From the journal Environmental Science: Atmospheres Peer review history

28-Aug-2022

Dear Dr Greaves:

Manuscript ID: EA-ART-07-2022-000089
TITLE: The Influence of Saturation on the Surface Structure of Mixed Fatty Acid-on-Water Aerosol: A Molecular Dynamics Study

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Associate Editor
Environmental Science: Atmospheres
Royal Society of Chemistry

************

Reviewer 1

This manuscript is mainly about a molecular dynamics study on the influence of saturation on the surface structure of mixed fatty acid-on-water aerosol. The authors tried to develop a way to physically mimic how organic layer grow on the water body. They use oleic/stearic acids and water as an example system. They highlight and explain the accessibility differences to ozone radicals between oleic and stearic acids in the aerosol. In general, I think the manuscript do provide some new physical insights in aerosol chemistry and can be accepted after addressing the following concerns.
(1) One of my major concerns is that the authors use “a slab of water that is extended infinitely in two dimensions” rather than using a droplet during their simulations. Since aerosols are not “infinite”, the authors should discuss more and justify whether the result also works for aerosol. I understand that the authors fully discussed the curvature effect in ESI section S2, however, are there more evidences to show that your approximation will not significantly affect the conclusion?
(2) What is the concentration of oleic/stearic acids in the atmosphere? Are there enough oleic/stearic acids in the atmosphere that can fully cover the water core, as shown in Fig. 2?
(3) Since the COOH group is pointed to water body, which does reflect accuracy of the parameter you generated, will oleic/stearic acids dissociate to protons and anions?
(4) More details (e.g. RMSD) of MD simulation should be provided in ESI.

Reviewer 2

Stewart et al. simulated a model aerosol system with C18 fatty acids partitioned into the vacuum-water interface. The interface is simulated as a monolayer containing different ratios of stearic acid and oleic acid. The object is to determine how the fully saturated stearic acid molecules affect the accessibility of alkene groups within oleic acid molecules to ozone radicals. They found that the presence of the ‘inert’ molecules (stearic acid) seemed not affect the accessibility of ozone molecules to the reactive alkene groups of oleic acid. As the processes and chemistry at the air-water interface can be very different from the bulk chemistry, this work is helpful to understand the effects of organic coatings on gas uptake by atmospheric water droplets. The manuscript is well written with solid results presented. I recommend the publication of this study after a major revision can be done. My detailed comments are as below.

Major comments:
1. My major concern is that this work mainly investigated the simulation results at the equilibrium conditions and lacked the kinetic analysis. The oxidation of a monolayer of oleic acid by ozone is a kinetic process including gas-phase diffusion, accommodation into the interface, and diffusion in the monolayer. The characteristic times of each kinetic step are different depending on various aerosol properties, including particle viscosity (Mai et al., 2015; Li and Shiraiwa, 2019). These characteristic times are associated to determining the rate-limiting step and controlling the lifetime of reactive species (King et al., 2020). The authors stated that only the fully equilibrated system was analyzed. Figure 2 analyzed the simulation results at the end of a 20 ns production run while Figure 3 shows the results at the end of a 10 ns production run. Figure 5 showed analysis with the final 6ns of production runs. Why these equilibrium times are different? I recommend add that how the authors determined the equilibration time and how they were certain that the model system had reached the equilibrium.

2. The authors concluded that the presence of the stearic acid would not hinder or increase the availability of reactive alkene groups in the interface, thus supported the experimental results that stearic acid was largely a bystander in the reaction of monolayers of oleic/stearic acid mixtures with ozone (King et al., 2020). The availability of alkene groups is not affected by the inert stearic acid molecules, which does not necessarily mean that the uptake of ozone is not affected. As the ozone uptake is a kinetic process as I explained in the major comment 1, I recommend that the authors add the kinetic analysis at non-equilibrium conditions, similar to Li et al., (2019). The mass accommodation coefficient, which is a key kinetic parameter in aerosol modelling (Shiraiwa and Pöschl, 2021), should be calculated to demonstrate if the increased stearic acid molecules could affect the ozone uptake (Li et al., 2019). From my personal view, as the stearic acid molecules increase, the viscosity of the monolayer might be significantly increased (O'brien et al., 2021), thus the interfacial transport of ozone molecules could be slowed down (Li and Shiraiwa, 2019; Mai et al., 2015), which, however, certainly requires future studies to examine the mechanisms of the interfacial transport associated with a viscous monolayer.

