 193: it is not clear what reference 2 is.

Table 1 and Figure 2: what are the experimental uncertainties. Can uncertainties on the fitting parameters be provided?

Regarding the results on the surface tension of mixtures – it could be interesting to apply some kind of mixing rule. It seems a bit surprising that the surface tension of SES does not stabilize as RH goes down (figure 3) but the surface tension of the mixtures with CA and NaCl does. Why could this be? 

Regarding the upper y-axis in figures 4 and 5. At a certain RH  - is the concentration of CA the same whether an organosulfate is present or does it apply only to the pure CA solution?  If so, this should be stated.

Discussion of Figure 5: explicit values should be given for the “typical surface tensions near the bulk CMC”. What are the indications that there could be synergistic interactions – what type of interactions?

Hygroscopicity, ternary systems:

Line 241: “using droplets of varying initial sizes” – these sizes should be given. Did the droplet size matter for the results?

Figure 6: the actual data points should be given. It should be stated how the lines shown were obtained. What was the reproducibility – it says that the experiments were repeated at least 5 times for each ternary system. 

A suggestion:  the authors could compare with growth factors predicted using the ZSR mixing rule. Estillore et al. [7] provides growth fators at 85 % RH for SMS and  SES – a mixing rule could be used to predict the corresponding growht factor for a mixture of CA ad SMS or SES and compared with the values meaured in this work at the same RH. 

In the paper by Bains et al. it says “Estillore et al. used a Multi-Analysis Aerosol Reactor System to measure the growth factor for a range of commercially available and synthesised organosulfate aerosol. The authors found that organosulfate aerosol does not undergo efflorescence/deliquescence behavior (except for samples that were suspected to be contaminated with NaCl) and retains an appreciable amount of water even at relative humidities (RHs) below 10%.”  How does this finding relate to the results in this work?

3.4: formation of partially engulfed particle.

I find this part of the manuscript highly interesting but also lacking some detail and explanation.

Regarding figure 7: why are there no data between 55 and 70% RH? 

Estillore et al. find a difference between deliquescence and effluresence brances of a NaCl SMS mixture – this could be discussed. Would the authors expect to see something similar for the NaCl SES mixture studied? 

What is meant with the statement: “Furthermore, the particle often remained opticalled trapped?  - this implies that they were not always remaining trapped, please provide some further explanation?

What exactly is meant with “partially engulfed” ? What is being engulfed and by what and how is that seen from the data? This could be better explained.

As I understand the authors tried to study surface tension of binary droplets containing salt and longer chain organo-sulfates but this was not technically possible. The reason is not entirely clear from the text. What is meant with “instability in aqueous systems” (line 223)?

1.    Li, Z., A.L. Williams, and M.J. Rood, Influence of Soluble Surfactant Properties on the Activation of Aerosol Particles Containing Inorganic Solute. Journal of the Atmospheric Sciences, 1998. 55(10): p. 1859-1866.

2.    Sorjamaa, R., et al., The role of surfactants in Köhler theory reconsidered. Atmos. Chem. Phys., 2004. 4(8): p. 2107-2117.

3.    Prisle, N.L., et al., Surfactants in cloud droplet activation: mixed organic-inorganic particles. Atmospheric Chemistry and Physics, 2010. 10(12): p. 5663-5683.

4.    Gantt, B. and N. Meskhidze, The physical and chemical characteristics of marine primary organic aerosol: a review. Atmos. Chem. Phys., 2013. 13(8): p. 3979-3996.

5.    Bain, A., M.N. Chan, and B.R. Bzdek, Physical properties of short chain aqueous organosulfate aerosol. Environmental Science: Atmospheres, 2023. 3(9): p. 1365-1373.

6.    Koda, S. and H. Nomura, Aqueous solutions of sodium methylsulfate by Raman scattering, NMR, ultrasound, and density measurements. Journal of Solution Chemistry, 1985. 14(5): p. 355-366.

7.    Estillore, A.D., et al., Water Uptake and Hygroscopic Growth of Organosulfate Aerosol. Environmental Science & Technology, 2016. 50(8): p. 4259-426