the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Insoluble lipid film mediates the transfer of soluble saccharides from the sea to the atmosphere: the role of hydrogen bonding
- Environment Research Institute, Shandong University, Binhai Road 72, Qingdao, 266237, China
- Environment Research Institute, Shandong University, Binhai Road 72, Qingdao, 266237, China
Abstract. Saccharides are a large group of organic matter in sea spray aerosol (SSA). Although they can affect climate-related properties of SSA, the mechanism through which saccharides are transferred from bulk seawater to the ocean surface and ultimately into SSA is still debated. Here, the transfer of small soluble saccharides was validated and quantified using a controlled plunging jet sea spray aerosol generator to better understand the wide range of particle properties produced by natural seawater mixed with model organic species, glucose and trehalose. Data show that both soluble saccharides can promote the production of SSA particles. Conversely, the role of the insoluble fatty acid film on the surface greatly reduced the production of SSA. The resulting inorganic-organic mixed particles identified by the transmission electron microscope (TEM) showed typical core-shell morphology. Langmuir model was used to parameterize the adsorption and distribution of saccharide into SSA across the bubble surface, while infrared reflection-absorption spectroscopy (IRRAS) combined with Langmuir isotherms were undertaken to examine the effects of aqueous subphase soluble saccharides on the phase behavior, structure and ordering of insoluble lipid monolayers absorbed at the air/water interface. Changes in alkyl chains and headgroups structure of mixed fatty acid monolayers under different saccharide concentrations in aqueous phase were reported. In seawater solution, the effects of dissolved saccharides on the ordering and organization of fatty acid chains were muted. Hydrogen bond analysis implied that soluble saccharide molecules displaced a large amount of water near the fatty acid polar headgroups. Saccharide-lipid interactions increased with increasing complexity of the saccharide in the order glucose < trehalose. Our results indicate that the interaction between soluble saccharides and insoluble fatty acid molecules through hydrogen bonds is an important component of the sea-air transfer mechanism of saccharides.
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Minglan Xu et al.
Status: closed
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RC1: 'Comment on acp-2022-628', Anonymous Referee #1, 31 Oct 2022
This manuscript presents a comprehensive study on the transfer of soluble saccharides at the sea-air interface, which affects the number concentration, chemical composition, and morphology of the resulting sea spray aerosols. I appreciate the fascinating experimental methodology of the study, and I enjoy digesting the results. The combination of Langmuir monolayer technology and infrared reflection-absorption spectroscopy helps to explain the interaction between soluble saccharides and insoluble fatty acids, and thus deduce a unique mechanism of hydrogen bonding, which is a novel technique and interesting results presentation, and certainly deserves more attention. Overall, the manuscript is well-written and most important the authors do a brilliant job in the reference list, very multidisciplinary and very updated. The authors have considered multiple views, and it is worth considering to be published in ACP after addressing some general issues.
I have two questions about the abstract. Whether insoluble lipid monolayers can be absorbed at the air/water interface or simply float on the surface? Regarding hydrogen bond analysis, I think the authors are more illustrated by the changes in the infrared spectra of the carbonyl region, and it is necessary to consider whether the expression here is appropriate.
Line 104: “A possible explanation for the SSA composition in saccharides involves”, whether or not the original meaning of expression should be the saccharides in the SSA composition, perhaps this sentence needs to be rewritten.
Line 148-149: It is best for the authors to explain why this molar ratio is used to obtain the mixed fatty acid solution in the method section, rather than in the results and discussion section below.
In the experiment of preparing Langmuir monolayer, whether the volatilization time of 15 min is enough to make chloroform volatilize completely?
Line 226: Whether R and R0 in the eq2 should be the surface covered by fatty acid monolayers and the surface of pure seawater solution?
Section 3.1 The authors have compared the SSA produced using the sea spray aerosol generator in this study with other laboratory devices, and obtained similar consistency, but lack a comparison with the particle number size distribution of the real SSA observed in the field. In other words, whether the device can fully simulate the real SSAs?
A drawback of the presented results is the lack of quantitative information describing the SSA production, specifically the variation of SSA number concentration. A clearer presentation of measured aerosol number concentrations would be most helpful.
Line 261-264: An ambiguous sentence. According to the above description, the author here should be referring to the comparison of SSA particles produced by artificial seawater solution containing such organic matter, rather than SSA particles containing these substances.
Line 325-326: Here the decrease in the lift-off area and molecular area of the stearic acid film should be relative to the previous palmitic acid. Therefore, this sentence should be correct only if it is added with respect to palmitic acid.
Line 355-360: In the Langmuir isotherms and infrared spectroscopy experiments, the authors designed concentrations about 3 orders of magnitude higher than in the real environment, and said that this high concentration is still environmentally relevant, how can it be better explained?
Some minor comments:
Line 43: It is better to use the generic name: humic-like substances (HULIS).
Line 157: “solution” should be changed to plural “solutions”.
Line 272: “change”-“changes”
Line 355: There are several instances in the manuscript where “Glucose and Trehalose” should be lowercase.
