From the journal Environmental Science: Atmospheres Peer review history

22-Jan-2021

Dear Dr Nizkorodov:

Manuscript ID: EA-ART-12-2020-000020
TITLE: Viscosity and liquid-liquid phase separation in healthy and stressed plant SOA

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

Reviewer 1

Review on Smith et al. 2021 ”Viscosity and liquid-liquid phase separation in healthy and stressed plant SOA”

This study investigates the chemical composition and several physical properties of Secondary Organic Aerosol (SOA) formed via photo-oxidation of mixtures of mono- and sesquiterpenes (MT, SQT). The chosen mixtures are representative for healthy and stressed scots pine trees. Two independent optical methods were used to investigate the phase state and viscosity of the formed SOA under a range of humidity conditions. Additionally, composition-based parameterizations were applied as well and compared to the experimental methods. The shifts in the emission profiles from healthy to stressed plants may appear subtle, but they have a major impact on the physical properties of the particles (namely volatility, hygroscopicity, and viscosity). The higher viscosity and different response to increased humidity will change the effect the particles have in the atmosphere and are relevant for air quality or climate modeling purposes. Furthermore, the importance of using realistic precursor mixtures rather than simple systems is shown as the authors can demonstrate that parameters derived from a-pinene cannot truly represent the studied properties.
The manuscript is well written, and the presented data supports the conclusions. I recommend publication in this journal after a few minor questions are addressed.

1) Ylisirniö et al. (2019) used the same Sigma Aldrich product labeled therein “SQT mixture” (product ID W383902). They analyzed it with GC-MS and found ~40% farnesene isomers, 40% bisabolene isomers, and 20% unidentified SQTs (see SI of study). It is possible that these ratios vary between production batches. However, if the authors do not have such an analysis of their batch, they should assume that these ratios are representative and highlight the significant contribution of bisabolenes in the methods section and not just mention it in the supplement material (footnote to Table SI1).

2) The authors make it sound as if the high resolving power of nano-DESI-MS enables them to identify individual compounds rather than just the sum formulas at a given exact m/z and the most likely candidate. There is a footnote hinting about this for Table S3. However, it should be stressed a bit more in the main text that there can be multiple isomers for each sum formula, especially since they have such a complex precursor mixture.

3) The authors should clarify why 250 Da was chosen as threshold for “high molecular mass” compounds. From the mass spectra in Fig 1, it looks like, sp-SOA has a stronger contribution in the range of 250 to 300 Da. The contribution of compounds with >300 Da seems to be higher for hp-SOA (at least in negative mode).

4) The authors discuss the “surprising” existence of signals with low molecular weight. First, it needs to be clarified that it is the presence in the particle phase that is unexpected, not the existence of these compounds in general. Especially with a high fraction of acyclic compounds, fragment with less than 10 C atoms are very likely. Even for MTs small compounds like oxalic, acetic or formic acid are produced in large quantities. But these should be too volatile to contribute considerably to the particle mass. Second, why do they choose 136 Da (mass of MT) as their threshold? Generally, MTs are not expected to partition into the particle phase, but neither are compounds with slightly higher molecular weight if the degree of oxidation is low. Shouldn’t this paragraph be linked to the estimated log10 C0 values instead and discuss why there are compounds with surprisingly high volatility in the particle phase? However, I do agree with the authors’ argument about fragmentation in the instrument causing these volatile compounds.

5) The hygroscopicity parameter (k) values taken from Zhao et al. (2017) were derived from CCN measurements. For calculations at 50% RH and lower, HTDMA measurement should provide a better estimate. k(HTDMA) for the SOA presented by Zhao et al. (2017) where in the range of 0.04 – 0.08 (private communication, co-author of Zhao et al. (2017)). However, the trend between MT and SQT dominate SOA stays the same. As the k(HTDMA) values are not published, I recommend to continue using the CCN based values, but highlight that the values of 0.07 and 0.15 are probably upper limits for this type of SOA and that the true water content at 50% RH may be lower but still follow the same trend.

6) The authors investigated the impact of the chosen hygroscopicity parameter value on the calculated viscosity values. Unfortunately, they only describe the results in Fig S8 compound by compound. This makes it difficult to understand how much the average particle viscosity, which is measured by the optical methods, would change. How much would the predicted viscosity vs RH curves (Fig 5) change if k=0.1 is used for both data sets?