Specific comments:
1. Abstract: I recommend add a sentence at the end of the abstract to show the atmospheric implications of the results found in this study.
2. Page 2 in the main text:’ It is thought, however, that reactive gaseous species such as ozone are often more likely to react at the surface of aerosol samples, rather than penetrating deeply into the bulk’. Weather the surface reactions dominate or the bulk reactions dominate depends largely on the phase state or the viscosity of aerosol particles (Shiraiwa et al., 2010), which should be clearly pointed out.
3. Page 2, the Method section, Please explain “NVT” at its first occurrence.
4. Page 3, near the end of the Methods section: Reference 66 is in preparation and this unpublished work in better not cited.
5. Page 3, ‘the total composition of the across the two surfaces was the same’. Do you mean ‘the total composition of the molecules’?
6. Page 4, Figure 4, the figure in the middle of the third panel should be with the oleic : stearic being 2:8 instead of 8:2.
References
King, M. D., Jones, S. H., Lucas, C. O. M., Thompson, K. C., Rennie, A. R., Ward, A. D., Marks, A. A., Fisher, F. N., Pfrang, C., Hughes, A. V., and Campbell, R. A.: The reaction of oleic acid monolayers with gas-phase ozone at the air water interface: the effect of sub-phase viscosity, and inert secondary components, Physical Chemistry Chemical Physics, 22, 28032-28044, 10.1039/D0CP03934A, 2020.
Li, W., Pak, C. Y., Wang, X., and Tse, Y.-L. S.: Uptake of Common Atmospheric Gases by Organic-Coated Water Droplets, The Journal of Physical Chemistry C, 123, 18924-18931, 10.1021/acs.jpcc.9b03252, 2019.
Li, Y. and Shiraiwa, M.: Timescales of secondary organic aerosols to reach equilibrium at various temperatures and relative humidities, Atmos. Chem. Phys., 19, 5959-5971, 10.5194/acp-19-5959-2019, 2019.
Mai, H., Shiraiwa, M., Flagan, R. C., and Seinfeld, J. H.: Under what conditions can equilibrium gas–particle partitioning be expected to hold in the atmosphere?, Environ. Sci. Technol., 49, 11485-11491, 10.1021/acs.est.5b02587, 2015.
O'Brien, R. E., Li, Y., Kiland, K. J., Katz, E. F., Or, V. W., Legaard, E., Walhout, E. Q., Thrasher, C., Grassian, V. H., DeCarlo, P. F., Bertram, A. K., and Shiraiwa, M.: Emerging investigator series: chemical and physical properties of organic mixtures on indoor surfaces during HOMEChem, Environmental Science: Processes & Impacts, 23, 559-568, 10.1039/D1EM00060H, 2021.
Shiraiwa, M. and Pöschl, U.: Mass accommodation and gas–particle partitioning in secondary organic aerosols: dependence on diffusivity, volatility, particle-phase reactions, and penetration depth, Atmos. Chem. Phys., 21, 1565-1580, 10.5194/acp-21-1565-2021, 2021.
Shiraiwa, M., Pfrang, C., and Pöschl, U.: Kinetic multi-layer model of aerosol surface and bulk chemistry (KM-SUB): the influence of interfacial transport and bulk diffusion on the oxidation of oleic acid by ozone, Atmos. Chem. Phys., 10, 3673-3691, 10.5194/acp-10-3673-2010, 2010.

Comments to the Author
This manuscript is mainly about a molecular dynamics study on the influence of saturation on the surface structure of mixed fatty acid-on-water aerosol. The authors tried to develop a way to physically mimic how organic layer grow on the water body. They use oleic/stearic acids and water as an example system. They highlight and explain the accessibility differences to ozone radicals between oleic and stearic acids in the aerosol. In general, I think the manuscript do provide some new physical insights in aerosol chemistry and can be accepted after addressing the following concerns.
(1) One of my major concerns is that the authors use “a slab of water that is extended infinitely in two dimensions” rather than using a droplet during their simulations. Since aerosols are not “infinite”, the authors should discuss more and justify whether the result also works for aerosol. I understand that the authors fully discussed the curvature effect in ESI section S2, however, are there more evidences to show that your approximation will not significantly affect the conclusion?