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AC1: 'Reply on RC1', Narcisse T. Tsona, 15 Nov 2022
The comment was uploaded in the form of a supplement: https://acp.copernicus.org/preprints/acp-2022-628/acp-2022-628-AC1-supplement.pdf
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AC1: 'Reply on RC1', Narcisse T. Tsona, 15 Nov 2022
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RC2: 'Comment on acp-2022-628', Kimberly Carter-Fenk, 14 Dec 2022
This manuscript presents an important contribution to the understanding of saccharide transfer and enrichment in sea spray aerosol. The authors conduct a thorough experimental investigation in which both particle properties and fundamental physicochemical properties of the air/water interface are investigated. The molecules used within this study represent important and abundant contributors to sea spray aerosol composition, and the composition was carefully selected to serve as a good model system. Additionally, the manuscript is well-written, and the results are summarized clearly and succinctly. While I think that the main arguments presented in the manuscript are reasonable, I believe that several clarifying details and/or a few additional control experiments will make the conclusions more convincing.
- What was the pH of the seawater solution at the beginning and end of the experiments? Basic solutions acidify over time when exposed to air due to atmospheric carbon dioxide, and pH changes can dramatically change the film morphology and saccharide adsorption to the monolayer. Carter-Fenk and Allen (Carter-Fenk, K. A.; Allen, H. C. Collapse Mechanisms of Nascent and Aged Sea Spray Aerosol Proxy Films. Atmosphere 2018, 9 (12), 503. https://doi.org/10.3390/atmos9120503.) demonstrate these pH-dependent changes using the same proxy monolayer mixture, 2 MA : 4 PA : 3 SA. The film morphology and isotherm change as a function of pH, and the myristic acid solubility decreases with decreasing pH. Consequently, any solution acidification could enhance myristic acid adsorption to the air/water interface, thereby expanding the monolayer and increasing the observed mean molecular area in the surface pressure-area isotherms. Carter-Fenk et al. also show how the subphase pH impacts saccharide co-adsorption to a palmitic acid and cetyl alcohol monolayer, albeit using different saccharides (Carter-Fenk, K. A.; Dommer, A. C.; Fiamingo, M. E.; Kim, J.; Amaro, R.; Allen, H. C. Calcium Bridging Drives Polysaccharide Co-Adsorption to a Proxy Sea Surface Microlayer. Chem. Chem. Phys. 2021, 23 (30), 16401–16416. https://doi.org/10.1039/D1CP01407B). The fatty acid carboxylic acid protonation state can change near seawater pH, and the overall monolayer protonation state impacts the intermolecular interactions between saccharides and monolayer headgroups at the air/water interface. For further discussion on the surface pKa of fatty acids at the air/water interface, see the following references: Wellen, B. A.; Lach, E. A.; Allen, H. C. Surface pKa of Octanoic, Nonanoic, and Decanoic Fatty Acids at the Air–Water Interface: Applications to Atmospheric Aerosol Chemistry. Phys. Chem. Chem. Phys. 2017, 19 (39), 26551–26558. https://doi.org/10.1039/C7CP04527A.; Zhang, T.; Brantley, S. L.; Verreault, D.; Dhankani, R.; Corcelli, S. A.; Allen, H. C. Effect of pH and Salt on Surface pKa of Phosphatidic Acid Monolayers. Langmuir 2018, 34 (1), 530–539. https://doi.org/10.1021/acs.langmuir.7b03579.
- The partial dissolution of myristic acid most likely accounts for the smaller mean molecular area observed in the proxy mixture isotherm compared to the palmitic acid and stearic acid isotherms (Figure 3). Myristic acid increases the fluidity of the monolayer, thereby expanding the surface pressure-area isotherm when myristic acid remains adsorbed to the surface (see https://doi.org/10.3390/atmos9120503 for further discussion).
- In line 325, the authors mention a “kink point” in the palmitic acid isotherm at ~40 mN/m. Palmitic acid should not have a phase transition at this point. It is possible that this “kink point” is caused by a contaminant that is being squeezed out upon monolayer compression. Does the “kink point” remain upon using a new palmitic acid and chloroform solution? Does the “kink point” disappear when compressing the barriers at 5 mm/min/barrier instead of 3 mm/min?
- In line 206, the barrier compression speed should be specified as 3 mm/min/barrier (if that is the case).
- In lines 353-355, the authors cite Vazquez de Vasquez et al., 2022 (https://doi.org/10.1021/acsearthspacechem.2c00066) for saccharide concentrations in the ocean. The authors should instead cite the original papers for these measurements: https://doi.org/10.1016/0304-4203(92)90020-B, https://doi.org/10.1021/cr500713g, and https://doi.org/10.1021/acsearthspacechem.9b00197. However, Vazquez de Vasquez et al. corroborate the authors’ argument that the saccharides interact with the monolayer headgroups and expand the monolayer. Additionally, Vazquez de Vasquez et al. argue that glucuronate intercalates into a stearic acid monolayer. Thus, a brief discussion and/or statement on the Vazquez de Vasquez et al. results is warranted in the context of this manuscript’s conclusions on the saccharide-carboxylic acid hydrogen bonding interactions. This statement/discussion would perhaps fit in with the discussion in lines 368-375.
- Lines 380-383: Due to the partial solubility of myristic acid at seawater pH, it is possible that higher concentrations of glucose or trehalose simply decrease the myristic acid solubility due to competitive hydration. In other words, the saccharides are weakly “salting out” the myristic acid from the seawater, enhancing myristic acid adsorption at the air/water interface and expanding the monolayer. I recommend conducting a control experiment in which the surface pressure of myristic acid alone is monitored as a function of saccharide concentration. Spread the same amount of myristic acid on the seawater surface, and test whether higher concentrations of saccharides increase the surface pressure (increase myristic acid adsorption). Normalize the change in surface pressure to any changes in the subphase surface tension due to the different concentrations of saccharides in the seawater. Alternatively, the authors could use deuterated myristic acid and track the C-D vibrational modes with IRRAS as a function of saccharide concentration. If the overall intensity of the C-D modes do not change with increasing saccharide concentration, then the saccharides are not impacting myristic acid adsorption at the air/water interface.