7) The possibility of evaporation of material from the particle phase during the optical measurements was investigated under dry conditions, and the authors correctly concluded that no significant evaporation occurred and then claim that that is also true for the higher RH values. I disagree with extending the findings for dry conditions to the wet ones. Particle evaporation studies have shown that even though particles may not evaporate significantly under dry conditions, they may lose considerable amounts of material under humid conditions due to the decrease in particle viscosity (Buchholz et al., 2019; Li et al., 2019; Yli-Juuti et al., 2017; Zaveri et al., 2020). The viscosity values at 50% RH and the derived mixing times (Fig 4) indicate that the plasticizing effect may have been strong enough to enhance the evaporation of semi-volatile material enough for apin- and hp-SOA. If the authors do not have experimental evidence that no particle evaporation occurred under wet conditions, they should clarify how that would impact the derived viscosity values.

8) The results from the LLPS measurements are not discussed enough. What does the LLPS occurring down to lower RH mean for the particle properties? Does the separate aqueous phase in the sp-SOA impact the viscosity? Could the particulate water be “confined” to its own phase and not available to act as a plasticizer for the organic phase? The authors should add some more discussion to the LLPS chapter and link it more to their other findings.

9) The authors show clearly that the phase state and viscosity of SOA from realistic plant emissions cannot be predicted from a-pinene measurements. One could argue that the structure of the SQTs is too different from a-pinene. Do comparable measurements exist for SQT derived SOA and how do they compare to the plant SOA? Could the complex mixtures be described as a combination of e.g. a-pinene and farnesene?

References
Buchholz, A., Lambe, A.T., Ylisirniö, A., Li, Z., Tikkanen, O.-P., Faiola, C., Kari, E., Hao, L., Luoma, O., Huang, W., Mohr, C., Worsnop, D.R., Nizkorodov, S.A., Yli-Juuti, T., Schobesberger, S., Virtanen, A., 2019. Insights into the O:C dependent mechanisms controlling the evaporation of a-pinene secondary organic aerosol particles. Atmos. Chem. Phys. Discuss. 1–21. https://doi.org/10.5194/acp-2018-1305

Li, Z., Tikkanen, O.-P., Buchholz, A., Hao, L., Kari, E., Yli-Juuti, T., Virtanen, A., 2019. Effect of Decreased Temperature on the Evaporation of α-Pinene Secondary Organic Aerosol Particles. ACS Earth Sp. Chem. 3, 2775–2785. https://doi.org/10.1021/acsearthspacechem.9b00240

Yli-Juuti, T., Pajunoja, A., Tikkanen, O.P., Buchholz, A., Faiola, C., Väisänen, O., Hao, L., Kari, E., Peräkylä, O., Garmash, O., Shiraiwa, M., Ehn, M., Lehtinen, K., Virtanen, A., 2017. Factors controlling the evaporation of secondary organic aerosol from a-pinene ozonolysis. Geophys. Res. Lett. 44, 2562–2570. https://doi.org/10.1002/2016GL072364

Ylisirniö, A., Buchholz, A., Mohr, C., Li, Z., Barreira, L., Lambe, A., Faiola, C., Kari, E., Yli-Juuti, T., Nizkorodov, S., Worsnop, D., Virtanen, A., Schobesberger, S., 2019. Composition and volatility of SOA formed from oxidation of real tree emissions compared to single VOC-systems. Atmos. Chem. Phys. Discuss. 1–29. https://doi.org/10.5194/acp-2019-939

Zaveri, R.A., Shilling, J.E., Zelenyuk, A., Zawadowicz, M.A., Suski, K., China, S., Bell, D.M., Veghte, D., Laskin, A., 2020. Particle-Phase Diffusion Modulates Partitioning of Semivolatile Organic Compounds to Aged Secondary Organic Aerosol. Environ. Sci. Technol. 54, 2595–2605. https://doi.org/10.1021/acs.est.9b05514

Zhao, D.F., Buchholz, A., Tillmann, R., Kleist, E., Wu, C., Rubach, F., Kiendler-Scharr, A., Rudich, Y., Wildt, J., Mentel, T.F., 2017. Environmental conditions regulate the impact of plants on cloud formation. Nat. Commun. 8, 14067. https://doi.org/10.1038/ncomms14067

Reviewer 2

Report on manuscript “Viscosity and liquid-liquid phase separation in healthy and stressed plant SOA” by Natalie R. Smith et al.

