From the journal Environmental Science: Atmospheres Peer review history

30-Sep-2022

Dear Dr Shiraiwa:

Manuscript ID: EA-ART-08-2022-000112
TITLE: Emerging investigator series: Quantifying the impact of relative humidity on human exposure to gas phase squalene ozonolysis products

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

Reviewer 1

The authors developed kinetic models to investigate the formation and concentrations of gas-phase and condensed phase squalene ozonolysis product under different indoor environments. Particularly, the authors found that relative humidity can play a role in governing the concentrations of some major gas-phase products, which could have strong health implications through the human exposure of indoor air pollutants. I support the publication with a few minor comments.

Page 9, “Mechanism simplification was performed by systematically testing the effect of removing certain reaction pathways and products on the evolution of the concentrations of the major gas-phase products.” Can the authors discuss why certain reaction pathways could be removed from the simulations? What are the justifications and reasons behind?

Page 12, “Although most parameters were consistent between scenarios, it was necessary to slightly change the values of some parameters between different scenarios (Table S3). This most likely reflects model simplifications such as missing processes or reactions and uncertainties in some of the measurements. Missing processes may impact the modeled scenarios in different ways due to the different conditions of the experiments leading to the apparent changes in the rate coefficients.” Can the authors elaborate how to justify the changes in values (e.g. how to explain an increase or a decrease in different parameters/processes under different sceneries?)

Page 14, “Levulinic acid and succinic acid are semi-volatile, but they were forced to remain in the condensed phase and not allowed to partition into the gas phase in the model in order to reproduce their measurements, which could also indicate that there may be a missing chemical or physical process in the model. For example, we speculate that the presence of even a small amount of water could lead to the formation of the levulinate and succinate conjugate bases. These species could also absorb to glass which may preferentially keep them on the surface” This is a nice explanation. Can the authors discuss why these processes do not include or consider in their model simulations?

Page 16, “However, the opposite trend is observed in the measurements, indicating that there may be other RH- dependent physical processes such as RH-dependent partitioning to room surfaces and competitive adsorption of water and squalene ozonolysis products.66-70 For example, we speculate that the partition coefficient of hydroxyacetone precursors may change for surface films containing different concentrations of water, leading to hydroxyacetone being formed at different overall rates and increasing its gas-phase concentration at higher RH. Hydroxyacetone molecules may also be displaced by water molecules on surfaces leading to it having lower surface concentrations and higher gas-phase concentrations at higher relative RH.”

“Similarly, the measurements show that 4-oxobutanoic acid, 5-hydroxy-4- oxopentanal and levulinic acid decreased with decreasing RH, which is the opposite of the expected trend based on the chemical mechanism”

Can the authors discuss why these processes do not include or consider in their model simulations in order to better explain the results?

Page 17, “These differences in parameters may indicate missing processes in the model and may also be related to calibration issues with instrumentation as discussed earlier. It should also be noted that people emit many compounds in their breath, such as acetone and isoprene, which has not been treated in the model. Hydroxyacetone may form from the OH initiated oxidation of isoprene which should be considered in future models.” Also, the question is how the importance of these missing processes determine or affect the performance of the model simulations given the models have already well captured the results at current settings?

Reviewer 2

General Comments:
The authors developed a chemical mechanism for squalene ozonolysis and implemented it into several models to improve our mechanistic understanding of the formation of products under different RHs. They then included the mechanism in kinetic models for ozone reaction system of squalene and skin oil. They found that RH has an influence on both the chemistry and partitioning processes.

Overall, this paper is well written and contains thorough discussion. However, it may be a bit long and has 6 multi-panel large figures. The authors should check if there is word count limit. They should make the paper more succinct with only a few straight-to-point major findings.

Further, there are many optimizations, adjustment, and simplification for the model parameters, and it is unclear why and how the authors choose the parameters. It reads like they chose these parameters only because it fits better to the experimental data. For example:
What are “relatively consistent parameters” in the abstract?
At the end of page 4, why is this number (k1 = 2.2 ×10-17 cm3 s-1 in the bulk) and used? Are there any references?
On page 5, the authors made a lot of assumptions on the model. They should give clearer reasons.
Page 6 beginning, what is the difference between simplified and non-simplified mechanism?
and the outcome.
What is the uncertainty of the model? And how can the authors evaluate the errors?

The authors mention that “Levulinic acid and succinic acid are relatively well reproduced by the model (note that these were treated as non-volatile, which may indicate that they are binding with bases present in skin oil)”. What type of base is in skin oil? What is the pH of skin oil?

“Hydroxyacetone molecules may also be displaced by water molecules on surfaces leading to it having lower surface concentrations and higher gas-phase concentrations at higher relative RH. Although this has not yet been investigated experimentally or theoretically for hydroxyacetone, measurements have shown that limonene can be displaced by water from hydroxylated SiO2,”
At higher RH, the aqueous film on surfaces is thicker, and polar/water soluble species like hydroxyacetone will partition more into aqueous phase and has lower gas phase concentration. This is different from the model results.
While, limonene is nonpolar and the effect of RH is less significant, and the change of the impact of aqueous film thickness is different from polar compounds.

