Measurement Report: Water diffusion in single suspended phase-separated aerosols

Tong, Yu-Kai; Wu, Zhijun; Hu, Min; Ye, Anpei

doi:https://doi.org/10.5194/egusphere-2023-1346

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https://doi.org/10.5194/egusphere-2023-1346

Preprints

12 Jul 2023

| 12 Jul 2023

Measurement Report: Water diffusion in single suspended phase-separated aerosols

Yu-Kai Tong, Zhijun Wu, Min Hu, and Anpei Ye

Abstract. Water diffusion is a typical thermodynamic process in ambient aerosols which plays pivotal roles in their physicochemical properties, atmospheric lifetime, and influences on the climate and human health. A fair amount of aerosols become phase-separated after experiencing atmospheric aging processes such as efflorescence, amorphization, and liquid-liquid phase separation. However, detecting the hygroscopicity of heterogeneous aerosols is quite intractable. Here, for the first time, we directly characterize the water diffusion in single suspended phase-separated aerosols via a self-constructed laser tweezers Raman spectroscopy (LTRS) system. The H₂O/D₂O isotope exchange is harnessed to trace the water diffusion in single laser-levitated homogenous/heterogeneous microdroplets. The time-resolved cavity-enhanced Raman spectra of the microdroplets is used to detect the diffusion process in real time. Two archetypes of phase-separated aerosols, i.e., partially engulfed and core-shell, are studied. Moreover, we quantify the dynamic water diffusion process by experimentally measuring the diffusion coefficients. The results show that compared with the homogenous aerosols, water diffusion limitations exist in the phase-separated aerosols. The incomplete diffusion may stem from both the hydrophobicity of the organics and the formation of certain molecule clusters. This work provides possible implications on the evolutions, especially the gas-particle partition, of the actual phase-separated atmospheric aerosols.

Received: 20 Jun 2023 – Discussion started: 12 Jul 2023

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07 Mar 2024

Measurement report: Water diffusion in single suspended phase-separated aerosols

Yu-Kai Tong, Zhijun Wu, Min Hu, and Anpei Ye

Atmos. Chem. Phys., 24, 2937–2950, https://doi.org/10.5194/acp-24-2937-2024,https://doi.org/10.5194/acp-24-2937-2024, 2024

Short summary

Yu-Kai Tong, Zhijun Wu, Min Hu, and Anpei Ye

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2023-1346', Anonymous Referee #1, 23 Jul 2023

Nice way to investigate mass transport limitations in non-homogeneous particles. However, there are problems with the presentation and analysis:
1. Can you show H2O/D2O exchange for just aqueous AS (or LiCl if efflorescence is an issue)? This would give some idea of the response time in your cell when switching between H2O and D2O. For instance, the characteristic time reported in Fig. 3 is probably just the time required for the cell to switch between H2O and D2O.
2. Throughout the paper you have this issue with \phi_{OD} not going to 0 or 1 as the experiment time becomes very long. This leads to some fairly implausible proposals (in my opinion) towards the end of the discussion. Isn't it simply the well-known issue that strong WGMs heavily distort vibrational bands, making their use in quantitative work difficult? The classic example is Anal. Chem. 2005, 77, 7148-7155 where the authors, who had previously been using an EDB, switched to glass slides because of this very issue (they state this in the introduction to that paper).
Although I suppose the argument given on lines 205-207 that there is condensation of the species being replaced somewhere on the walls is also possible.
3. Can you list the fitted droplet size where applicable, e.g. in Fig. 3.
4. Why are the WGMs shifting in Fig. 3. Isn't the droplet in equilibrium with the surroundings? Is some other parameter changing?
5. What are the uncertainties associated with the measured \tau reported in Table 1? Were any of these measurements repeated more than once?
Awkward writing:
"surplus protons are considered to have appreciable impacts..." This is a weird way of saying low pH.
"panoramically presents a panorama"
"equilibrated in D2O in ambiance"

Citation: https://doi.org/10.5194/egusphere-2023-1346-RC1
- AC1: 'Reply on RC1', Yu-Kai Tong, 23 Aug 2023
  
  The point-to-point responses can be seen in the attachment.
  