The authors report about molecular composition, viscosity and the range of RH for which they observe LLPS for surrogate SOA. The SOA is produce in an environmental smog chamber from VOCs and meant to mimic emission from Scots pine trees under healthy and stress conditions. The authors find a broader humidity range for LLPS for the stressed plant SOA, at intermediate humidities the “stressed SOA” exhibits about one order of magnitude larger viscosity compared to the “healthy SOA”. Estimation of viscosity based on the molecular composition support the measurements.

This is a careful study and it fits well for Environmental Science: Atmosphere. The manuscript is well written and supported by adequate figures. It is a detailed characterization study showing that a more complex SOA than that derived from alpha-pinene leads to a more complex morphology and higher viscosity. My recommendation is to publish it, but I would like the authors to consider the following comments.

Page 4, right column:
There is no discussion on mixing times for smaller (oxidant) molecules nor water. The authors point to the fractional Stokes-Einstein equation, but do not use it to make a (probably crude) estimate. Such an estimate could nevertheless be helpful.

Page 6, conditioning for viscosity measurements:
As 1.5 hours seems to be sufficient to establish equilibrium even at low RH, even at 0 %RH, you may give a lower limit for water diffusivity based on this observation. How does this compare with fractional Stokes-Einstein estimates on water diffusivity?

Results section, molecular composition:
A detailed comparison between the SOA compositions with the one of Faiola et al. (2019) would be very helpful. For me it is difficult to get an idea how the molecular composition of the SOAs compare.

Results section, LLPS:
I am missing a discussion on how LLPS may influence the viscosity measurement. You poke through the outer phase first; is the hole you produce basically covered by the outer phase or does it cut through both phases?
In addition, the reader is left with the fact that the particles are phase separated, but it remains unclear how different the physicochemical properties of the two phases are.

Referee: 1
Comment:
Comments to the Author
Review on Smith et al. 2021 “Viscosity and liquid-liquid phase separation in healthy and stressed plant
SOA”
This study investigates the chemical composition and several physical properties of Secondary Organic
Aerosol (SOA) formed via photo-oxidation of mixtures of mono- and sesquiterpenes (MT, SQT). The
chosen mixtures are representative for healthy and stressed scots pine trees. Two independent optical
methods were used to investigate the phase state and viscosity of the formed SOA under a range of
humidity conditions. Additionally, composition-based parameterizations were applied as well and
compared to the experimental methods. The shifts in the emission profiles from healthy to stressed plants
may appear subtle, but they have a major impact on the physical properties of the particles (namely
volatility, hygroscopicity, and viscosity). The higher viscosity and different response to increased
humidity will change the effect the particles have in the atmosphere and are relevant for air quality or
climate modeling purposes. Furthermore, the importance of using realistic precursor mixtures rather than
simple systems is shown as the authors can demonstrate that parameters derived from a-pinene cannot
truly represent the studied properties.
The manuscript is well written, and the presented data supports the conclusions. I recommend publication
in this journal after a few minor questions are addressed.
1) Ylisirniö et al. (2019) used the same Sigma Aldrich product labeled therein “SQT mixture” (product ID
W383902). They analyzed it with GC-MS and found ~40% farnesene isomers, 40% bisabolene isomers,
and 20% unidentified SQTs (see SI of study). It is possible that these ratios vary between production
batches. However, if the authors do not have such an analysis of their batch, they should assume that
these ratios are representative and highlight the significant contribution of bisabolenes in the methods
section and not just mention it in the supplement material (footnote to Table SI1).

Response:
The main text (VOC mixtures methods section) and the note in Figure S1 were updated to reflect the fact
that bisabolenes and other sesquiterpenes could be present in addition to the farnesene isomers.

Comment:
2) The authors make it sound as if the high resolving power of nano-DESI-MS enables them to identify
individual compounds rather than just the sum formulas at a given exact m/z and the most likely
candidate. There is a footnote hinting about this for Table S3. However, it should be stressed a bit more in
the main text that there can be multiple isomers for each sum formula, especially since they have such a
complex precursor mixture.

Response:
The main text (Molecular composition of SOA particles results section) was updated to clarify these are
tentative assignments and exact structures have not been confirmed but could represent various isomers.

Comment:
3) The authors should clarify why 250 Da was chosen as threshold for “high molecular mass”
compounds. From the mass spectra in Fig 1, it looks like, sp-SOA has a stronger contribution in the range
of 250 to 300 Da. The contribution of compounds with >300 Da seems to be higher for hp-SOA (at least
in negative mode).

Response:
This is an excellent point. The main text (Molecular composition of SOA particles results section) was
updated to clarify that 250 Da was chosen to designate “high molecular mass” compounds because 250
Da is usually a transition region between monomers and dimers for monoterpene SOA (apin, limonene,
etc.). The focus for the analysis was on larger intensity peaks which would have the most impact on
particle properties.

Comment:
4) The authors discuss the “surprising” existence of signals with low molecular weight. First, it needs to
be clarified that it is the presence in the particle phase that is unexpected, not the existence of these
compounds in general. Especially with a high fraction of acyclic compounds, fragment with less than 10
C atoms are very likely. Even for MTs small compounds like oxalic, acetic or formic acid are produced in
large quantities. But these should be too volatile to contribute considerably to the particle mass. Second,
why do they choose 136 Da (mass of MT) as their threshold? Generally, MTs are not expected to partition
into the particle phase, but neither are compounds with slightly higher molecular weight if the degree of
oxidation is low. Shouldn’t this paragraph be linked to the estimated log10 C0 values instead and discuss
why there are compounds with surprisingly high volatility in the particle phase? However, I do agree with
the authors’ argument about fragmentation in the instrument causing these volatile compounds.

Response:
Thank you for this very useful insight! The paragraph in the Molecular composition of SOA particles
results section was completely re-written to focus on the portion of compounds with intermediate
volatility and their surprising presence in the particle phase. We previously attributed it to in-source
fragmentation during analysis but now we recognize that it is possible these more volatile organic species
could be trapped in the particle due to higher viscosity at low RH.

Comment:
5) The hygroscopicity parameter (k) values taken from Zhao et al. (2017) were derived from CCN
measurements. For calculations at 50% RH and lower, HTDMA measurement should provide a better
estimate. k(HTDMA) for the SOA presented by Zhao et al. (2017) where in the range of 0.04 – 0.08
(private communication, co-author of Zhao et al. (2017)). However, the trend between MT and SQT
dominate SOA stays the same. As the k(HTDMA) values are not published, I recommend to continue
using the CCN based values, but highlight that the values of 0.07 and 0.15 are probably upper limits for
this type of SOA and that the true water content at 50% RH may be lower but still follow the same trend.

Response:
This is another great suggestion from this reviewer. The main text was updated (viscosity predictions
methods section) to state that our kappa values likely represent upper limits on the hygroscopicity
parameters.

Comment:
6) The authors investigated the impact of the chosen hygroscopicity parameter value on the calculated
viscosity values. Unfortunately, they only describe the results in Fig S8 compound by compound. This
makes it difficult to understand how much the average particle viscosity, which is measured by the optical
methods, would change. How much would the predicted viscosity vs RH curves (Fig 5) change if k=0.1 is
used for both data sets?

Response:
A sensitivity calculation was done using k= 0.1 for both data sets and is shown the figure below (added as
new Figure S10). Overall, when the same k value is used, the difference between viscosity predictions for
both data sets decrease to about an order of magnitude instead of several orders when using differ k
values, indicating the predictions are quite sensitive to k values used and should be investigated in future
studies. The manuscript and SI were updated to reflect these changes.

Comment:
7) The possibility of evaporation of material from the particle phase during the optical measurements was
investigated under dry conditions, and the authors correctly concluded that no significant evaporation
occurred and then claim that that is also true for the higher RH values. I disagree with extending the
findings for dry conditions to the wet ones. Particle evaporation studies have shown that even though
particles may not evaporate significantly under dry conditions, they may lose considerable amounts of
material under humid conditions due to the decrease in particle viscosity (Buchholz et al., 2019; Li et al.,
2019; Yli-Juuti et al., 2017; Zaveri et al., 2020). The viscosity values at 50% RH and the derived mixing
times (Fig 4) indicate that the plasticizing effect may have been strong enough to enhance the evaporation
of semi-volatile material enough for apin- and hp-SOA. If the authors do not have experimental evidence
that no particle evaporation occurred under wet conditions, they should clarify how that would impact the
derived viscosity values.

Response:
The main text (viscosity measurements experimental section) was updated to clarify that viscosity did not
change significantly when using conditioning times between 1.5- 24 hours, even at 50% RH (Figure S4),
therefore any evaporative loss at higher RH would not have a large impact on overall particle viscosity.
Although this is indirect evidence, we regard it as a strong confirmation of the validity of our
measurements.

Comment:
8) The results from the LLPS measurements are not discussed enough. What does the LLPS occurring
down to lower RH mean for the particle properties? Does the separate aqueous phase in the sp-SOA
impact the viscosity? Could the particulate water be “confined” to its own phase and not available to act
as a plasticizer for the organic phase? The authors should add some more discussion to the LLPS chapter
and link it more to their other findings.

Response:
The main text (Experimental viscosity and mixing times results section) was updated with a note that we
did not consider the presence of two phases in the particles and the viscosity calculations represent a
combination of both phases. The individual phases may have different viscosities than those reported
here. It is expected the inner aqueous-rich phase would be less viscous due to the plasticizing effect of
water and the organic-rich phase likely has a higher viscosity than reported.

Comment:
9) The authors show clearly that the phase state and viscosity of SOA from realistic plant emissions
cannot be predicted from a-pinene measurements. One could argue that the structure of the SQTs is too
different from a-pinene. Do comparable measurements exist for SQT derived SOA and how do they
compare to the plant SOA? Could the complex mixtures be described as a combination of e.g. a-pinene
and farnesene?

Response:
A paragraph was added to the viscosity predictions results section comparing the results of this work with
similar measurements for caryophyllene ozonolysis SOA. The fact that simulated plant mixtures produce
a higher viscosity is a surprising observation, and we are looking forward to investigating this topic
further.

Comment:
References
Buchholz, A., Lambe, A.T., Ylisirniö, A., Li, Z., Tikkanen, O.-P., Faiola, C., Kari, E., Hao, L., Luoma,
O., Huang, W., Mohr, C., Worsnop, D.R., Nizkorodov, S.A., Yli-Juuti, T., Schobesberger, S., Virtanen,
A., 2019. Insights into the O:C dependent mechanisms controlling the evaporation of a-pinene secondary
organic aerosol particles. Atmos. Chem. Phys. Discuss. 1–21. https://doi.org/10.5194/acp-2018-1305
Li, Z., Tikkanen, O.-P., Buchholz, A., Hao, L., Kari, E., Yli-Juuti, T., Virtanen, A., 2019. Effect of
Decreased Temperature on the Evaporation of α-Pinene Secondary Organic Aerosol Particles. ACS Earth
Sp. Chem. 3, 2775–2785. https://doi.org/10.1021/acsearthspacechem.9b00240
Yli-Juuti, T., Pajunoja, A., Tikkanen, O.P., Buchholz, A., Faiola, C., Väisänen, O., Hao, L., Kari, E.,
Peräkylä, O., Garmash, O., Shiraiwa, M., Ehn, M., Lehtinen, K., Virtanen, A., 2017. Factors controlling
the evaporation of secondary organic aerosol from a-pinene ozonolysis. Geophys. Res. Lett. 44, 2562–
2570. https://doi.org/10.1002/2016GL072364
Ylisirniö, A., Buchholz, A., Mohr, C., Li, Z., Barreira, L., Lambe, A., Faiola, C., Kari, E., Yli-Juuti, T.,
Nizkorodov, S., Worsnop, D., Virtanen, A., Schobesberger, S., 2019. Composition and volatility of SOA
formed from oxidation of real tree emissions compared to single VOC-systems. Atmos. Chem. Phys.
Discuss. 1–29. https://doi.org/10.5194/acp-2019-939
Zaveri, R.A., Shilling, J.E., Zelenyuk, A., Zawadowicz, M.A., Suski, K., China, S., Bell, D.M., Veghte,
D., Laskin, A., 2020. Particle-Phase Diffusion Modulates Partitioning of Semivolatile Organic
Compounds to Aged Secondary Organic Aerosol. Environ. Sci. Technol. 54, 2595–
2605. https://doi.org/10.1021/acs.est.9b05514
Zhao, D.F., Buchholz, A., Tillmann, R., Kleist, E., Wu, C., Rubach, F., Kiendler-Scharr, A., Rudich, Y.,
Wildt, J., Mentel, T.F., 2017. Environmental conditions regulate the impact of plants on cloud formation.
Nat. Commun. 8, 14067. https://doi.org/10.1038/ncomms14067

Referee: 2
Comment:

Comments to the Author
Report on manuscript “Viscosity and liquid-liquid phase separation in healthy and stressed plant SOA” by
Natalie R. Smith et al.
The authors report about molecular composition, viscosity and the range of RH for which they observe
LLPS for surrogate SOA. The SOA is produce in an environmental smog chamber from VOCs and meant
to mimic emission from Scots pine trees under healthy and stress conditions. The authors find a broader
humidity range for LLPS for the stressed plant SOA, at intermediate humidities the “stressed SOA”
exhibits about one order of magnitude larger viscosity compared to the “healthy SOA”. Estimation of
viscosity based on the molecular composition support the measurements.
This is a careful study and it fits well for Environmental Science: Atmosphere. The manuscript is well
written and supported by adequate figures. It is a detailed characterization study showing that a more
complex SOA than that derived from alpha-pinene leads to a more complex morphology and higher
viscosity. My recommendation is to publish it, but I would like the authors to consider the following
comments.
1.) Page 4, right column:
There is no discussion on mixing times for smaller (oxidant) molecules nor water. The authors point to
the fractional Stokes-Einstein equation, but do not use it to make a (probably crude) estimate. Such an
estimate could nevertheless be helpful.

Response:
The topic of mixing times of water is rather long and it would distract the reader from the main focus of
this paper. We will be present this topic in a future paper currently in preparation (A. M. Maclean, G.
Crescenzo, Y. Li, N. R. Smith, M. Shiraiwa, S. A. Nizkorodov, A. K. Bertram, Global Distribution of The Phase
State and Mixing Times Within Secondary Organic Aerosol Particles in The Troposphere Based on RoomTemperature Viscosity Measurements. For now, we have done limited estimations and included them in
the SI section as described below.

Comment:
2.) Page 6, conditioning for viscosity measurements:
As 1.5 hours seems to be sufficient to establish equilibrium even at low RH, even at 0 %RH, you may
give a lower limit for water diffusivity based on this observation. How does this compare with fractional
Stokes-Einstein estimates on water diffusivity?

Response:
For the hp-SOA and sp-SOA at 25% RH and less, we only report lower limits to viscosity measurements.
For these cases, if the water content in the particles was still decreasing at times longer than 1.5 hours, we
would not see a change in the lower limits to the viscosity measurements. Hence, we cannot be sure that
equilibrium is established with the surrounding at 1.5 hours for RH values of 25% and less. At 40 and
50% RH the data is consistent with equilibrium established in 1.5 hours. This is within uncertainty of our
calculations based on the fractional Stokes-Einstein equation, which have now been added to the revised
manuscript (new Figure S6 and text preceding it).

Comment:
3.) Results section, molecular composition: A detailed comparison between the SOA compositions with
the one of Faiola et al. (2019) would be very helpful. For me it is difficult to get an idea how the
molecular composition of the SOAs compare.

Response:
Faiola et al. (2019) only report high resolution mass spectrometry data for the gas-phase oxidation
products using an acetate-CIMS. The volatility distributions shown in that paper are gas-phase
measurements where the predicted fraction of condensed material in each volatility bin is indicated with
green bars. Those were predictions, not measurements. We agree a comparison between the SOA
composition data in these two studies would be very helpful, but unfortunately the only SOA composition
data from the Faiola et al. (2019) study is bulk SOA data from the Aerodyne, Inc. HR-ToF-AMS, which
is not comparable to the Nano-DESI-HRMS data presented in our paper.

Comment:
4.) Results section, LLPS:
I am missing a discussion on how LLPS may influence the viscosity measurement. You poke through the
outer phase first; is the hole you produce basically covered by the outer phase or does it cut through both
phases? In addition, the reader is left with the fact that the particles are phase separated, but it remains
unclear how different the physicochemical properties of the two phases are.

Response:
The main text (Experimental viscosity and mixing times results section) was updated with a note that
when calculating the viscosity, we did not consider the heterogeneity of the particle (i.e., the presence of
both organic-rich and water-rich phases). The viscosity calculations represent a combination of both
phases. The individual phases may have different viscosities than those reported here. It is expected the
inner aqueous-rich phase would be less viscous due to the plasticizing effect of water and the organic-rich
phase likely has a higher viscosity than reported.

02-Mar-2021

Dear Dr Nizkorodov:

Manuscript ID: EA-ART-12-2020-000020.R1
TITLE: Viscosity and liquid-liquid phase separation in healthy and stressed plant SOA

Thank you for submitting your revised manuscript to Environmental Science: Atmospheres. After considering the changes you have made, 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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Reviewer 2

The questions and comments raised in the original review have been adequately addressed by the authors in their revised manuscript and answers to my questions.
My recommendation is publish as is.