- Lines 386-389: Another change in isotherm slope, or “kink point”, is observed in the proxy monolayer mixtures with saccharides. Is it possible that this change in slope is due to the same “kink point” observed in the palmitic acid isotherm (especially due to the large mole fraction of palmitic acid within the mixture)? Was the same palmitic acid sample used in the preparation of the proxy mixture? Is it possible that some contaminant is making its way into both the palmitic acid + chloroform solution and the proxy mixture + chloroform solution?
- The sentence in lines 456-457 (“This band component is put down to the conformation with the carbonyl group almost parallel to the water surface.”) is unclear.
- How are the center frequencies of the IRRAS peaks being determined? Are the peaks being fitted to Gaussian functions? It would be helpful to have a table (perhaps in the Supplement) of the carbonyl peak center frequencies at the various saccharide concentrations to more readily understand how the vibrational frequencies are changing.
- The sentence in lines 465-467 states: “We believe that saccharides displace water surrounding the fatty acid polar headgroups and interact strongly with both water and lipid headgroups, resulting in a slight increase in hydration near the monolayer interface.” Shouldn’t saccharide adsorption decrease hydration near the monolayer interface due to saccharides displacing water to interact with the monolayer headgroups?
- Lines 487-488: Again, how were the center frequencies determined for the carboxylate stretching modes? Are there three peaks if individual spectra are decomposed into Gaussians? Secondly, the shifts to higher frequencies in the carboxylate stretching modes might be indicative of carboxylate dehydration upon interactions with saccharides.
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AC2: 'Reply on RC2', Narcisse T. Tsona, 13 Jan 2023
The comment was uploaded in the form of a supplement: https://acp.copernicus.org/preprints/acp-2022-628/acp-2022-628-AC2-supplement.pdf
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RC3: 'Comment on acp-2022-628', Anonymous Referee #3, 15 Dec 2022
The manuscript under consideration presents a compelling model for describing the transfer of saccharides into the aerosol phase through artificial breaking waves. The authors use a combination of state-of-the-art methods to support their model: SMPS, Langmuir isotherms, PM-IRRAS, and TEM imaging all serve as a basis for exploring the interactions within a ternary system of saccharides, seawater and insoluble fatty acids. The scientific arguments are logically sequenced, and well-referenced to previous work in the discipline. Overall, I expect that this work will be well-received by the scientific community as it provides interesting insight into the transfer of organic material from the SML into the aerosol phase.
My main criticism of the work in its present form is the authors' use of PM-IRRAS to elucidate the role of hydrogen bonding between the saccharide and fatty acid layer. In particular, I would like to see stronger evidence that there was a shift in the v(C=O) frequency, as it is not abundantly clear from Figure 5 in its present form. In addition, there are some finer points of the authors' scientific arguments that could be expanded upon. I think that these should be addressed before the manuscript is accepted, as it helps to contextualize their results.
Line 35: “SSA represents the major source of aerosol particle populations”. I think this is a complicated assertion to make. While SSA emission per annum is the greatest of all sources with respect to mass (Textor et al, 2006), the same can’t be said about number: even in the Southern Ocean, where sea spray production is rampant, SSA is outnumbered by sulfate aerosols (Quinn et al, 2017). As the sentence goes on to describe effects relating to CCN and IN, I think the statement should be softened to:
“SSA represents a major source of aerosol particle populations”.
Textor, C., Schulz, M., Guibert, S., Kinne, S., Balkanski, Y., Bauer, S., Berntsen, T., Berglen, T., Boucher, O., Chin, M. and Dentener, F., 2006. Analysis and quantification of the diversities of aerosol life cycles within AeroCom. Atmospheric Chemistry and Physics, 6(7), pp.1777-1813.
Quinn, P.K., Coffman, D.J., Johnson, J.E., Upchurch, L.M. and Bates, T.S., 2017. Small fraction of marine cloud condensation nuclei made up of sea spray aerosol. Nature Geoscience, 10(9), pp.674-679.
Line 152: “surface seawater was obtained by dipping an HDPE container through the seawater surface.” I think it would be misleading to describe your collection method as sampling only surface water as this manuscript often references the SML, which is <1 mm thick. There are specific glass plate sampling methods for collecting SML which would have required a glass plate. The method you described (which is fine, in principle), might be better described as having collected both “surface and near-surface seawater.”
Line 252: I think you need to provide more evidence that plunging jets are similar to breaking waves. The previous work that you cited (Christiansen et al, 2019) does not provide any data on the bubble size distribution, nor does the reference work for your apparatus (Liu et al, 2022). In particular, Prather et al (2013) only described similarities between real breaking waves and plunging sheets (which are different from plunging jets). Looking at a similar plunging jet apparatus described by Salter et al (2014) shows that the bubble size distribution produced by a plunging jet is broadly similar to the plunging sheet shown in Prather et al (2013) and Stokes et al (2013). However, note that the exponents of the power law described by Stokes et al (2013) for the plunging sheet apparatus and real waves are larger than for the plunging jet described by Salter et al (2014) (See Table 4 in Salter and Figure 4 in Stokes). Thus, I would suspect that your bubble size distribution was much broader than for true breaking waves. This ought to be discussed with slightly more nuance in the present manuscript. Larger bubbles have a smaller surface-area-to-volume ratio, which ultimately influences the relative production of film drops versus jet drops. Jet drops, whose composition is more strongly tied to the subsurface below the SML are likely depleted in OM.
Salter, M.E., Nilsson, E.D., Butcher, A. and Bilde, M., 2014. On the seawater temperature dependence of the sea spray aerosol generated by a continuous plunging jet. Journal of Geophysical Research: Atmospheres, 119(14), pp.9052-9072.
Stokes, M.D., Deane, G.B., Prather, K., Bertram, T.H., Ruppel, M.J., Ryder, O.S., Brady, J.M. and Zhao, D., 2013. A Marine Aerosol Reference Tank system as a breaking wave analogue for the production of foam and sea-spray aerosols. Atmospheric Measurement Techniques, 6(4), pp.1085-1094.
Line 287: You describe the stability of the surface layer in the presence of fatty acids, but you are constantly disrupting the surface with your plunging jet which is mixing the SML into the subsurface waters. You describe later on (Line 312) that the collapse of the 2D film is itself an irreversible process. Part of my concern with your sampling method is that it does not allow for any transient redevelopment of the SML. There is a time constant related to the development of the SML after being perturbed. In the real ocean, waves rarely ever break the same surface twice. Plunging sheet methods (Stokes et al, 2013) and wave chambers (Prather et al, 2013) allow for the redevelopment of an SML between wave-breaking events. I think it is worth discussing within your manuscript that the transfer of saccharides to the aerosol phase may actually have been limited by the continuous mixing of the SML into the subsurface.
Section 3-2: I just wanted to comment that I found this entire section well-written and illuminating.
Lines 462-465: Here you are describing a shift in the vibrational frequency as evidence of hydrogen bonding. While this is not my specific area of expertise, I am having a hard time seeing a systematic shift in the peak of v(C=O) in either Figure 5a or b. Unless I am gravely misinterpreting these plots, the peak appears to go back and forth between the dashed lines you highlighted as the saccharide concentration increased, rather than one peak systematically outweighing the others as the concentration increased. Case in point, the dominant peak for the carbonyl stretch mode v(C=O) appears to be 1732 cm-1 for both seawater AND your highest concentration of Glucose in Figure 5a. Perhaps you could add an inset to Figure 5 that zooms in on this band and better describes the phenomena you are observing. This is a key observation that you repeatedly use throughout the remainder of the manuscript to support evidence of hydrogen bonding between the saccharide and fatty acid. It ought to be crystal clear to the reader.
Lines 487-488: Again, I had to look quite closely to see the trifurcation of the vas(COO) peak. This is more obvious upon closer inspection than my previous comment about v(C=O), but an inset of Figure 5 that focuses on the 1500-1600 cm-1 region might be helpful to the reader.
Figure 6: This is a beautiful figure, but one of my concerns is that you have analyzed (and are thus comparing) particles of different sizes. There are many studies which suggest that the fraction of organic matter within the generated aerosol can be highly size-dependent for particles produced from the same bulk water composition. This complicates your comparison somewhat and ought to be discussed with more nuance in this section; particularly, as you reference Estillore et al (2017)’s finding that the core-shell morphology is highly dependent on the salt-organic ratio. I think that your qualitative argument is fine, but some additional citations and discussion of the inherent limitations of comparing different-sized particles are needed.
Line 575: “poor”. I think this is a bit of a harsh way of phrasing the scope of this study. Suggest softening "poor" to “limited”.
Line 582-585: While this is the general view, there is some nuance to this assertion specifically for CCN. The hygroscopicity of a composite aerosol is generally well-modelled according to a linear mixture model based on volume fraction (Petters and Kreidenweiss, 2007). The hygroscopicity of your aerosol was likely between that of glucose (k=0.17; Ziemann, Kreidenweiss and Petters, 2013) and that of sea salt (k=1-1.25; Zieger et al, 2017). Further, you combine De Vasquez et al (2022) and (Quinn et al 2015; Hasenacz et al 2019) to conclude that the oceanic concentration of saccharides is just 0.14 mg/L, which is substantially lower than the concentrations observed here. So, how considerable of an effect is this going to have on hygroscopicity? Here is some back-of-the-envelope math:
Ocean Salinity (g/L): 35
Bulk Saccharide Concentration (g/L): 0.00014
Density of Glucose (g/cm3) ~ 1.56
Density of Salt (g/cm3) ~ 2
Enrichment factor (Zeppenfeld et al, 2021): <167000
Mass Ratio of saccharide in aerosol (g/g): (0.00014/35)*167000 = 0.67
Volume Ratio (L/L): 0.67*2/1.5 = 0.89
k = 0.89*0.17 + 0.11*1.1 = 0.27
This is likely a lower limit of the resulting hygroscopicity of your mixed aerosol since it assumed that the enrichment factor is on the largest end of the factors reported by Zeppenfeld et al (2021). Relating this to the sc-Dd curve presented by Petters and Kreidenweiss (2007) in Figure 2, a supersaturation of just 0.1% is required to activate >50% of your particle size distribution as CCN. Consider that 0.1% is the lower end of supersaturations experienced during cloud formation and consider that the calculation above is likely an upper limit of the abundance of the saccharide within the aerosol. Case in point, at a supersaturation of 1.0% virtually your entire particle size distribution could act as CCN. This could add a little more nuance to your discussion of climatic effects.
Ziemann, Paul J., Kreidenweis, Sonia M., and Petters, Markus D.. Quantifying the Relationship between Organic Aerosol Composition and Hygroscopicity/CCN Activity. United States: N. p., 2013. Web. doi:10.2172/1086826.
Petters, M.D. and Kreidenweis, S.M., 2007. A single parameter representation of hygroscopic growth and cloud condensation nucleus activity. Atmospheric Chemistry and Physics, 7(8), pp.1961-1971.
Zieger, P., Väisänen, O., Corbin, J.C., Partridge, D.G., Bastelberger, S., Mousavi-Fard, M., Rosati, B., Gysel, M., Krieger, U.K., Leck, C. and Nenes, A., 2017. Revising the hygroscopicity of inorganic sea salt particles. Nature Communications, 8(1), pp.1-10.
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AC3: 'Reply on RC3', Narcisse T. Tsona, 13 Jan 2023
The comment was uploaded in the form of a supplement: https://acp.copernicus.org/preprints/acp-2022-628/acp-2022-628-AC3-supplement.pdf
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AC3: 'Reply on RC3', Narcisse T. Tsona, 13 Jan 2023
Status: closed
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RC1: 'Comment on acp-2022-628', Anonymous Referee #1, 31 Oct 2022
This manuscript presents a comprehensive study on the transfer of soluble saccharides at the sea-air interface, which affects the number concentration, chemical composition, and morphology of the resulting sea spray aerosols. I appreciate the fascinating experimental methodology of the study, and I enjoy digesting the results. The combination of Langmuir monolayer technology and infrared reflection-absorption spectroscopy helps to explain the interaction between soluble saccharides and insoluble fatty acids, and thus deduce a unique mechanism of hydrogen bonding, which is a novel technique and interesting results presentation, and certainly deserves more attention. Overall, the manuscript is well-written and most important the authors do a brilliant job in the reference list, very multidisciplinary and very updated. The authors have considered multiple views, and it is worth considering to be published in ACP after addressing some general issues.
I have two questions about the abstract. Whether insoluble lipid monolayers can be absorbed at the air/water interface or simply float on the surface? Regarding hydrogen bond analysis, I think the authors are more illustrated by the changes in the infrared spectra of the carbonyl region, and it is necessary to consider whether the expression here is appropriate.
Line 104: “A possible explanation for the SSA composition in saccharides involves”, whether or not the original meaning of expression should be the saccharides in the SSA composition, perhaps this sentence needs to be rewritten.
Line 148-149: It is best for the authors to explain why this molar ratio is used to obtain the mixed fatty acid solution in the method section, rather than in the results and discussion section below.
In the experiment of preparing Langmuir monolayer, whether the volatilization time of 15 min is enough to make chloroform volatilize completely?
Line 226: Whether R and R0 in the eq2 should be the surface covered by fatty acid monolayers and the surface of pure seawater solution?
Section 3.1 The authors have compared the SSA produced using the sea spray aerosol generator in this study with other laboratory devices, and obtained similar consistency, but lack a comparison with the particle number size distribution of the real SSA observed in the field. In other words, whether the device can fully simulate the real SSAs?
A drawback of the presented results is the lack of quantitative information describing the SSA production, specifically the variation of SSA number concentration. A clearer presentation of measured aerosol number concentrations would be most helpful.
Line 261-264: An ambiguous sentence. According to the above description, the author here should be referring to the comparison of SSA particles produced by artificial seawater solution containing such organic matter, rather than SSA particles containing these substances.
Line 325-326: Here the decrease in the lift-off area and molecular area of the stearic acid film should be relative to the previous palmitic acid. Therefore, this sentence should be correct only if it is added with respect to palmitic acid.
Line 355-360: In the Langmuir isotherms and infrared spectroscopy experiments, the authors designed concentrations about 3 orders of magnitude higher than in the real environment, and said that this high concentration is still environmentally relevant, how can it be better explained?
Some minor comments:
Line 43: It is better to use the generic name: humic-like substances (HULIS).
Line 157: “solution” should be changed to plural “solutions”.
Line 272: “change”-“changes”
Line 355: There are several instances in the manuscript where “Glucose and Trehalose” should be lowercase.
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AC1: 'Reply on RC1', Narcisse T. Tsona, 15 Nov 2022
The comment was uploaded in the form of a supplement: https://acp.copernicus.org/preprints/acp-2022-628/acp-2022-628-AC1-supplement.pdf
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AC1: 'Reply on RC1', Narcisse T. Tsona, 15 Nov 2022
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RC2: 'Comment on acp-2022-628', Kimberly Carter-Fenk, 14 Dec 2022
This manuscript presents an important contribution to the understanding of saccharide transfer and enrichment in sea spray aerosol. The authors conduct a thorough experimental investigation in which both particle properties and fundamental physicochemical properties of the air/water interface are investigated. The molecules used within this study represent important and abundant contributors to sea spray aerosol composition, and the composition was carefully selected to serve as a good model system. Additionally, the manuscript is well-written, and the results are summarized clearly and succinctly. While I think that the main arguments presented in the manuscript are reasonable, I believe that several clarifying details and/or a few additional control experiments will make the conclusions more convincing.
- What was the pH of the seawater solution at the beginning and end of the experiments? Basic solutions acidify over time when exposed to air due to atmospheric carbon dioxide, and pH changes can dramatically change the film morphology and saccharide adsorption to the monolayer. Carter-Fenk and Allen (Carter-Fenk, K. A.; Allen, H. C. Collapse Mechanisms of Nascent and Aged Sea Spray Aerosol Proxy Films. Atmosphere 2018, 9 (12), 503. https://doi.org/10.3390/atmos9120503.) demonstrate these pH-dependent changes using the same proxy monolayer mixture, 2 MA : 4 PA : 3 SA. The film morphology and isotherm change as a function of pH, and the myristic acid solubility decreases with decreasing pH. Consequently, any solution acidification could enhance myristic acid adsorption to the air/water interface, thereby expanding the monolayer and increasing the observed mean molecular area in the surface pressure-area isotherms. Carter-Fenk et al. also show how the subphase pH impacts saccharide co-adsorption to a palmitic acid and cetyl alcohol monolayer, albeit using different saccharides (Carter-Fenk, K. A.; Dommer, A. C.; Fiamingo, M. E.; Kim, J.; Amaro, R.; Allen, H. C. Calcium Bridging Drives Polysaccharide Co-Adsorption to a Proxy Sea Surface Microlayer. Chem. Chem. Phys. 2021, 23 (30), 16401–16416. https://doi.org/10.1039/D1CP01407B). The fatty acid carboxylic acid protonation state can change near seawater pH, and the overall monolayer protonation state impacts the intermolecular interactions between saccharides and monolayer headgroups at the air/water interface. For further discussion on the surface pKa of fatty acids at the air/water interface, see the following references: Wellen, B. A.; Lach, E. A.; Allen, H. C. Surface pKa of Octanoic, Nonanoic, and Decanoic Fatty Acids at the Air–Water Interface: Applications to Atmospheric Aerosol Chemistry. Phys. Chem. Chem. Phys. 2017, 19 (39), 26551–26558. https://doi.org/10.1039/C7CP04527A.; Zhang, T.; Brantley, S. L.; Verreault, D.; Dhankani, R.; Corcelli, S. A.; Allen, H. C. Effect of pH and Salt on Surface pKa of Phosphatidic Acid Monolayers. Langmuir 2018, 34 (1), 530–539. https://doi.org/10.1021/acs.langmuir.7b03579.
- The partial dissolution of myristic acid most likely accounts for the smaller mean molecular area observed in the proxy mixture isotherm compared to the palmitic acid and stearic acid isotherms (Figure 3). Myristic acid increases the fluidity of the monolayer, thereby expanding the surface pressure-area isotherm when myristic acid remains adsorbed to the surface (see https://doi.org/10.3390/atmos9120503 for further discussion).
- In line 325, the authors mention a “kink point” in the palmitic acid isotherm at ~40 mN/m. Palmitic acid should not have a phase transition at this point. It is possible that this “kink point” is caused by a contaminant that is being squeezed out upon monolayer compression. Does the “kink point” remain upon using a new palmitic acid and chloroform solution? Does the “kink point” disappear when compressing the barriers at 5 mm/min/barrier instead of 3 mm/min?
- In line 206, the barrier compression speed should be specified as 3 mm/min/barrier (if that is the case).
- In lines 353-355, the authors cite Vazquez de Vasquez et al., 2022 (https://doi.org/10.1021/acsearthspacechem.2c00066) for saccharide concentrations in the ocean. The authors should instead cite the original papers for these measurements: https://doi.org/10.1016/0304-4203(92)90020-B, https://doi.org/10.1021/cr500713g, and https://doi.org/10.1021/acsearthspacechem.9b00197. However, Vazquez de Vasquez et al. corroborate the authors’ argument that the saccharides interact with the monolayer headgroups and expand the monolayer. Additionally, Vazquez de Vasquez et al. argue that glucuronate intercalates into a stearic acid monolayer. Thus, a brief discussion and/or statement on the Vazquez de Vasquez et al. results is warranted in the context of this manuscript’s conclusions on the saccharide-carboxylic acid hydrogen bonding interactions. This statement/discussion would perhaps fit in with the discussion in lines 368-375.
- Lines 380-383: Due to the partial solubility of myristic acid at seawater pH, it is possible that higher concentrations of glucose or trehalose simply decrease the myristic acid solubility due to competitive hydration. In other words, the saccharides are weakly “salting out” the myristic acid from the seawater, enhancing myristic acid adsorption at the air/water interface and expanding the monolayer. I recommend conducting a control experiment in which the surface pressure of myristic acid alone is monitored as a function of saccharide concentration. Spread the same amount of myristic acid on the seawater surface, and test whether higher concentrations of saccharides increase the surface pressure (increase myristic acid adsorption). Normalize the change in surface pressure to any changes in the subphase surface tension due to the different concentrations of saccharides in the seawater. Alternatively, the authors could use deuterated myristic acid and track the C-D vibrational modes with IRRAS as a function of saccharide concentration. If the overall intensity of the C-D modes do not change with increasing saccharide concentration, then the saccharides are not impacting myristic acid adsorption at the air/water interface.
- Lines 386-389: Another change in isotherm slope, or “kink point”, is observed in the proxy monolayer mixtures with saccharides. Is it possible that this change in slope is due to the same “kink point” observed in the palmitic acid isotherm (especially due to the large mole fraction of palmitic acid within the mixture)? Was the same palmitic acid sample used in the preparation of the proxy mixture? Is it possible that some contaminant is making its way into both the palmitic acid + chloroform solution and the proxy mixture + chloroform solution?
- The sentence in lines 456-457 (“This band component is put down to the conformation with the carbonyl group almost parallel to the water surface.”) is unclear.
- How are the center frequencies of the IRRAS peaks being determined? Are the peaks being fitted to Gaussian functions? It would be helpful to have a table (perhaps in the Supplement) of the carbonyl peak center frequencies at the various saccharide concentrations to more readily understand how the vibrational frequencies are changing.
- The sentence in lines 465-467 states: “We believe that saccharides displace water surrounding the fatty acid polar headgroups and interact strongly with both water and lipid headgroups, resulting in a slight increase in hydration near the monolayer interface.” Shouldn’t saccharide adsorption decrease hydration near the monolayer interface due to saccharides displacing water to interact with the monolayer headgroups?
- Lines 487-488: Again, how were the center frequencies determined for the carboxylate stretching modes? Are there three peaks if individual spectra are decomposed into Gaussians? Secondly, the shifts to higher frequencies in the carboxylate stretching modes might be indicative of carboxylate dehydration upon interactions with saccharides.
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AC2: 'Reply on RC2', Narcisse T. Tsona, 13 Jan 2023
The comment was uploaded in the form of a supplement: https://acp.copernicus.org/preprints/acp-2022-628/acp-2022-628-AC2-supplement.pdf
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RC3: 'Comment on acp-2022-628', Anonymous Referee #3, 15 Dec 2022
The manuscript under consideration presents a compelling model for describing the transfer of saccharides into the aerosol phase through artificial breaking waves. The authors use a combination of state-of-the-art methods to support their model: SMPS, Langmuir isotherms, PM-IRRAS, and TEM imaging all serve as a basis for exploring the interactions within a ternary system of saccharides, seawater and insoluble fatty acids. The scientific arguments are logically sequenced, and well-referenced to previous work in the discipline. Overall, I expect that this work will be well-received by the scientific community as it provides interesting insight into the transfer of organic material from the SML into the aerosol phase.
My main criticism of the work in its present form is the authors' use of PM-IRRAS to elucidate the role of hydrogen bonding between the saccharide and fatty acid layer. In particular, I would like to see stronger evidence that there was a shift in the v(C=O) frequency, as it is not abundantly clear from Figure 5 in its present form. In addition, there are some finer points of the authors' scientific arguments that could be expanded upon. I think that these should be addressed before the manuscript is accepted, as it helps to contextualize their results.
Line 35: “SSA represents the major source of aerosol particle populations”. I think this is a complicated assertion to make. While SSA emission per annum is the greatest of all sources with respect to mass (Textor et al, 2006), the same can’t be said about number: even in the Southern Ocean, where sea spray production is rampant, SSA is outnumbered by sulfate aerosols (Quinn et al, 2017). As the sentence goes on to describe effects relating to CCN and IN, I think the statement should be softened to:
“SSA represents a major source of aerosol particle populations”.
Textor, C., Schulz, M., Guibert, S., Kinne, S., Balkanski, Y., Bauer, S., Berntsen, T., Berglen, T., Boucher, O., Chin, M. and Dentener, F., 2006. Analysis and quantification of the diversities of aerosol life cycles within AeroCom. Atmospheric Chemistry and Physics, 6(7), pp.1777-1813.
Quinn, P.K., Coffman, D.J., Johnson, J.E., Upchurch, L.M. and Bates, T.S., 2017. Small fraction of marine cloud condensation nuclei made up of sea spray aerosol. Nature Geoscience, 10(9), pp.674-679.
Line 152: “surface seawater was obtained by dipping an HDPE container through the seawater surface.” I think it would be misleading to describe your collection method as sampling only surface water as this manuscript often references the SML, which is <1 mm thick. There are specific glass plate sampling methods for collecting SML which would have required a glass plate. The method you described (which is fine, in principle), might be better described as having collected both “surface and near-surface seawater.”
Line 252: I think you need to provide more evidence that plunging jets are similar to breaking waves. The previous work that you cited (Christiansen et al, 2019) does not provide any data on the bubble size distribution, nor does the reference work for your apparatus (Liu et al, 2022). In particular, Prather et al (2013) only described similarities between real breaking waves and plunging sheets (which are different from plunging jets). Looking at a similar plunging jet apparatus described by Salter et al (2014) shows that the bubble size distribution produced by a plunging jet is broadly similar to the plunging sheet shown in Prather et al (2013) and Stokes et al (2013). However, note that the exponents of the power law described by Stokes et al (2013) for the plunging sheet apparatus and real waves are larger than for the plunging jet described by Salter et al (2014) (See Table 4 in Salter and Figure 4 in Stokes). Thus, I would suspect that your bubble size distribution was much broader than for true breaking waves. This ought to be discussed with slightly more nuance in the present manuscript. Larger bubbles have a smaller surface-area-to-volume ratio, which ultimately influences the relative production of film drops versus jet drops. Jet drops, whose composition is more strongly tied to the subsurface below the SML are likely depleted in OM.
Salter, M.E., Nilsson, E.D., Butcher, A. and Bilde, M., 2014. On the seawater temperature dependence of the sea spray aerosol generated by a continuous plunging jet. Journal of Geophysical Research: Atmospheres, 119(14), pp.9052-9072.
Stokes, M.D., Deane, G.B., Prather, K., Bertram, T.H., Ruppel, M.J., Ryder, O.S., Brady, J.M. and Zhao, D., 2013. A Marine Aerosol Reference Tank system as a breaking wave analogue for the production of foam and sea-spray aerosols. Atmospheric Measurement Techniques, 6(4), pp.1085-1094.
Line 287: You describe the stability of the surface layer in the presence of fatty acids, but you are constantly disrupting the surface with your plunging jet which is mixing the SML into the subsurface waters. You describe later on (Line 312) that the collapse of the 2D film is itself an irreversible process. Part of my concern with your sampling method is that it does not allow for any transient redevelopment of the SML. There is a time constant related to the development of the SML after being perturbed. In the real ocean, waves rarely ever break the same surface twice. Plunging sheet methods (Stokes et al, 2013) and wave chambers (Prather et al, 2013) allow for the redevelopment of an SML between wave-breaking events. I think it is worth discussing within your manuscript that the transfer of saccharides to the aerosol phase may actually have been limited by the continuous mixing of the SML into the subsurface.
Section 3-2: I just wanted to comment that I found this entire section well-written and illuminating.
Lines 462-465: Here you are describing a shift in the vibrational frequency as evidence of hydrogen bonding. While this is not my specific area of expertise, I am having a hard time seeing a systematic shift in the peak of v(C=O) in either Figure 5a or b. Unless I am gravely misinterpreting these plots, the peak appears to go back and forth between the dashed lines you highlighted as the saccharide concentration increased, rather than one peak systematically outweighing the others as the concentration increased. Case in point, the dominant peak for the carbonyl stretch mode v(C=O) appears to be 1732 cm-1 for both seawater AND your highest concentration of Glucose in Figure 5a. Perhaps you could add an inset to Figure 5 that zooms in on this band and better describes the phenomena you are observing. This is a key observation that you repeatedly use throughout the remainder of the manuscript to support evidence of hydrogen bonding between the saccharide and fatty acid. It ought to be crystal clear to the reader.
Lines 487-488: Again, I had to look quite closely to see the trifurcation of the vas(COO) peak. This is more obvious upon closer inspection than my previous comment about v(C=O), but an inset of Figure 5 that focuses on the 1500-1600 cm-1 region might be helpful to the reader.
Figure 6: This is a beautiful figure, but one of my concerns is that you have analyzed (and are thus comparing) particles of different sizes. There are many studies which suggest that the fraction of organic matter within the generated aerosol can be highly size-dependent for particles produced from the same bulk water composition. This complicates your comparison somewhat and ought to be discussed with more nuance in this section; particularly, as you reference Estillore et al (2017)’s finding that the core-shell morphology is highly dependent on the salt-organic ratio. I think that your qualitative argument is fine, but some additional citations and discussion of the inherent limitations of comparing different-sized particles are needed.
Line 575: “poor”. I think this is a bit of a harsh way of phrasing the scope of this study. Suggest softening "poor" to “limited”.
Line 582-585: While this is the general view, there is some nuance to this assertion specifically for CCN. The hygroscopicity of a composite aerosol is generally well-modelled according to a linear mixture model based on volume fraction (Petters and Kreidenweiss, 2007). The hygroscopicity of your aerosol was likely between that of glucose (k=0.17; Ziemann, Kreidenweiss and Petters, 2013) and that of sea salt (k=1-1.25; Zieger et al, 2017). Further, you combine De Vasquez et al (2022) and (Quinn et al 2015; Hasenacz et al 2019) to conclude that the oceanic concentration of saccharides is just 0.14 mg/L, which is substantially lower than the concentrations observed here. So, how considerable of an effect is this going to have on hygroscopicity? Here is some back-of-the-envelope math:
Ocean Salinity (g/L): 35
Bulk Saccharide Concentration (g/L): 0.00014
Density of Glucose (g/cm3) ~ 1.56
Density of Salt (g/cm3) ~ 2
Enrichment factor (Zeppenfeld et al, 2021): <167000
Mass Ratio of saccharide in aerosol (g/g): (0.00014/35)*167000 = 0.67
Volume Ratio (L/L): 0.67*2/1.5 = 0.89
k = 0.89*0.17 + 0.11*1.1 = 0.27
This is likely a lower limit of the resulting hygroscopicity of your mixed aerosol since it assumed that the enrichment factor is on the largest end of the factors reported by Zeppenfeld et al (2021). Relating this to the sc-Dd curve presented by Petters and Kreidenweiss (2007) in Figure 2, a supersaturation of just 0.1% is required to activate >50% of your particle size distribution as CCN. Consider that 0.1% is the lower end of supersaturations experienced during cloud formation and consider that the calculation above is likely an upper limit of the abundance of the saccharide within the aerosol. Case in point, at a supersaturation of 1.0% virtually your entire particle size distribution could act as CCN. This could add a little more nuance to your discussion of climatic effects.
Ziemann, Paul J., Kreidenweis, Sonia M., and Petters, Markus D.. Quantifying the Relationship between Organic Aerosol Composition and Hygroscopicity/CCN Activity. United States: N. p., 2013. Web. doi:10.2172/1086826.
Petters, M.D. and Kreidenweis, S.M., 2007. A single parameter representation of hygroscopic growth and cloud condensation nucleus activity. Atmospheric Chemistry and Physics, 7(8), pp.1961-1971.
Zieger, P., Väisänen, O., Corbin, J.C., Partridge, D.G., Bastelberger, S., Mousavi-Fard, M., Rosati, B., Gysel, M., Krieger, U.K., Leck, C. and Nenes, A., 2017. Revising the hygroscopicity of inorganic sea salt particles. Nature Communications, 8(1), pp.1-10.
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AC3: 'Reply on RC3', Narcisse T. Tsona, 13 Jan 2023
The comment was uploaded in the form of a supplement: https://acp.copernicus.org/preprints/acp-2022-628/acp-2022-628-AC3-supplement.pdf
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AC3: 'Reply on RC3', Narcisse T. Tsona, 13 Jan 2023
Minglan Xu et al.
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