  Citation: https://doi.org/10.5194/egusphere-2023-1346-AC1
RC2:
'Comment on egusphere-2023-1346', Anonymous Referee #2, 28 Jul 2023
This manuscript reports measurements of water diffusion in homogeneous and phase-separated particles using an isotope exchange method in conjunction with Raman spectroscopy in an aerosol optical tweezers (AOT). This method was first demonstrated in an AOT by Davies and Wilson (https://doi.org/10.1021/acs.analchem.5b04315) and subsequently in an electrodynamic balance (EDB) by Nadler et al. (https://doi.org/10.1039/C8CP07052K). In the current work, the authors describe the application of the technique towards studying water diffusion in phase separated particles - ammonium sulfate solution droplets partially engulfed by oleic acid, and core shell ammonium sulfate droplets coated with diethyl-L-tartrate and 1,2,6-hexanetriol. The results indicate both forms of phase separation lead to slow water diffusion, with core-shell particles exhibiting the slowest rate of water transport. This work is interesting and there are clear motivations towards exploring the physical chemistry of phase separated particles. The figures are well made and clarify key points of discussion from the text. However, in its current form, I do not think the manuscript conclusively demonstrates water diffusion limitations are responsible for the slow exchange. My comments below include several suggestions for additional commentary, clarifications, and possible measurements that would strengthen the arguments made in the manuscript.
The figures showing the isotope exchange (Figure 3 in particular) indicate the WGM’s move significantly during the process. Is this due to a change in the RH when switching between H2O and D2O? The SI indicates some correction to account for this, but it is unclear how effective this method is given the observations.

The WGM’s in the spectra are very broad, up to 1 nm in width. These features on particles of the size reported should be narrow, <<1 nm. Typically, in AOT systems, spectrometer resolution of <0.1nm are used to resolve 1st order modes. Can the authors confirm the grating and the resolution of their spectrometer?

The spectrum for a core-shell particle will depend on the thickness of the shell, and whether the WGM’s penetrate deeply enough to interfere with the internal interface. A core-shell particle retains spherical symmetry, so while the position of the WGM’s in the spectrum may differ from a homogeneous particle, the peaks generally should appear well resolved. A deviation for spherical symmetry would lead to the spectrum degrading, which may occur if the shell thickness is not uniform.

Were diffusion coefficients measured at lower RH for a well-characterized system, such as CA, to validate the accuracy against previous data?

What is the chamber response time? How much does this vary due to the nebulization process depositing more material into the chamber? Was the chamber cleaned out between measurements?

How was the oleic acid mixture nebulized, given the insoluble nature of oleic acid? How much OA was in the levitated particles? Previously, OA has been introduced as a vapor into AOT’s and allowed to condensed on existing particles (such as NaCl, in the work of Dennis-Smither et al. (https://doi.org/10.1029/2012JD018163)).

Were measurements performed for water diffusion in 1,2,6-hexanetriol alone, under the same RH conditions? Based on its hygroscopicity and fluidity, it seems unlikely major diffusion limitations would be observed for water in this system. Does the presence of ammonium sulfate in particles in the present work lead to some kind of change in the rheology that leads to more diffusion limitations?

How much oleic acid is present in the particles? From the SI figure, the coating is small compared to the size of the droplet and the reduced surface area of the aqueous phase relative to a pure aqueous droplet would be small.

Table 1 indicates the addition of acid leads to homogeneous morphology, but the rate of diffusion is still much slower than for a homogeneous system. Further the influence of lowered pH in this system would lead to protonated organic molecules that would be less polar, rather than more polar, suggesting miscibility should decrease rather than increase. Perhaps the pH influences the proton exchanges rates on the alcohol groups, which affects the timescales?

For the core-shell fitting, can the authors provide simulated spectra to compare with the measured core-shell spectra? More details on the use of this and the outputs would be useful.
Citation: https://doi.org/10.5194/egusphere-2023-1346-RC2
- AC2: 'Reply on RC2', Yu-Kai Tong, 23 Aug 2023
  
  The point-to-point response can be seen in the attachment.
  
  Citation: https://doi.org/10.5194/egusphere-2023-1346-AC2

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2023-1346', Anonymous Referee #1, 23 Jul 2023

Nice way to investigate mass transport limitations in non-homogeneous particles. However, there are problems with the presentation and analysis:
1. Can you show H2O/D2O exchange for just aqueous AS (or LiCl if efflorescence is an issue)? This would give some idea of the response time in your cell when switching between H2O and D2O. For instance, the characteristic time reported in Fig. 3 is probably just the time required for the cell to switch between H2O and D2O.
2. Throughout the paper you have this issue with \phi_{OD} not going to 0 or 1 as the experiment time becomes very long. This leads to some fairly implausible proposals (in my opinion) towards the end of the discussion. Isn't it simply the well-known issue that strong WGMs heavily distort vibrational bands, making their use in quantitative work difficult? The classic example is Anal. Chem. 2005, 77, 7148-7155 where the authors, who had previously been using an EDB, switched to glass slides because of this very issue (they state this in the introduction to that paper).
Although I suppose the argument given on lines 205-207 that there is condensation of the species being replaced somewhere on the walls is also possible.
3. Can you list the fitted droplet size where applicable, e.g. in Fig. 3.
4. Why are the WGMs shifting in Fig. 3. Isn't the droplet in equilibrium with the surroundings? Is some other parameter changing?
5. What are the uncertainties associated with the measured \tau reported in Table 1? Were any of these measurements repeated more than once?
Awkward writing:
"surplus protons are considered to have appreciable impacts..." This is a weird way of saying low pH.
"panoramically presents a panorama"
"equilibrated in D2O in ambiance"

Citation: https://doi.org/10.5194/egusphere-2023-1346-RC1
- AC1: 'Reply on RC1', Yu-Kai Tong, 23 Aug 2023
  
  The point-to-point responses can be seen in the attachment.
  
  Citation: https://doi.org/10.5194/egusphere-2023-1346-AC1
RC2:
'Comment on egusphere-2023-1346', Anonymous Referee #2, 28 Jul 2023
This manuscript reports measurements of water diffusion in homogeneous and phase-separated particles using an isotope exchange method in conjunction with Raman spectroscopy in an aerosol optical tweezers (AOT). This method was first demonstrated in an AOT by Davies and Wilson (https://doi.org/10.1021/acs.analchem.5b04315) and subsequently in an electrodynamic balance (EDB) by Nadler et al. (https://doi.org/10.1039/C8CP07052K). In the current work, the authors describe the application of the technique towards studying water diffusion in phase separated particles - ammonium sulfate solution droplets partially engulfed by oleic acid, and core shell ammonium sulfate droplets coated with diethyl-L-tartrate and 1,2,6-hexanetriol. The results indicate both forms of phase separation lead to slow water diffusion, with core-shell particles exhibiting the slowest rate of water transport. This work is interesting and there are clear motivations towards exploring the physical chemistry of phase separated particles. The figures are well made and clarify key points of discussion from the text. However, in its current form, I do not think the manuscript conclusively demonstrates water diffusion limitations are responsible for the slow exchange. My comments below include several suggestions for additional commentary, clarifications, and possible measurements that would strengthen the arguments made in the manuscript.
The figures showing the isotope exchange (Figure 3 in particular) indicate the WGM’s move significantly during the process. Is this due to a change in the RH when switching between H2O and D2O? The SI indicates some correction to account for this, but it is unclear how effective this method is given the observations.

The WGM’s in the spectra are very broad, up to 1 nm in width. These features on particles of the size reported should be narrow, <<1 nm. Typically, in AOT systems, spectrometer resolution of <0.1nm are used to resolve 1st order modes. Can the authors confirm the grating and the resolution of their spectrometer?

The spectrum for a core-shell particle will depend on the thickness of the shell, and whether the WGM’s penetrate deeply enough to interfere with the internal interface. A core-shell particle retains spherical symmetry, so while the position of the WGM’s in the spectrum may differ from a homogeneous particle, the peaks generally should appear well resolved. A deviation for spherical symmetry would lead to the spectrum degrading, which may occur if the shell thickness is not uniform.

Were diffusion coefficients measured at lower RH for a well-characterized system, such as CA, to validate the accuracy against previous data?

What is the chamber response time? How much does this vary due to the nebulization process depositing more material into the chamber? Was the chamber cleaned out between measurements?

How was the oleic acid mixture nebulized, given the insoluble nature of oleic acid? How much OA was in the levitated particles? Previously, OA has been introduced as a vapor into AOT’s and allowed to condensed on existing particles (such as NaCl, in the work of Dennis-Smither et al. (https://doi.org/10.1029/2012JD018163)).

Were measurements performed for water diffusion in 1,2,6-hexanetriol alone, under the same RH conditions? Based on its hygroscopicity and fluidity, it seems unlikely major diffusion limitations would be observed for water in this system. Does the presence of ammonium sulfate in particles in the present work lead to some kind of change in the rheology that leads to more diffusion limitations?

How much oleic acid is present in the particles? From the SI figure, the coating is small compared to the size of the droplet and the reduced surface area of the aqueous phase relative to a pure aqueous droplet would be small.

Table 1 indicates the addition of acid leads to homogeneous morphology, but the rate of diffusion is still much slower than for a homogeneous system. Further the influence of lowered pH in this system would lead to protonated organic molecules that would be less polar, rather than more polar, suggesting miscibility should decrease rather than increase. Perhaps the pH influences the proton exchanges rates on the alcohol groups, which affects the timescales?

For the core-shell fitting, can the authors provide simulated spectra to compare with the measured core-shell spectra? More details on the use of this and the outputs would be useful.
Citation: https://doi.org/10.5194/egusphere-2023-1346-RC2
- AC2: 'Reply on RC2', Yu-Kai Tong, 23 Aug 2023
  
  The point-to-point response can be seen in the attachment.
  
