the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 15 May 2025
Surface tension and hygroscopicity analysis of aerosols containing organosulfate surfactants
Abstract. Organosulfate (OS) surfactants can influence cloud condensation nuclei (CCN) activation and hygroscopic growth by reducing the surface tension of aerosol particles. We investigate the surface tension and hygroscopicity of aerosols containing short- and long-chain OSs under supersaturated conditions using an electrodeformation method coupled with Raman spectroscopy. For short-chain OSs, the surface tension continues to decrease even under dry, viscous conditions. Sodium ethyl sulfate (SES) lowered surface tension to approximately 30 mN m-1, a value lower than that of sodium dodecyl sulfate (SDS) at its critical micelle concentration. We also studied ternary systems containing OSs with citric acid (CA) or sodium chloride (NaCl). Even small amounts of SDS, with a molar ratio of 10-3 relative to CA, reduce surface tension by up to 40 % at low relative humidity (RH) compared to CA alone. Despite strong surface tension reduction, ternary OS–CA–water systems show hygroscopicity nearly identical to binary CA–water systems, suggesting that surface tension does not influence water uptake under subsaturated conditions. Ternary systems containing NaCl and OS undergo efflorescence at 47 % RH, but the crystallized NaCl becomes partially engulfed. If the RH is subsequently increased, the coating takes up water. At the deliquescence point (72 % RH), the particle becomes homogeneous again. These findings improve our understanding of particle growth and cloud drop formation processes, which influence cloud properties like albedo and lifetime.
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RC1: 'Comment on egusphere-2025-2170', Anonymous Referee #1, 20 Jun 2025
- AC1: 'Reply on RC1', Thomas Preston, 23 Aug 2025
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CC1: 'Comment on egusphere-2025-2170', Jussi Malila, 26 Jun 2025
Referee report for “Surface tension and hygroscopicity analysis of aerosols containing organosulfate surfactants”
The manuscript provides surface tension measurements of a series of sodium alkyl sulfates (SMS, SES, SOS, SDeS and SDS) in binary and ternary (mixtures with citric acid or NaCl) performed using electrodeformation of optically trapped droplets probed with Raman spectroscopy. The measured values are used to infer the hygroscopic growth factors and kappa-parameters. As the members of the studied series are often used proxies for real atmospheric surfactants – also others than atmospheric organosulfates, as SDS has been used in this context at least since Li et al. (1998) – and also the citric acid can be seen as a proxy various highly-oxidised organic molecules, the reported measurements are relevant for the readers of this journal and especially the study of a series of homologous molecules provides molecular-level insight. The manuscript is well written and suitable for publication in ACP after following comments have been considered.
The major concern comes from the stated purities of the compounds used, especially SMS (>92%) and SOS (>95%): as even a small amount of, say, SDS could significantly affect the surface tensions here, can it be confirmed that no strongly surface-active impurities are present?
Minor issues:
- Line 5 (abstract): “the surface tension continues decrease” – I assume that this refers to the hydrocarbon length, but I can be wrong as well. Please reformulate this sentence to be less ambiguous.
- Line 20: Perhaps the most recent IPCC assessment report could be cited instead?
- Line 39: Study of Li and Jang (2013) did not pass the peer review for ACP (as apparent from the reference), please reconsider this.
- Line 56: For Kelly et al. (2009), please cite the corresponding GMD article instead of the preprint, if there is no special reason for the latter.
- Line 96: For consistency, please indicate also the instrument(s) used for gravimetry.
- Equations (1)-(3) and Table 1: These kinds of data are very valuable to test different hypothesis and compare theories of CCN activation. To facilitate it, authors have kindly included the measurement data as a supplement. However, also reporting measurement uncertainties (either in the supplement or at least in Fig. 2) would further facilitate such use.
- Line 115: Please provide also original references for the Szyszkowski-Langmuir and Eberhardt models.
- Equation (5): It might be good to stress that the mole fraction here is the total mole fraction of the solution, as a similar type of equation has been used in related context (e.g. Bzdek et al., 2020) with surface mole fractions.
- Please provide a reference for Eq. (7).
- Lines 256-261: The conclusions hold, as the literature values refer either to kappas calculated assuming the surface tension of water (Han et al., 2022) or deduced directly from measured growth factors (Marsh et al., 2017). This could be spelled out clearly.
- Figure 7 (c) and (d) and related discussion: Could at least part of the water signal originate from an adsorbed layer instead of residual water within the particle? Such adsorbed layers have been extensively studied for nanosized NaCl and NaCl-SDS particles (e.g. Harmon et al., 2010, and references therein).
Very minor issues:
- Line 36: From US to China is not more than half of the Northern Hemisphere, thus “globally” sounds like an overstatement. Maybe either part of the sentence could be reworded?
- I know that I am already presenting a minority view at this point, but I would prefer authors to make a clear distinction between “aerosol” and “aerosol particle”.
Additional references:
Harmon, C W., R. L. Grimm, T. M. McIntire, M. D. Peterson, B. Njegic, V. M. Angel, A. Alshawa, J. S. Underwood, D. J. Tobias, R. B. Gerber, M. S. Gordon, J. C. Hemminger, and S. A. Nizkorodov, 2010: Hygroscopic Growth and Deliquescence of NaCl Nanoparticles Mixed with Surfactant SDS. J. Phys. Chem. B, 114, 2435-2449.
Li, Z., A. L. Williams, and M. J. Rood, 1998: Influence of Soluble Surfactant Properties on the Activation of Aerosol Particles Containing Inorganic Solute. J. Atmos. Sci., 55, 1859–1866.