While there have been a number of computational studies carried out in which aerosols have been simulated as droplets, it is only possible to simulate the very smallest sizes of aerosol by this method, due to the immense computational costs of simulating most sizes of aerosol particle. Our work focusses on simulating significantly larger aerosols that it would be impossible to simulate as full droplets. These aerosols, as demonstrated in Section S4 (previously S2) of the ESI are large enough that any curvature effect is not noticeable on an atomic scale. This means that the molecules can pack together more closely and will not be splayed out, as on a very small aerosol core. As such the surfaces of larger aerosols more closely resemble a flat slab than a small droplet. There are other studies that have also used the strategy of simulating the surfaces of larger aerosols using infinite slabs,1,2 or that have used self-assembled monolayers as aerosol mimics3,4 and the use of periodic boundary conditions in our simulations build on this idea. We have added references to these works to the Curvature Effects section of the ESI to provide more evidence to support the use of this methodology.

(2) What is the concentration of oleic/stearic acids in the atmosphere? Are there enough oleic/stearic acids in the atmosphere that can fully cover the water core, as shown in Fig. 2?

The authors are by no means suggesting that there are likely to be atmospheric aerosols containing only oleic and/or stearic acid on water core and they have mentioned in the introduction that the actual compositions of aerosols are varied and highly complex. However, there have been noted to be significant amounts of oleic and stearic acid present within atmospheric aerosols5,6 and as such oleic acid in particular has been the subject of many experimental studies as a proxy for the unsaturated component within aerosols. Our study aims to give insights into how typical saturated and unsaturated components may interact within an aerosol, as well as providing a theoretical benchmark to compare to existing experimental work7 (reference 43 of the main paper). Within works that look at aerosol mimics, the organic layer investigated is commonly looked at as a monolayer coverage. The authors intend in future studies to investigate the effects of sub- and super-monolayer coverages, but they note here that aerosols come in a wide range of coverages and compositions and the focus here is the commonly investigated monolayer film.

(3) Since the COOH group is pointed to water body, which does reflect accuracy of the parameter you generated, will oleic/stearic acids dissociate to protons and anions?

The authors note that there is the possibility that the fatty acid molecules could be present alongside their dissociated counterparts and it is likely that there will be some kind of equilibrium formed between the two. The exact proportion of acid and conjugate ions present will depend on a number of factors, including the pH of the droplet, and as such could vary significantly between different aerosols in the atmosphere. However, the authors note that the majority of studies of fatty acid-on-water aerosols have focussed on the acids in their protonated form, and amongst those that have investigated the effects of protonation on aerosol structure, there has not been found to be vast differences in the overall structure of the aerosol mimics, for example in a study by Johann et al.8 of arachidic acid (the C20 analogue of stearic acid) on water it was found that the average tilt angle of the organic molecules only changed by a few degrees on changing the pH between 5.6 and 12.0, a range that is more extreme than would be expected to be found in most atmospheric conditions. In addition, the main conclusions of our study were a result of the overwhelming preference for COOH groups to be directed inwards towards the water core, leaving the non-polar parts of the chains pointing outwards, and as COO- is also likely to interact strongly with water this overall structural feature, and thus the main conclusions of this work, are unlikely to be significantly altered by the protonation or deprotonation of the acid groups.

(4) More details (e.g. RMSD) of MD simulation should be provided in ESI.

The authors have added sections on the RMSD of the simulations and further information about the equilibration procedure to the ESI.


Referee: 2

Comments to the Author
Stewart et al. simulated a model aerosol system with C18 fatty acids partitioned into the vacuum-water interface. The interface is simulated as a monolayer containing different ratios of stearic acid and oleic acid. The object is to determine how the fully saturated stearic acid molecules affect the accessibility of alkene groups within oleic acid molecules to ozone radicals. They found that the presence of the ‘inert’ molecules (stearic acid) seemed not affect the accessibility of ozone molecules to the reactive alkene groups of oleic acid. As the processes and chemistry at the air-water interface can be very different from the bulk chemistry, this work is helpful to understand the effects of organic coatings on gas uptake by atmospheric water droplets. The manuscript is well written with solid results presented. I recommend the publication of this study after a major revision can be done. My detailed comments are as below.

Major comments:
1. My major concern is that this work mainly investigated the simulation results at the equilibrium conditions and lacked the kinetic analysis. The oxidation of a monolayer of oleic acid by ozone is a kinetic process including gas-phase diffusion, accommodation into the interface, and diffusion in the monolayer. The characteristic times of each kinetic step are different depending on various aerosol properties, including particle viscosity (Mai et al., 2015; Li and Shiraiwa, 2019). These characteristic times are associated to determining the rate-limiting step and controlling the lifetime of reactive species (King et al., 2020). The authors stated that only the fully equilibrated system was analyzed. Figure 2 analyzed the simulation results at the end of a 20 ns production run while Figure 3 shows the results at the end of a 10 ns production run. Figure 5 showed analysis with the final 6ns of production runs. Why these equilibrium times are different? I recommend add that how the authors determined the equilibration time and how they were certain that the model system had reached the equilibrium.

The authors apologise for confusion caused by the caption of Fig. 2, this was a typographical error. This image was taken at the end of a 10 ns production run. The caption of the figure has now been changed to reflect this. The reason that Fig. 5 refers to the last 6 ns of production runs is because this analysis has been carried out over all frames in the trajectory at times 4-10 ns, whereas the images in Fig. 2 and 3 are snapshots taken from only one frame. The total length of the equilibration step was selected based on running simulations of different lengths and calculating the partial densities and SASA values of different groups over different segments of the trajectories. At times above 10 ns there was found to be no significant change in either the partial density or fractional SASA values, even on running the simulations for a further 20 ns and it was therefore concluded that 10 ns was a sufficient time for the slab to be fully equilibrated, prior to starting the production runs. Equilibration time analysis was carried out on slabs of different compositions and the results of analysis of one of these have been added to section S2 of the ESI. This equilibration process was carried out to ensure that there were no computational artifacts from the set-up of the slabs that could bias the results.

2. The authors concluded that the presence of the stearic acid would not hinder or increase the availability of reactive alkene groups in the interface, thus supported the experimental results that stearic acid was largely a bystander in the reaction of monolayers of oleic/stearic acid mixtures with ozone (King et al., 2020). The availability of alkene groups is not affected by the inert stearic acid molecules, which does not necessarily mean that the uptake of ozone is not affected. As the ozone uptake is a kinetic process as I explained in the major comment 1, I recommend that the authors add the kinetic analysis at non-equilibrium conditions, similar to Li et al., (2019). The mass accommodation coefficient, which is a key kinetic parameter in aerosol modelling (Shiraiwa and Pöschl, 2021), should be calculated to demonstrate if the increased stearic acid molecules could affect the ozone uptake (Li et al., 2019). From my personal view, as the stearic acid molecules increase, the viscosity of the monolayer might be significantly increased (O'brien et al., 2021), thus the interfacial transport of ozone molecules could be slowed down (Li and Shiraiwa, 2019; Mai et al., 2015), which, however, certainly requires future studies to examine the mechanisms of the interfacial transport associated with a viscous monolayer.

The authors thank the reviewer for this point and have made some clarifications to this in the text (red). It has been suggested by previous studies that the rate of reaction between aerosols and ozone occurs preferentially at the atmosphere-aerosol interface,9,10 and therefore if there were an enhanced or decreased HC=CH presence at the interface this could have led to significant changes in the rate of aerosol aging, and this was the main point that the authors were trying to make in carrying out SASA analysis. However, it is correct that the overall structure of the aerosol monolayer will also affect the rate of oxidation by allowing or hindering the penetration of oxidants into the slab and therefore the tighter packing of the chains could cause an overall decrease in oxidation, even if the surface rate is the same. The authors have added references and modified the discussion and conclusions section of the main text to highlight this point to the readers. A full kinetic analysis is, however, unfortunately beyond the scope of this paper but could be an interesting and important subject of future studies.


Specific comments:
1. Abstract: I recommend add a sentence at the end of the abstract to show the atmospheric implications of the results found in this study.
The authors have added an additional sentence at the end of the abstract highlighting this.

2. Page 2 in the main text:’ It is thought, however, that reactive gaseous species such as ozone are often more likely to react at the surface of aerosol samples, rather than penetrating deeply into the bulk’. Weather the surface reactions dominate or the bulk reactions dominate depends largely on the phase state or the viscosity of aerosol particles (Shiraiwa et al., 2010), which should be clearly pointed out.
The authors believe that this point has been covered in answering major comment 2.

3. Page 2, the Method section, Please explain “NVT” at its first occurrence.
The authors have included a definition of this acronym.