  Citation: https://doi.org/10.5194/egusphere-2023-1346-AC2

Peer review completion

AR: Author's response | RR: Referee report | ED: Editor decision | EF: Editorial file upload

AR by Yu-Kai Tong on behalf of the Authors (25 Aug 2023) Author's response Author's tracked changes Manuscript

ED: Reconsider after major revisions (02 Sep 2023) by Thomas Berkemeier

Dear Dr. Tong and Co-Authors,

Thank you for submitting your revised manuscript to ACP. We appreciate the effort you have made to address the comments of our peer reviewers.

After a thorough evaluation of the revisions, it has been determined that the manuscript will require further major revisions before it can be accepted for publication in ACP. While we recognize that some changes have been made, they do not fully address the comments and concerns raised by the reviewers in the first round of review.

---

The main issues are as follows:

- The revision does not yet convincingly address and incorporate the reviewers' questions and suggestions into the main text. Please adjust your responses to the following reviewer comments to be reflected in the revised manuscript: Reviewer 1's comments #2 and #3, Reviewer 2's comments #2, #4, #6, #8, and #9. In your point-by-point response, indicate clearly how you addressed the reviewer comments.

- Both reviewers commented on the shifting of whispering-gallery modes (WGMs). The response is rather brief and not sufficiently substantiated by experimental evidence. Please expand your response to this important point brought up by the reviewers.

- Please thoroughly discuss the reasons and implications of \phi_{OD} not going to 0 or 1 (Reviewer 1, comment #2).

- The discussions in the manuscript could use better referencing. It is sometimes not clearly indicated what is a known fact (provide reference to literature), an observation (provide reference to data), or speculation (indicate in your wording).

---

Please also address the following more specific comments:

- Consider adding the H2O + CA data (new in supplement) to main manuscript.

- Response to reviewer 2, comment #3: Please discuss in more detail how this may affect the conclusions of the manuscript.

- l. 249 of revised manuscript: "the added sulfuric acid increased droplet viscosity, the plasticizing effect of which may limit local concentration fluctuations and promote homogeneity" - Please provide proper reference or indicate speculation. Also clarify which molecule's plasticizing effect is referred to here.

- l. 250 of revised manuscript: "Notably, what the characteristic diffusion time directly mirrors is diffusion rate of molecules in the size of water molecule into such droplet." - The meaning of the sentence is unclear, please revise.

- l. 252 of revised manuscript: "However, the equilibrium water vapor pressure at the surface of acidified homogenous droplet may be different from that of neutral homogenous droplet at the same RH level." - This is unclear. Please provide reference or more detailed explanation for this statement. Are you suggesting that the relationship of water activity and vapor pressure may differ?

---

It is crucial that you consult the reviewer comments in detail and make more comprehensive adjustments to your manuscript. Please understand that failing to make these major revisions will likely result in the rejection of your manuscript. The revised manuscript will undergo another round of peer review to assess whether the issues have been adequately resolved.

Thank you for considering ACP as the platform for your research. We hope that you will find the reviewer feedback constructive and that it will assist you in enhancing the quality of your work.

Sincerely,

Thomas Berkemeier
Editor

Hide

AR by Yu-Kai Tong on behalf of the Authors (23 Nov 2023) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (23 Nov 2023) by Thomas Berkemeier

RR by Anonymous Referee #2 (27 Nov 2023)

ED: Publish subject to minor revisions (review by editor) (28 Nov 2023) by Thomas Berkemeier

AR by Yu-Kai Tong on behalf of the Authors (29 Nov 2023) Author's response Author's tracked changes Manuscript

ED: Publish as is (25 Dec 2023) by Thomas Berkemeier

AR by Yu-Kai Tong on behalf of the Authors (27 Dec 2023) Manuscript

Journal article(s) based on this preprint

07 Mar 2024

Measurement report: Water diffusion in single suspended phase-separated aerosols

Yu-Kai Tong, Zhijun Wu, Min Hu, and Anpei Ye

Atmos. Chem. Phys., 24, 2937–2950, https://doi.org/10.5194/acp-24-2937-2024,https://doi.org/10.5194/acp-24-2937-2024, 2024

Short summary

Yu-Kai Tong, Zhijun Wu, Min Hu, and Anpei Ye

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https://doi.org/10.5194/egusphere-2023-1346-supplement

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Dataset for “Measurement Report: Water diffusion in single suspended phase-separated aerosols” Yu-Kai Tong, Zhijun Wu, Min Hu, and Anpei Ye https://doi.org/10.18170/DVN/LJMWYV

Yu-Kai Tong, Zhijun Wu, Min Hu, and Anpei Ye

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The interplay between aerosols and moisture is one of the most crucial atmospheric processes. However, till date, the literature results on the influence of phase separation on water diffusion in aerosols are divergent. This work directly unveiled the water diffusion process in single suspended phase-separated microdroplets and quantitatively analyzed the diffusion rate and extent. The results show that diffusion limitations and certain molecule clusters exist in the phase-separated aerosols.

Measurement Report: Water diffusion in single suspended phase-separated aerosols

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