Citation: https://doi.org/10.5194/egusphere-2025-2170-CC1 - AC3: 'Reply on CC1', Thomas Preston, 23 Aug 2025
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AC4: 'Reply on CC1', Thomas Preston, 23 Aug 2025
Publisher’s note: this comment is a copy of AC3 and its content was therefore removed on 26 August 2025.
Citation: https://doi.org/10.5194/egusphere-2025-2170-AC4
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RC2: 'Comment on egusphere-2025-2170', Anonymous Referee #2, 29 Jul 2025
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. 4259-426Citation: https://doi.org/10.5194/egusphere-2025-2170-RC2 - AC2: 'Reply on RC2', Thomas Preston, 23 Aug 2025
Status: closed
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RC1: 'Comment on egusphere-2025-2170', Anonymous Referee #1, 20 Jun 2025
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:
- 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.
- Figure 2a: sodium methyl sulfate is misspelled in the figure legend.
- 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%).
- 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.
- 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?
- 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?
- 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.
Citation: https://doi.org/10.5194/egusphere-2025-2170-RC1 - AC1: 'Reply on RC1', Thomas Preston, 23 Aug 2025
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CC1: 'Comment on egusphere-2025-2170', Jussi Malila, 26 Jun 2025
Referee report for “Surface tension and hygroscopicity analysis of aerosols containing organosulfate surfactants”
The manuscript provides surface tension measurements of a series of sodium alkyl sulfates (SMS, SES, SOS, SDeS and SDS) in binary and ternary (mixtures with citric acid or NaCl) performed using electrodeformation of optically trapped droplets probed with Raman spectroscopy. The measured values are used to infer the hygroscopic growth factors and kappa-parameters. As the members of the studied series are often used proxies for real atmospheric surfactants – also others than atmospheric organosulfates, as SDS has been used in this context at least since Li et al. (1998) – and also the citric acid can be seen as a proxy various highly-oxidised organic molecules, the reported measurements are relevant for the readers of this journal and especially the study of a series of homologous molecules provides molecular-level insight. The manuscript is well written and suitable for publication in ACP after following comments have been considered.
The major concern comes from the stated purities of the compounds used, especially SMS (>92%) and SOS (>95%): as even a small amount of, say, SDS could significantly affect the surface tensions here, can it be confirmed that no strongly surface-active impurities are present?
Minor issues:
- Line 5 (abstract): “the surface tension continues decrease” – I assume that this refers to the hydrocarbon length, but I can be wrong as well. Please reformulate this sentence to be less ambiguous.
- Line 20: Perhaps the most recent IPCC assessment report could be cited instead?
- Line 39: Study of Li and Jang (2013) did not pass the peer review for ACP (as apparent from the reference), please reconsider this.
- Line 56: For Kelly et al. (2009), please cite the corresponding GMD article instead of the preprint, if there is no special reason for the latter.
- Line 96: For consistency, please indicate also the instrument(s) used for gravimetry.
- Equations (1)-(3) and Table 1: These kinds of data are very valuable to test different hypothesis and compare theories of CCN activation. To facilitate it, authors have kindly included the measurement data as a supplement. However, also reporting measurement uncertainties (either in the supplement or at least in Fig. 2) would further facilitate such use.
- Line 115: Please provide also original references for the Szyszkowski-Langmuir and Eberhardt models.
- Equation (5): It might be good to stress that the mole fraction here is the total mole fraction of the solution, as a similar type of equation has been used in related context (e.g. Bzdek et al., 2020) with surface mole fractions.
- Please provide a reference for Eq. (7).
- Lines 256-261: The conclusions hold, as the literature values refer either to kappas calculated assuming the surface tension of water (Han et al., 2022) or deduced directly from measured growth factors (Marsh et al., 2017). This could be spelled out clearly.
- Figure 7 (c) and (d) and related discussion: Could at least part of the water signal originate from an adsorbed layer instead of residual water within the particle? Such adsorbed layers have been extensively studied for nanosized NaCl and NaCl-SDS particles (e.g. Harmon et al., 2010, and references therein).
Very minor issues:
- Line 36: From US to China is not more than half of the Northern Hemisphere, thus “globally” sounds like an overstatement. Maybe either part of the sentence could be reworded?
- I know that I am already presenting a minority view at this point, but I would prefer authors to make a clear distinction between “aerosol” and “aerosol particle”.
Additional references:
Harmon, C W., R. L. Grimm, T. M. McIntire, M. D. Peterson, B. Njegic, V. M. Angel, A. Alshawa, J. S. Underwood, D. J. Tobias, R. B. Gerber, M. S. Gordon, J. C. Hemminger, and S. A. Nizkorodov, 2010: Hygroscopic Growth and Deliquescence of NaCl Nanoparticles Mixed with Surfactant SDS. J. Phys. Chem. B, 114, 2435-2449.
Li, Z., A. L. Williams, and M. J. Rood, 1998: Influence of Soluble Surfactant Properties on the Activation of Aerosol Particles Containing Inorganic Solute. J. Atmos. Sci., 55, 1859–1866.
Citation: https://doi.org/10.5194/egusphere-2025-2170-CC1 - AC3: 'Reply on CC1', Thomas Preston, 23 Aug 2025
-
AC4: 'Reply on CC1', Thomas Preston, 23 Aug 2025
Publisher’s note: this comment is a copy of AC3 and its content was therefore removed on 26 August 2025.
Citation: https://doi.org/10.5194/egusphere-2025-2170-AC4
-
RC2: 'Comment on egusphere-2025-2170', Anonymous Referee #2, 29 Jul 2025
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.
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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: