From the journal Environmental Science: Atmospheres Peer review history

19-Apr-2022

Dear Dr McPherson Donahue:

Manuscript ID: EA-ART-03-2022-000017
TITLE: Single particle measurements of mixing between mimics for biomass burning and aged secondary organic aerosols

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

Reviewer 1

Review of “Single particle measurements of mixing between mimics for biomass burning and aged secondary organic aerosols” by Habib and Donahue.

The authors present measurements of the mixing times of particles containing black carbon coated with erythritol and ammonium sulfate particles mixed with non-volatile sugars. The black carbon particles coated with erythritol were used as a proxy for fresh biomass burning aerosol and the ammonium sulfate particles mixed with non-volatile sugars were used as proxies of highly aged aerosol in the atmosphere. The work was carried out to determine if these two types of aerosol populations will mix in the atmosphere, with important implications for predicting the physical and chemical properties of atmospheric particles.

These studies are at the forefront of this research field and are novel. This research is especially important considering the abundance and impact of biomass burning aerosol in the atmosphere. This work also builds nicely on previous work by Donahue, a leader in this area of research. Nevertheless, I have several comments that should be addressed before publication.

Major Comments:

1) Abstract. The authors state the following: “Here we describe experiments on carefully prepared particle populations representing glassy aged organic particles and fresh biomass burning particles to develop a model phase space for organic aerosol systems and better understand when particle glassiness impedes mixing.” However, the authors did not provide evidence that the particles they studies were in a glass state. I suggest the authors change “glassiness” to “highly viscous” throughout the paper unless they have evidence indicating that the mixtures they studied were in a glass state. As pointed out by the authors, the non-volatile sugars were mixed with ammonium sulfate, which can lower the glass transition temperature by large amounts. Some discussion on how much ammonium sulfate may lower the glass transition temperature would be useful (Zobrist et al. 2008, Song et al. 2021, Lilek and Zuend 2022).

2) Throughout the document, the authors used erythritol to represent fresh biomass burning particles. Please add some discussion on why this system was chosen and discuss if this is a realistic proxy for fresh biomass burning particles. Erythritol has a low viscosity and low glass transition temperature, while the viscosity and glass transition temperature of biomass burning particles may be higher (DeRieux et al. 2018). This is relevant since erythritol likely lowered the viscosity of the non-volatile sugars mixed with ammonium sulfate by acting as a plasticizer, as pointed out by the authors.

3) Please add more information on viscosities and glass transition temperatures of the organics studied (erythritol and the non-volatile sugars). If there is information on how much the presence of ammonium sulfate (or other inorganic salts0 will lower these viscosities and glass transition temperatures, please add it to the document (Zobrist et al. 2008, Song et al. 2021, Lilek and Zuend 2022).

4) At the end of the Introduction, the authors state “this enables us to observe conditions at which a glass transition occurs, which can give us an indirect estimate of diffusivity at certain conditions.” Please modify this sentence, since I don’t think the authors observed conditions at which a glass transition occurs.

5) What were the sizes of the particles in this study? Were the particles monodispersed or polydispersed? What was the thickness of the coatings? This information is needed to relate the current results to atmospheric conditions.

6) Figure 4. The authors state that erythritol does not absorb on ammonium sulfate (no green appears in the panel on the right), but there is a green peak around 50 minutes, corresponding to roughly 20% of the ammonium sulfate mass. Please address this point in the revised manuscript. Does this change any of the conclusions in the paper?

7) On page 6, the authors discuss possible explanations for why erythritol mixes with glucose-containing particles. In addition to the arguments presented by the authors, I think it would help to point out that erythritol is a smaller molecule than glucose (as well as sucrose and raffinose). Hence erythritol can “percolate” through the glucose-ammonium sulfate mixture. In this case, diffusion will be faster than expected based on the Stokes-Einstein equation (Price et al. 2014, Marshall et al. 2016, Evoy et al. 2020).

8) The authors state “if the model systems represent real-world wildfire smoke…”. The authors have used erythritol as a proxy for fresh biomass burning aerosol. However, erythritol has a low viscosity, low glass transition temperature, and small molecular radius, which all favor fast mixing. The authors should point this out and make it clear that biomass burning aerosol may have a higher viscosity, higher glass transition temperature, and larger molecular radius, which may result in longer mixing times.

Minor comments:

9) There are several mistakes in the reference list.

10) Experimental. RH was either very low (2-5%) or high (approximately 90%). Is this before or after adding the particles to the chamber? Also, does the RH correspond to room temperature or the temperature within the chamber?

11) Page 2. There appears to be a typo when discussing the composition of the particles studied.

12) Were the SP mode and the tungsten vaporization done separately or at the same time.

13) Was a significant amount of the erythritol material externally mixed with black carbon, or was it all associated with black carbon particles?

14) Please add more information about the components in the setup used – brands, specifications, etc. For example, please add details on the brands and specifications of the AMS, SMPS, atomizers, and diffusion dryers. Also, please add information on where the chemicals were bought and their purity.

15) Page 5 and Figure Caption 5. The authors mention cross-talk between sucrose and glucose. Please be more specific on what this means. I assume the authors mean that the mass spectrometric technique could not completely distinguish between glucose and sucrose. Does this have any implications for the conclusions in this paper?


References:

DeRieux, W.-S. W., Y. Li, P. Lin, J. Laskin, A. Laskin, A. K. Bertram, S. A. Nizkorodov and M. Shiraiwa (2018). "Predicting the glass transition temperature and viscosity of secondary organic material using molecular composition." Atmospheric Chemistry and Physics 18(9): 6331-6351.
Evoy, E., S. Kamal, G. N. Patey, S. T. Martin and A. K. Bertram (2020). "Unified Description of Diffusion Coefficients from Small to Large Molecules in Organic–Water Mixtures." The Journal of Physical Chemistry A 124(11): 2301-2308.
Lilek, J. and A. Zuend (2022). "A predictive viscosity model for aqueous electrolytes and mixed organic–inorganic aerosol phases." Atmospheric Chemistry and Physics 22(5): 3203-3233.
Marshall, F. H., R. E. H. Miles, Y. C. Song, P. B. Ohm, R. M. Power, J. P. Reid and C. S. Dutcher (2016). "Diffusion and reactivity in ultraviscous aerosol and the correlation with particle viscosity." Chem Sci 7(2): 1298-1308.
Price, H. C., B. J. Murray, J. Mattsson, amp, apos, D. Sullivan, T. W. Wilson, K. J. Baustian and L. G. Benning (2014). "Quantifying water diffusion in high-viscosity and glassy aqueous solutions using a Raman isotope tracer method." Atmospheric Chemistry and Physics 14(8): 3817-3830.
Song, Y. C., J. Lilek, J. B. Lee, M. N. Chan, Z. J. Wu, A. Zuend and M. J. Song (2021). "Viscosity and phase state of aerosol particles consisting of sucrose mixed with inorganic salts." Atmospheric Chemistry and Physics 21(13): 10215-10228.
Zobrist, B., C. Marcolli, D. A. Pedernera and T. Koop (2008). "Do atmospheric aerosols form glasses?" Atmospheric Chemistry and Physics 8(17): 5221-5244.

Reviewer 2

General comments:
The topic of this article is of general interest to the atmospheric (aerosol) chemistry community. The understanding of diffusive mixing within potentially viscous particles as well as that of mixing within distinct particle populations is a current topic of interest for various reasons pointed out in the introduction of the article, including that of the timescale for mixing compared to typical assumptions made in atmospheric chemistry models.
• The experimental design of this study is interesting and has provided the authors with a set of measurements that help to interpret the issue of mixing of semivolatile organics in/on aerosols via gas phase diffusion. I find the selection of compounds and distinct aerosols cores, the application of the event trigger mode with an AMS, and the data analysis to be sound and novel.
Figs. 2–4 provide evidence that the method is at least qualitatively able to distinguish between different particle cores and between simple adsorption to particles vs absorption into particle coatings.

• My main comments concern some of the interpretation of the results and the qualitative vs quantitative aspects of the detection of SVOC diffusion to and mixing with other particles, as addressed in the following. In part this comes back to the definition of what constitutes substantial mixing.
• In the Conclusions section it is stated: “we never observed substantial limitations to mixing”. One could argue that this strongly depends on the applied definition of mixing and the timescale of interest. If many hours are of interest, that may be true. But in that case one could also argue that Bones et al. (2012, PNAS) did not find substantial limitations on the > 3 hour timescale for water equilibration into glassy particles; however, those authors consider it a substantial limitation to mixing.
I think the authors agree that a timescale of > 60 min is far from quasi-instantaneous. However, in atmospheric chemistry models, the time steps and assumption of instantaneous mixing are usually on a much shorter sub-1-hour timescale (probably sub-10-min), so there would be implications of assuming quasi-equilibrium internal mixtures of aerosol populations in such models. I recommend to account for this question of definition in the phrasing of such statements and to put such conclusions into context of the timescales of relevance.
• Also in the Conclusions section, it is stated: “These results demonstrate the importance of probing particle behavior directly, wherever possible, rather than relying on proxy properties, such as viscosity or an estimated diffusivity”. I agree that direct measurements are great and certainly helpful, but the experiments did not measure diffusivity nor viscosity and as such cannot be used to directly disproof those particle properties or conclusions derived from particle viscosity measurements on diffusive mixing timescales. The results from this study are only semi-quantitative and may not fully rule out substantial limitations to complete mixing. If the authors disagree with this view, I would appreciate a more detailed discussion of the quantitative vs qualitative nature of mixing behavior and equilibration measured, e.g. more detailed, quantified particle compositions over the course of an experiment. I think the authors agree that what leads to complete mixing (no concentration gradients within a particle layer/phase) will depend on diffusion fluxes and hence molecular diffusivity. Instead, these experiments may provide evidence for important pathways of (partial) mixing, e.g. via absorption into the near-surface layers of a particle, lowering its viscosity and enhancing its absorptivity. Certainly nice results to ponder.
Did the authors consider comparison with experiments on significantly larger particles, which, collectively with the data for the smaller particles, may provide data to assess the mixing depth as a function of time?Please consider revising the wording of the Conclusions section.

• If a particle (or at least its sugar-coating) is truly in a glassy state, absorption into said glassy phase would be expected to be severely suppressed. Therefore, full mixing throughout the entire organic (initially glassy) sugar phase would be expected to be rather slow – on the order of many hours to days or longer. The authors do seem to agree with that assumption, which motivated their experiments. However, the authors did not determine the viscosity of the phases probed in these experiments, so it is not clear whether they were glassy or perhaps of substantially lower viscosity. Clearly, if they were glassy *and remain glassy* under low RH conditions, one would expect different outcomes from measurements at higher RH of ~90% compared to dry conditions, but the results shown in Figs. 7 and 8 do seem to indicate that there is no difference despite a rather large RH difference. This is unexpected given the hygroscopic nature of the sugar compounds involved. Perhaps it is indicative of the semi-quantitative nature of these measurements. For example, the erythritol signal in Fig. 8 (right panel) is somewhat lower than in Fig. 7. What explains that? Is it simply the uncertainty of the measurements, or competition for erythritol by other particles with larger condensed phase absorptive mass in the high-RH case? A brief discussion of this would be of interest.
• It would be useful if the authors could also discuss the expected minimum mixing timescale when considering the gas phase diffusion limitation of erythritol while assuming liquid-like absorptive particles. Some related points are mentioned in the following.
• Page 7: “However, there are no striking differences between the low RH and high RH experiments. This suggests that there truly are no barriers to mixing at any relative humidity at the temperatures we studied for the erythritol – glucose system.” Consider that this conclusion is only qualitatively correct, in part because the measurements do not seem to allow determining whether the resulting particles were well mixed internally – except for the distinct cores – or whether only a relatively thin layer of absorbed erythritol mixed with some glucose and water was present in the initially SVOC-free particles. Please discuss.

• The above comment seems to apply in general to the interpretation of these experiments also with other sugars. While I do not doubt the measurements, I have questions about their interpretation. Namely, whether the particles were sufficiently well mixed in the “sugar” shell phase, including the SVOC, during the experiment time of a few hours, such that we could call this a nearly equilibrated aerosol population. For the low-RH experiments: would it not be possible that the SVOC first adsorbed at the surface of the sugar phase, then relatively fast diffusion into the first few nm of the non-volatile sugar phase took place, which may have softened that layer to a viscosity much below that of a glassy state and allowed for accelerated absorptive partitioning of SVOC (and some water) into that phase. This could then allow that layer to grow via absorptive partitioning and lead to a notable signal in the mass spectra. This despite the SVOC not diffusing through the full thickness of the sugar coating. In such a case, what is measured would be a partially mixed state, not considered to be equilibrated. This would still be an interesting insight and result from this work, but statements like “no barriers to mixing” may need to be toned down, since those types of experiments may not be able to detect the true level of internal mixing.
To some extent, this comes down to how we define “mixing” within a single particle and at what point we would say it is sufficiently well mixed, e.g. via an e-folding equilibration time.

Specific comments:
• Abstract text: why put glassy in quotes? Glassy material is a phase state defined as an amorphous solid (that has undergone a glass transition), so no need for quotes here. Please consider revising similar “glassy” instances throughout the introduction section.
• Abstract text: particle “glassiness” – again, either a particle is in a glassy state or not; to this reviewer’s knowledge there is no spectrum of glassiness, perhaps the authors mean viscosity here.
• Page 2, third paragraph: check the units, there seems to be one minus sign too many.
• Page 2, top right: should 6 g of Aquadag be 0.6 g?
• Page 4 end of second paragraph on right: I appreciate the discussion and consideration of adsorption of erythritol as a potentially confounding effect. Your tests show that it does not seem to absorb into dry ammonium sulfate (AS) particles. What the test does not fully show is whether erythritol may do multilayer adsorption on the non-volatile sugar phases (rather than on AS). Have you considered that option? Given that erythritol is an SVOC under the given conditions, with its melting point at far higher temperatures, multilayer adsorption may be possible; although I agree that it would be difficult to explain why it did not also adsorb on AS surfaces.
• Fig. 5 and associated text on page 5: please explain what is meant by cross talk in a bit more detail.
• Page 5: “Tg is often used as a proxy for diffusivity” – this is a convoluted statement; Tg is a proxy for viscosity, not directly diffusivity. I suggest rephrasing.
• Fig. 6, right panel: why is the erythritol signal decreasing with time after its peak near 60 min? Is it loss to the chamber walls? If absorptive partitioning were dominant, wouldn't we expect an increase or stabilization at higher levels with time? What is the error on those curves/measurements? Also, why is the Erythritol signal vs AS increasing already prior to t_mix start time (same in Fig. 7)? That seems odd.

Dear Editor,

Please find attached a manuscript describing mixing experiments between different particle populations carried out using a single-particle soot-particle aerosol mass spectrometer. We made particle populations to mimic interaction of fresh biomass burning soot with aged background aerosols by coating black carbon with a volatile sugar (erythritol) and by coating ammonium sulfate with various non-volatile sugars (glucose, sucrose, and raffinose). These complement earlier experiments from our group using much more rich mixtures of secondary organic aerosol to test mixing with a refined design using materials with well known properties. We also add single-particle detection of black-carbon soot cores using an infrared vaporizing laser as well as measurements at lower temperature, where particles are more viscous.

Our experiments are designed to address the hot topic of “glassy” aerosols (originally proposed using water uptake into very cold glucose), and we extend our finding that, even though particles can be very viscous and bouncy, as measured by various probes, when we directly test whether vapors can condense onto and diffuse into the particles, they generally do. Here we find that erythritol will evaporate from black carbon particles and diffuse into even cold (trisacharide) raffinose. While this may be in part because the raffinose itself has retained some water (with or without assistance from the ammonium sulfate core), our experiments strong suggest that diffusion limitations are hard to come by in the real world.

The results of this study suggest that vitrification of these sugar particles is not directly related to the vitrification behavior of these sugars in the bulk. If particles in the atmosphere are accurately represented by these mimics, the results also suggest that there are not prohibitive mixing limitations at the conditions we tested. Ultimately, the methods used in this system can be applied to increasingly complicated systems to better approximate the mixing behavior of atmospherically relevant particle systems. We feel that the methods are novel – to our knowledge this is the first instance of the soot-particle single-particle measurements being used in this way, as well as the first example of our mixing experiments at reduced temperature. We also feel that the results are of broad interest to the environmental science community, especially given the rapidly increasing incidence of wildfires.

This manuscript (original manuscript ID EA-ART-03-2022-000017) is being resubmitted after a request for major revisions from the two reviewers. Below, the reviewer comments are included along with our responses and the locations of relevant edits made to the manuscript.

REVIEWER 1
1. Abstract. The authors state the following: “Here we describe experiments on carefully prepared particle populations representing glassy aged organic particles and fresh biomass burning particles to develop a model phase space for organic aerosol systems and better understand when particle glassiness impedes mixing.” However, the authors did not provide evidence that the particles they studies were in a glass state. I suggest the authors change “glassiness” to “highly viscous” throughout the paper unless they have evidence indicating that the mixtures they studied were in a glass state. As pointed out by the authors, the non-volatile sugars were mixed with ammonium sulfate, which can lower the glass transition temperature by large amounts. Some discussion on how much ammonium sulfate may lower the glass transition temperature would be useful (Zobrist et al. 2008, Song et al. 2021, Lilek and Zuend 2022).
a. We agree with the reviewer that we did not directly assess the phase state or quantify the viscosity of our model particle systems for the purposes of this study. Our aim was to directly probe the mixing behavior of particles regardless of estimated, or expected, phase state. This was meant to illustrate that even in particle populations where existing literature would suggest that the viscosity of the particle could lead to diffusive limitations to various aerosol phenomena, direct observations of those phenomena may not line up with those expectations. We have addressed this comment in the manuscript in a number of places, outlined below.
i. In the abstract, we added “highly viscous or potentially” and replaced “’glassiness’” with “phase state” in the sentence that previously read “Here we describe experiments on carefully prepared particle populations representing “glassy” aged organic particles (non-volatile sugars 13C-glucose, sucrose, and raffinose with ammonium sulfate seeds) and fresh biomass burning particles (erythritol with black carbon seeds) to develop a model phase space for organic aerosol systems and better understand when particle “glassiness” impedes mixing.”
1. The new sentence reads “Here we describe experiments on carefully prepared particle populations representing highly viscous or potentially “glassy” aged organic particles (non-volatile sugars 13C-glucose, sucrose, and raffinose with ammonium sulfate seeds) and fresh biomass burning particles (erythritol with black carbon seeds) to develop a model phase space for organic aerosol systems and better understand when particle phase state impedes mixing.”
ii. In line 105 on page 2 on the right column, we added “highly viscous and potentially” to the sentence describing what our ammonium sulfate and non-volatile sugar particle populations represent. Previously “…particles with various effectively non-volatile sugars coating ammonium sulfate represent aged, ambient, and glassy, SOA (LV-OOA).”
1. The new sentence reads “…particles with various effectively non-volatile sugars coating ammonium sulfate represent aged, ambient, highly viscous and potentially glassy, SOA (LV-OOA).”
iii. In line 330 on page 7 in the right column, we added “highly viscous, and potentially” to a sentence describing our expectations of the phase state of 13C-glucose and ammonium sulfate particles. Previously “…and even though we expect the particles to be glassy, erythritol evaporation is followed by absorption into the glucose coating.”
1. The new sentence reads “…and even though we expect the particles to be highly viscous, and potentially glassy, erythritol evaporation is followed by absorption into the glucose coating.”
iv. In lines 461-462 on page 9 in the left column, we added “high viscosity or potentially” when describing potential inhibitors to mixing. Previously “…without obvious inhibitions due to glassy behavior.”
1. The new sentence reads “…without obvious inhibitions due to high viscosity or potentially glassy behavior.”
v. In lines 364-367 on page 8 in the left column, we added a sentence briefly discussing how much inorganic material can reduce the viscosity of particles containing organic material, compared to pure organic material.
1. The new sentence reads “There is evidence in existing literature that organic-inorganic particle material mixtures can reduce viscosity of the particles compared to pure organic particles by 2 or more orders of magnitude52.” And the new citation is for Song, et al. 2021.
2. Throughout the document, the authors used erythritol to represent fresh biomass burning particles. Please add some discussion on why this system was chosen and discuss if this is a realistic proxy for fresh biomass burning particles. Erythritol has a low viscosity and low glass transition temperature, while the viscosity and glass transition temperature of biomass burning particles may be higher (DeRieux et al. 2018). This is relevant since erythritol likely lowered the viscosity of the non-volatile sugars mixed with ammonium sulfate by acting as a plasticizer, as pointed out by the authors.
a. First, to address the relevance of erythritol as a marker of biomass burning particles, there is evidence in literature that erythritol is a relevant component of biomass burning particles in a number of contexts (Vincenti et al. 2022). Ultimately erythritol was chosen because it is semi-volatile, present in biomass burning, and readily available to us in the laboratory. Most importantly, erythritol is a sugar, so it should be readily miscible with any non-volatile sugar that we chose; this was important to our study so we could limit any potential mixing limitations to diffusive limitations rather than thermodynamic limitations. With regard to the viscosity and glass transition of erythritol, this was not directly relevant to this study because we were never probing the mixing into erythritol (and, therefore, the phase state or glass transition). Actual fresh biomass burning particles would, in fact, have a much more complex mix of organic compounds, including low volatility components. We have addressed this comment in the manuscript in a number of places, outlined below.
i. We added references 49-51 around line 102 on page 2 in the right column.
ii. We added a paragraph on page 3 on lines 172-181 in the right column. This paragraph clarifies the roles that each organic fraction plays when we were deciding on what model particle populations to use in our experiments. This new paragraph reads:
1. Additionally, we decided that one population will have a non-volatile sugar fraction and the other with a semi-volatile sugar fraction. We can specifically track mixing into one population from the other, without expecting any mixing to happen in the opposite direction. Thus, we are only tracking mixing, and any potential mixing limitations, into the non-volatile sugar populations. Using two distinct sugars, we can also be quite sure that if we encounter any mixing limitations between the particle populations, they would be due to diffusion and not miscibility.
3. Please add more information on viscosities and glass transition temperatures of the organics studied (erythritol and the non-volatile sugars). If there is information on how much the presence of ammonium sulfate (or other inorganic salts) will lower these viscosities and glass transition temperatures, please add it to the document (Zobrist et al. 2008, Song et al. 2021, Lilek and Zuend 2022).
a. First, to address a previous comment we have added a comment on how inorganics can affect the glass transition of organic materials. Next, to address the viscosity and glass transition of erythritol we have discussed above how we did not take those into account because erythritol was meant to be mixing into the non-volatile sugars. Next, we included a statement about the glass transition of glucose when we begin to introduce the experiment results on page 6 from lines 302-306 “Various estimates of the glass transition behavior of glucose suggest that its glass transition temperature, Tg, is typically at or below room temperature, depending on the water content in the sugar and relative humidity of the environment 52,53.” Otherwise, we have added explicit information about the glass transition temperatures of the other two sugars studied: sucrose and raffinose. The goal of our study was to probe the mixing behavior of organic material with well described glass transitions (commonly used sugars), to test if we could observe a glass transition similar to literature values. Thus, we never make a quantitative estimate of the viscosity of any of the sugars based on our results, and didn’t rely on estimates of their viscosity directly either. We have addressed this comment in the manuscript in a number of places, outlined below.
i. We added a sentence about the glass transition temperature of sucrose on page 8 in lines 388-391 in the left column. The new sentence reads:
1. Pure sucrose has well described glass transition behavior showing a glass transition significantly above room temperature, somewhere between 50 – 80 ºC, depending on the conditions of the test52,55.
ii. We added a sentence about the glass transition temperature of raffinose on page 8 in lines 373-376 in the right column. The new sentence reads:
1. Pure raffinose has an even higher glass transition temperature than sucrose, around 100 ºC56.
2. The new reference is Kajiwara and Franks, 1997.
4. At the end of the Introduction, the authors state “this enables us to observe conditions at which a glass transition occurs, which can give us an indirect estimate of diffusivity at certain conditions.” Please modify this sentence, since I don’t think the authors observed conditions at which a glass transition occurs.
a. We agree that in this paper we do not observe a glass transition; however the purpose of this sentence was to illustrate the possibility of this experimental setup (observe a glass transition directly in the particle phase, rather than estimate diffusivity or viscosity and make claims about the phase that way). However, to be more clear, we have changed the last sentence of the introduction on page 2 lines 91-94 in the left column.
i. The sentence now reads: “This choice makes it possible to observe conditions where the phase state or viscosity of our organic fractions inhibit mixing between the particle populations, if those conditions exist within our experimental ranges.”
5. What were the sizes of the particles in this study? Were the particles monodispersed or polydispersed? What was the thickness of the coatings? This information is needed to relate the current results to atmospheric conditions.
a. Particle size information exists in the SI document that accompanies this manuscript. All of our particle populations were polydisperse. Typically, the black carbon and erythritol populations had a particle size mode around or slightly below 100 nm with a wide distribution and the ammonium sulfate and non-volatile sugar populations had a particle size mode around 120-150 nm with a smaller size range, but still polydisperse. We never directly observed the thickness of the coatings, but these are estimated by the relative difference in signal between the respective particle seed and organic fraction.
6. Figure 4. The authors state that erythritol does not absorb on ammonium sulfate (no green appears in the panel on the right), but there is a green peak around 50 minutes, corresponding to roughly 20% of the ammonium sulfate mass. Please address this point in the revised manuscript. Does this change any of the conclusions in the paper?
a. We agree that this wording was misleading, and did not address the small bump around 50 minutes. To address this, we changed the word “detectable” to significant on page 5 lines 283 in the left column, and also added a few sentences explaining why this result still supports our claim that when erythritol signal ends up in the non-volatile sugar/ammonium sulfate population, this is due to mixing with the sugar and not ammonium sulfate. We also changed “adsorb onto” to “diffuse into” on page 5 line 276 in the right column. Those new sentences are on page 5 in lines 284-287 in the left column and 274-275 in the right column and they read:
i. There is a small increase in erythritol that reaches a maximum near 20% of the ammonium sulfate signal some 50 minutes after mixing; however, this small and short-lived signal is likely due to experimental noise or a small amount of erythritol briefly condensing onto the ammonium sulfate.
7. On page 6, the authors discuss possible explanations for why erythritol mixes with glucose-containing particles. In addition to the arguments presented by the authors, I think it would help to point out that erythritol is a smaller molecule than glucose (as well as sucrose and raffinose). Hence erythritol can “percolate” through the glucose-ammonium sulfate mixture. In this case, diffusion will be faster than expected based on the Stokes-Einstein equation (Price et al. 2014, Marshall et al. 2016, Evoy et al. 2020).
a. We agree with this comment and that adding something like this to that argument will make it stronger. We addressed this comment on pages 7 (lines 342-343 in the right column) and 8 (lines 359-361 in the left column) with the following sentence:
i. There are a number of studies demonstrating how smaller organic molecules can diffuse into highly viscous organic material more rapidly than diffusion models estimate (i.e., Stokes-Einstein relation) and reduce the local viscosity54–56.
ii. The added citations were those suggested by the reviewer comment.
8. The authors state “if the model systems represent real-world wildfire smoke…”. The authors have used erythritol as a proxy for fresh biomass burning aerosol. However, erythritol has a low viscosity, low glass transition temperature, and small molecular radius, which all favor fast mixing. The authors should point this out and make it clear that biomass burning aerosol may have a higher viscosity, higher glass transition temperature, and larger molecular radius, which may result in longer mixing times.
a. Again, this is a good suggestion by the reviewer that will clarify and contextualize the results of this study and how it would compare to more complex atmospherically relevant systems. To address this, we have added the following sentences to the end of the conclusion on page 9 lines 462-468 in the left column:
i. Real world biomass burning particles likely have a more complex mixture of organic material with a wide range of volatility and viscosity, which would certainly affect the progress towards an internal mixture between those particles and an ambient, aged particle population. Increasingly complex model systems will more closely approximate the behavior of real-world systems.
9. There are several mistakes in the reference list.
a. The following references were edited for correctness:
i. Reference 2: Ye et al 2020 – removed DOI and included volume number. Page numbers were not included in the citation.
ii. Reference 3: Marcolli et al 2004 – removed article title and replaced it with journal name, added page numbers.
iii. Reference 9: IPCC report – removed long title and publisher information.
iv. Reference 14 – Replaced article name with journal name and added page numbers.
v. Reference 16 – added page numbers
vi. Reference 18 – removed the article title and added page numbers
vii. Reference 19 – removed the article title and added page numbers
viii. Reference 20 – added URL and access date
ix. Reference 22 – removed DOI, added volume number. Page numbers not included in the citation.
x. Reference 25 – removed article title, added journal title, added page numbers.
xi. Reference 27 – added publisher location
xii. Reference 28 – removed (1979() from journal title
xiii. Reference 29 – removed article title, added journal title, added page numbers.
xiv. Reference 30 – removed article title, added page numbers.
xv. Reference 32 – removed DOI, added volume number. Page numbers not included in the citation.
10. Experimental. RH was either very low (2-5%) or high (approximately 90%). Is this before or after adding the particles to the chamber? Also, does the RH correspond to room temperature or the temperature within the chamber?
a. All experimental conditions were established before introducing particles into the chamber. RH always corresponded to the temperature within the chamber. This was reflected in a number of places in the manuscript
i. On page 2 lines 105-106 in the left column we added “(i.e. temperature and relative humidity)”
ii. On page 2 lines 53 and 54 in the right column we added “within the chamber”
11. Page 2. There appears to be a typo when discussing the composition of the particles studied.
a. We believe this typo is now on page 3 line 176 in the left column where we mistakenly refer to the organic fraction of all of the particle populations as “semi-volatile” when we just meant “organic” – this has been addressed.
12. Were the SP mode and the tungsten vaporization done separately or at the same time.
a. The SP laser and tungsten vaporizer were used simultaneously for all AMS data collected in this study. To clarify this, we added a sentence to lines 95-96 on the right column of page 2:
i. “Whenever we collected data in the ET mode, the SP laser and tungsten filament were used simultaneously.”
13. Was a significant amount of the erythritol material externally mixed with black carbon, or was it all associated with black carbon particles?
a. We’re not totally sure what this question is asking, but to clarify the source of erythritol in our experiments: any erythritol injected into the chamber started in a liquid dispersion with liquid aquadag (black carbon graphite), erythritol, and water. This dispersion was then directly atomized into our chamber setup. Any erythritol introduced to the chamber was associated with black carbon particles, and we observed no signal consistent with particles consisting of pure erythritol.
14. Please add more information about the components in the setup used – brands, specifications, etc. For example, please add details on the brands and specifications of the AMS, SMPS, atomizers, and diffusion dryers. Also, please add information on where the chemicals were bought and their purity.
a. This information has been added for all chemicals used, AMS, SMPS, clean air generator, and atomizers on the first and second paragraphs in the right column on page 2.
15. Page 5 and Figure Caption 5. The authors mention cross-talk between sucrose and glucose. Please be more specific on what this means. I assume the authors mean that the mass spectrometric technique could not completely distinguish between glucose and sucrose. Does this have any implications for the conclusions in this paper?
a. The reviewer is correct here – by cross-talk we mean similarities between the mass spectra. We do not believe this has implications for the conclusions we drew in this paper because other than this control experiment we did not run any mixing experiments between two non-volatile sugars, and there was never significant cross-talk between the erythritol and the non-volatile sugars. Additionally, the purpose of this control experiment in Figure 5 was to see if the concentration of the opposing organic fraction changed over the course of the experiment, and what we observed was a non-zero, but constant, signal of sucrose in the glucose/ammonium sulfate spectra, which suggests to us that it was in fact cross-talk and not mixing.
i. To clarify this, we added “mass spectra” to the end of the sentence talking about cross-talk in the figure 5 caption.

REVIEWER 2
1. In the Conclusions section it is stated: “we never observed substantial limitations to mixing”. One could argue that this strongly depends on the applied definition of mixing and the timescale of interest. If many hours are of interest, that may be true. But in that case one could also argue that Bones et al. (2012, PNAS) did not find substantial limitations on the > 3 hour timescale for water equilibration into glassy particles; however, those authors consider it a substantial limitation to mixing.
I think the authors agree that a timescale of > 60 min is far from quasi-instantaneous. However, in atmospheric chemistry models, the time steps and assumption of instantaneous mixing are usually on a much shorter sub-1-hour timescale (probably sub-10-min), so there would be implications of assuming quasi-equilibrium internal mixtures of aerosol populations in such models. I recommend to account for this question of definition in the phrasing of such statements and to put such conclusions into context of the timescales of relevance.
a. We agree that we did not observe quasi-instantaneous mixing in a number of our mixing experiments, in fact we arguably only see “rapid mixing” (by this reviewer’s definition) in the first two reported mixing experiments with glucose. However, the goal of our study was to ultimately answer a binary question: is mixing observed at all? We define our mixing timescale of interest in the introduction to be on the order of a few hours, which is also why our experiments were typically run over the course of a few hours. To address this, we added a clarifying statement to our conclusions where we state that there were not substantial limitations to mixing in our system.
i. Page 9 lines 440-441 in the left column we added “within our defined mixing timeline on the order of hours”
2. Also in the Conclusions section, it is stated: “These results demonstrate the importance of probing particle behavior directly, wherever possible, rather than relying on proxy properties, such as viscosity or an estimated diffusivity”. I agree that direct measurements are great and certainly helpful, but the experiments did not measure diffusivity nor viscosity and as such cannot be used to directly disproof those particle properties or conclusions derived from particle viscosity measurements on diffusive mixing timescales. The results from this study are only semi-quantitative and may not fully rule out substantial limitations to complete mixing. If the authors disagree with this view, I would appreciate a more detailed discussion of the quantitative vs qualitative nature of mixing behavior and equilibration measured, e.g. more detailed, quantified particle compositions over the course of an experiment. I think the authors agree that what leads to complete mixing (no concentration gradients within a particle layer/phase) will depend on diffusion fluxes and hence molecular diffusivity. Instead, these experiments may provide evidence for important pathways of (partial) mixing, e.g. via absorption into the near-surface layers of a particle, lowering its viscosity and enhancing its absorptivity. Certainly nice results to ponder.
Did the authors consider comparison with experiments on significantly larger particles, which, collectively with the data for the smaller particles, may provide data to assess the mixing depth as a function of time? Please consider revising the wording of the Conclusions section.
a. We agree that we did not directly disprove any results from studies that draw conclusions about particle behaviors from properties such as viscosity or diffusivity. The purpose of this statement was that we chose to probe particle behavior (in this case mixing between organic fractions of distinct particle populations) directly, rather than draw conclusions based on viscosity and diffusivity models. We tried to draw conclusions that were relevant to studies that use viscosity or diffusivity by choosing organic materials that, based on literature values, could exist as highly viscous or glassy at conditions we tested in the chamber (various sugars important to food science and industry, with very well defined rheological properties). That being said, we did not make any direct estimates of particle properties. We did not consider experiments with larger particles because as we mentioned in the previous comment, the scope of this study was to establish if mixing happens, not necessarily to quantitatively define the completeness of this mixing. This would be an interesting follow-up to this study and would definitely add nuance and depth to the results we found. To clarify this sentiment, we added to the end of the first paragraph, starting in the sentence called out by this comment, shown below.
i. On page 9 lines 444-452 in the left column “These results demonstrate the importance of probing particle behavior directly wherever possible, rather than relying solely on proxy properties, such as viscosity or an estimated diffusivity, to draw conclusions about particle behavior. Our observations indicate that these sugars either do not actually exist as glasses or highly viscous at the expected conditions in the particle phase or that they were plasticized by the erythritol, ammonium sulfate, and/or water vapor in the chamber. Directly comparing these results to diffusivity and viscosity measurements of the same components under the same conditions could further illuminate the mixing pathway for this system.”
3. If a particle (or at least its sugar-coating) is truly in a glassy state, absorption into said glassy phase would be expected to be severely suppressed. Therefore, full mixing throughout the entire organic (initially glassy) sugar phase would be expected to be rather slow – on the order of many hours to days or longer. The authors do seem to agree with that assumption, which motivated their experiments. However, the authors did not determine the viscosity of the phases probed in these experiments, so it is not clear whether they were glassy or perhaps of substantially lower viscosity. Clearly, if they were glassy *and remain glassy* under low RH conditions, one would expect different outcomes from measurements at higher RH of ~90% compared to dry conditions, but the results shown in Figs. 7 and 8 do seem to indicate that there is no difference despite a rather large RH difference. This is unexpected given the hygroscopic nature of the sugar compounds involved. Perhaps it is indicative of the semi-quantitative nature of these measurements. For example, the erythritol signal in Fig. 8 (right panel) is somewhat lower than in Fig. 7. What explains that? Is it simply the uncertainty of the measurements, or competition for erythritol by other particles with larger condensed phase absorptive mass in the high-RH case? A brief discussion of this would be of interest.
a. This discrepancy in the erythritol signal showing up in the glucose particles (dashed light green lines on the right panels of figures 7 and 8) is certainly in part due to uncertainty and noise associated with our ET data from the AMS, which we smooth during data processing. However, our interpretation of this is that the difference in magnitude is more to do with the difference in evaporated erythritol from the black carbon particles (solid light green line in the left panels of figures 7 and 8), resulting in less erythritol available in the gas-phase to condense onto the glucose particles. In figure 8, the erythritol signal decreases from a ratio to black carbon of ~6:1 to ~3.5:1; in figure 7, that ratio decreases from ~5:1 to ~1:1. Generally speaking, we saw slightly slower evaporation of erythritol from black carbon in the high RH experiments compared to low RH experiments.
4. It would be useful if the authors could also discuss the expected minimum mixing timescale when considering the gas phase diffusion limitation of erythritol while assuming liquid-like absorptive particles. Some related points are mentioned in the following.
a. In the supplemental material we present the condensation sink of the suspensions. That, and not gas-phase diffusion per se, is the fundamental limitation to equilibration in these systems. This has been addressed at length in the literature (Saleh, Donahue advances article, add to citations) and so we have also added citations here. An important design constraint in these experiments is for the condensation sink of the “semi volatile” chamber population (the population containing erythritol) to be significantly faster than the collision frequency of vapors with the chamber walls (15-20 minutes here). That ensures that the gas-phase activity of the semi-volatile constituents will approach an asymptotic limit with equal activities in the aerosol and gas phases (in this case we expect that limit to be a = 1 because the coatings are pure erythritol). Once that condition is met, the system will remain at equilibrium with an ever decreasing burden of suspended aerosol phase erythritol as the vapors are slowly lost to wall absorption until the system is finally exhausted and the wall loss of those chamber particles draws the condensation sink below the wall collision frequency. We have used this balance to measure the wall loss of semi-volatile vapors in P. Ye et al (it is analogous to the coated bead column method of vapor pressure measurement). The only other element of gas-phase diffusion at play is diffusion through the surface layer near larger particles (with Kn < 1), but the timescale there is sub microsecond.
b. To address this discussion briefly in the manuscript, we have added a paragraph to the end of the results and discussion section describing how we came to our interpretation of these results, with the relevant citations added. This addition can be found on pages 8 and 9, lines 387-393 in the right column of page 8 and lines 429-433 in the left column on page 9.
i. The initial rapid evaporation of erythritol from the black carbon seeds, subsequent uptake and stabilization into the non-volatile sugar/ammonium sulfate particles, which was sometimes followed by the slow steady decline in the signal for the rest of the experiment is consistent with other modelled and measured mixing behavior between organic particle fractions published in relevant literature60-62. Based on the particle surface area condensation sinks (available in the SI) compared to the collision frequency of vapors to the chamber walls (approximately 15-20 minutes for our system), our results indicate that we are observing evolution towards an internal mixtures in each of the experiments discussed here.
5. Page 7: “However, there are no striking differences between the low RH and high RH experiments. This suggests that there truly are no barriers to mixing at any relative humidity at the temperatures we studied for the erythritol – glucose system.” Consider that this conclusion is only qualitatively correct, in part because the measurements do not seem to allow determining whether the resulting particles were well mixed internally – except for the distinct cores – or whether only a relatively thin layer of absorbed erythritol mixed with some glucose and water was present in the initially SVOC-free particles. Please discuss.
a. This is something we considered deeply when interpreting our results. It is true that we did not directly determine how complete the mixing was between the organic fractions of our systems. However, we believe that we collected enough evidence to be sure that we were not just observing a thin (or mono-) layer of erythritol condensing onto the non-volatile sugars. First, we believe that we may have observed this in figure 4 when we tested if erythritol would mix into ammonium sulfate alone. The small peak around 50 minutes, which depletes again by 100 minutes, was likely due to a thin layer of condensed erythritol that ultimately re-evaporated as the particle condensation sink decreased and the wall condensation sink became more dominant. Comparing this result to the resulting erythritol signal strength and signal persistence in the mixing experiments with non-volatile sugars, it seems unlikely to us that we are observing a similarly thin layer.
6. The above comment seems to apply in general to the interpretation of these experiments also with other sugars. While I do not doubt the measurements, I have questions about their interpretation. Namely, whether the particles were sufficiently well mixed in the “sugar” shell phase, including the SVOC, during the experiment time of a few hours, such that we could call this a nearly equilibrated aerosol population. For the low-RH experiments: would it not be possible that the SVOC first adsorbed at the surface of the sugar phase, then relatively fast diffusion into the first few nm of the non-volatile sugar phase took place, which may have softened that layer to a viscosity much below that of a glassy state and allowed for accelerated absorptive partitioning of SVOC (and some water) into that phase. This could then allow that layer to grow via absorptive partitioning and lead to a notable signal in the mass spectra. This despite the SVOC not diffusing through the full thickness of the sugar coating. In such a case, what is measured would be a partially mixed state, not considered to be equilibrated. This would still be an interesting insight and result from this work, but statements like “no barriers to mixing” may need to be toned down, since those types of experiments may not be able to detect the true level of internal mixing.
To some extent, this comes down to how we define “mixing” within a single particle and at what point we would say it is sufficiently well mixed, e.g. via an e-folding equilibration time.
a. We agree with the assessment, and possible mixing pathway suggested here by the reviewer. In all cases we believe that we observed progress towards internal mixing, not necessarily complete mixing, because we didn’t take any steps to assess the completeness of mixing.
7. Abstract text: why put glassy in quotes? Glassy material is a phase state defined as an amorphous solid (that has undergone a glass transition), so no need for quotes here. Please consider revising similar “glassy” instances throughout the introduction section.
a. We chose to put glassy in quotes because we never directly probe the phase state of our particle systems. We agree that the glass phase is a distinct phase. To address this, we revised the manuscript abstract and introduction to refer to particles as glassy (without quotes) when speaking about particles generally, and “glassy” when discussing particles that we studied directly.
8. Abstract text: particle “glassiness” – again, either a particle is in a glassy state or not; to this reviewer’s knowledge there is no spectrum of glassiness, perhaps the authors mean viscosity here.
a. Same response as above, and also made some adjustments from a similar comment from reviewer 1.
9. Page 2, third paragraph: check the units, there seems to be one minus sign too many.
a. We think the confusion here comes from the way we wrote the units “cm2-s-1” where the hyphen was meant to separate the two units. To avoid confusion, we removed the hyphen so it now reads “cm2 s-1”
10. Page 2, top right: should 6 g of Aquadag be 0.6 g?
a. This should be 6 g. To address a comment from reviewer 1, we have added purity information for all of the materials we used, but to clarify here our sample of Aquadag was approximately 90% water and 10% graphite, so to get 0.6 g black carbon we added 6 g of Aquadag.
11. Page 4 end of second paragraph on right: I appreciate the discussion and consideration of adsorption of erythritol as a potentially confounding effect. Your tests show that it does not seem to absorb into dry ammonium sulfate (AS) particles. What the test does not fully show is whether erythritol may do multilayer adsorption on the non-volatile sugar phases (rather than on AS). Have you considered that option? Given that erythritol is an SVOC under the given conditions, with its melting point at far higher temperatures, multilayer adsorption may be possible; although I agree that it would be difficult to explain why it did not also adsorb on AS surfaces.
a. We discussed this a bit before in a previous comment from this reviewer, and ultimately found it hard to believe that this was the case given that we did not observe it on the AS surfaces at all. This was our main test into whether or not this was the case, so it is certainly still possible, but we do not think it’s likely.
12. Fig. 5 and associated text on page 5: please explain what is meant by cross talk in a bit more detail.
a. We addressed this in a comment from reviewer 1 – we meant that there were similarities between the two mass spectra that our analysis could not fully distinguish between because the ET data is unit mass resolution.
13. Page 5: “Tg is often used as a proxy for diffusivity” – this is a convoluted statement; Tg is a proxy for viscosity, not directly diffusivity. I suggest rephrasing.
a. This is reasonable. What we meant here was that Tg is a proxy for viscosity, which is a proxy for diffusivity (in studies probing this kind of behavior), but we have clarified this in the sentence. Due to edits, this is now on page 6 and lines 312-313 in the right column, and it now reads:
i. Tg is often used as a proxy for viscosity, which is related to diffusivity via relations such as Stokes-Einstein, and it is argued that for T < Tg, diffusive mixing can be extremely slow even in 100 nm diameter particles29–45.
14. Fig. 6, right panel: why is the erythritol signal decreasing with time after its peak near 60 min? Is it loss to the chamber walls? If absorptive partitioning were dominant, wouldn't we expect an increase or stabilization at higher levels with time? What is the error on those curves/measurements? Also, why is the Erythritol signal vs AS increasing already prior to t_mix start time (same in Fig. 7)? That seems odd.
a. We believe this is due to the dominant condensation sink shifting from particle surface area to chamber wall surface area. In all of these experiments we are conducting a linear combination of reference mass spectra with the observed unit mass resolution ET mass spectra of the two distinct particle populations (black carbon seed and ammonium sulfate seed populations) to estimate the percent contribution of each component to the resulting spectrum. There is a residual for these calculations typically of order 10-3. Then we divide each percentage by the relevant seed to get signal strength relative to the inorganic, totally non-volatile seed and ultimately we smooth the data using a weighted average data smoother in MATLAB. That smoothing is why some of the figures show erythritol increasing prior to t_mix.
Thank you for considering this research article that probes the mixing state of mimics for atmospherically relevant particle populations with changing temperature and relative humidity conditions.

28-May-2022

Dear Dr McPherson Donahue:

Manuscript ID: EA-ART-03-2022-000017.R1
TITLE: Single particle measurements of mixing between mimics for biomass burning and aged secondary organic aerosols

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Reviewer 1

The authors have adequately addressed my comments/questions. Thank you.