4. Page 3, near the end of the Methods section: Reference 66 is in preparation and this unpublished work in better not cited.
The authors have removed this citation from the paper.

5. Page 3, ‘the total composition of the across the two surfaces was the same’. Do you mean ‘the total composition of the molecules’?
The authors apologise for this typographical error and have corrected it.

6. Page 4, Figure 4, the figure in the middle of the third panel should be with the oleic : stearic being 2:8 instead of 8:2.
The authors apologise for this error and have corrected it.

References
1. O. Ozgurel, D. Duflot, M. Masella, F. Réal, and C. Toubin, A molecular scale investigation of organic/inorganic ion selectivity at the air−liquid interface, ACS Earth and Space Chemistry, 2022, 6, 7, 1698-1716.

2. S. Takahama and L. M. Russell, A Molecular Dynamics Study of Water Mass Accommodation on Condensed Phase Water Coated by Fatty Acid Monolayers, J.Geophys.Res., 2011, 116, D02203(1-14).


3. Y. Dubowski, J. Vieceli, D. J. Tobias, A. Gomez, A. Lin, S. A. Nizkorodov, T. M. McIntire, and B. J. Finlayson-Pitts, Interaction of Gas-Phase Ozone at 296 K with Unsaturated Self-Assembled Monolayers: A New Look at an Old System, J. Phys. Chem. A 2004, 108, 10473-10485.

4. J. Park, A. L. Gomez, M. L. Walser, A. Lin and S. A. Nizkorodov, Ozonolysis and Photolysis of Alkene-Terminated Self-Assembled Monolayers on Quartz Nanoparticles: Implications for Photochemical Aging of Organic Aerosol Particles, Phys. Chem. Chem. Phys., 2006, 2506–2512.

5. M. King, A. Rennie, K. Thompson, F. Fisher, C. Dong, R. Thomas, C. Pfrang and A. Hughes, Oxidation of Oleic Acid at the Air–Water Interface and its Potential Effects on Cloud Critical Supersaturation, Phys. Chem. Chem. Phys., 2009, 11, 7699-7707.

6. W. F. Rogge, L. M. Hildemann, M. A. Mazurek, G. R. Cass, B. R. T. Slmoneit, Sources of Fine Organic Aerosol. 1. Charbroilers and Meat Cooking Operations, Environmental Science and Technology, 1991, 25, 1112-1125.

7. M. King, S. H. Jones, C. O. M. Lucas, K. C. Thompson, A. R. Rennie, A. D. Ward, A. A. Marks, F. N. Fisher, C. Pfrang, A. V. Hughes and R. A. Campbell, The Reaction of Oleic Acid Monolayers with Gas-Phase Ozone at the Air Water Interface: the Effect of Sub-Phase Viscosity, and Inert Secondary Components.

8. R. Johann, D. Vollhardt, H. Möhwald, Study of the pH Dependence of Head Group Bonding in Arachidic Acid Monolayers by Polarization Modulation Infrared Reflection Absorption Spectroscopy, Colloids and Surfaces, 2000, 182, 311-320.

9. J. D. Hearn, A. J. Lovett, G. D. Smith, Ozonolysis of Oleic Acid Particles: Evidence for a Surface Reaction and Secondary Reactions Involving Criegee Intermediates, Phys. Chem. Chem. Phys., 2005, 7, 501–511.

10. Y. Katrib, G. Biskos, P. R. Buseck, P. Davidovits, J. T. Jayne, M. Mochida, M. E. Wise, D. R. Worsnop, S. T. Martin, Ozonolysis of Mixed Oleic-Acid/Stearic-Acid Particles: Reaction Kinetics and Chemical Morphology, J. Phys. Chem. A., 2005, 109, 10910–10919.

10-Oct-2022

Dear Dr Greaves:

Manuscript ID: EA-ART-07-2022-000089.R1
TITLE: The Influence of Saturation on the Surface Structure of Mixed Fatty Acid-on-Water Aerosol: A Molecular Dynamics Study

Thank you for submitting your revised manuscript to Environmental Science: Atmospheres. I am pleased to accept your manuscript for publication in its current form. I have copied any final comments from the reviewer(s) below.

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Environmental Science: Atmospheres
Royal Society of Chemistry

Reviewer 1

Accept as is.

Reviewer 2

I appreciate that the authors have considered and properly addressed my comments. I recommend the publication of the revised version of this study.