This text has been copied from the PDF response to reviewers and does not include any figures, images or special characters.

Referee: 1
Comments to the Author
The authors developed kinetic models to investigate the formation and concentrations of gas-phase and
condensed phase squalene ozonolysis product under different indoor environments. Particularly, the authors
found that relative humidity can play a role in governing the concentrations of some major gas-phase
products, which could have strong health implications through the human exposure of indoor air pollutants.
I support the publication with a few minor comments.
We thank the reviewer for the positive evaluation of our manuscript, and we address their comments below.
Page 9, “Mechanism simplification was performed by systematically testing the effect of removing certain
reaction pathways and products on the evolution of the concentrations of the major gas-phase products.”
Can the authors discuss why certain reaction pathways could be removed from the simulations? What are
the justifications and reasons behind?
As noted in the manuscript, mechanism simplification was required when including it in the KM-SUBSkin-Clothing model as including all reactions would have been too computationally expensive. Reactions
were systematically tested to check whether their removal would have a large impact on the concentrations
of gas-phase species. If a certain reaction had a negligible or small impact on the concentrations of gasphase species, it was removed. There are three main reasons why reactions had a negligible impact on gasphase concentrations of species. Firstly, the rate of a reaction may be slow compared to other reactions.
This could occur due to small rate coefficients or low concentrations of the reactants. Secondly, the reaction
could mainly lead to non-volatile and non-reactive products which we were not of interest in the KM-SUBSkin-Clothing model at this time. Thirdly, reactions where the yield of volatile products was low, for
example due to low branching ratios, meant that these products were only formed in small quantities and
that these reactions could therefore be removed.
The following text has been added to the manuscript:
“Any reactions that only had a negligible or very small impact on gas-phase products were removed from
the mechanism. Reactions may not impact the concentration of gas-phase species significantly if their rate
is slow compared to other reactions due to small rate coefficients or low concentrations of reactants. Some
reactions that only lead to non-volatile and non-reactive products were also removed. The yield of volatile
products from certain reactions was low, allowing these reactions to be removed without impacting their
gas-phase concentrations.”
Page 12, “Although most parameters were consistent between scenarios, it was necessary to slightly change
the values of some parameters between different scenarios (Table S3). This most likely reflects model
simplifications such as missing processes or reactions and uncertainties in some of the measurements.
Missing processes may impact the modeled scenarios in different ways due to the different conditions of
the experiments leading to the apparent changes in the rate coefficients.” Can the authors elaborate how to
justify the changes in values (e.g. how to explain an increase or a decrease in different parameters/processes
under different sceneries?)
The example that we currently give in the manuscript as a potential missing process is the reversible and
non-reversible loss of species at the chamber wall or on flow tube surfaces. Wall losses of a compound may
vary significantly depending on the surface-to-volume ratio in the chamber or flow tube, the composition
of the surface and any species already deposited on the surface that could bind with or react with the
compound, the porosity of the surface, the thickness of any grime on the surface and the partitioning of the
compound into it, the flow conditions in the chamber or flow tube, and environmental factors such as
temperature and relative humidity. If there is a wall loss of certain compounds, it is likely to be different
for different scenarios due to the factors mentioned above. By not including wall losses in the model, the
reported product formation rates are effectively lowered to compensate for the additional loss term. The
extent of this effect is different in each model scenario. Wall losses have not been included as they are
currently totally unconstrained and it should be noted that there may be other missing processes and
reactions in the model.
The following text has been added to the manuscript:
“There are several factors that could influence these losses including the surface-to-volume ratio, the
composition and reactivity of the surface, the porosity of the surface and the flow conditions. By not
including wall losses in the model, the reported product formation rates are effectively lowered to
compensate for the additional loss term. The extent of this effect is different in each model scenario.”
Page 14, “Levulinic acid and succinic acid are semi-volatile, but they were forced to remain in the
condensed phase and not allowed to partition into the gas phase in the model in order to reproduce their
measurements, which could also indicate that there may be a missing chemical or physical process in the
model. For example, we speculate that the presence of even a small amount of water could lead to the
formation of the levulinate and succinate conjugate bases. These species could also absorb to glass which
may preferentially keep them on the surface” This is a nice explanation. Can the authors discuss why these
processes do not include or consider in their model simulations?
We did not include these processes in the model as they are purely speculative. We are unsure as to whether
any water is present, and we can also only speculate as to whether levulinic acid and succinic acid can bind
strongly enough to glass to allow a signal to be observed. It is possible that there could also be another
explanation for the observed signals and we do not want to include a complex process in our model which
is speculative at this point.
The following text was added to the manuscript.
“However, due to uncertainty regarding whether a sufficient amount of water was present in the system
and the binding strengths of these acids to glass, we have not tried to model these processes explicitly.”
Page 16, “However, the opposite trend is observed in the measurements, indicating that there may be other
RH- dependent physical processes such as RH-dependent partitioning to room surfaces and competitive
adsorption of water and squalene ozonolysis products.66-70 For example, we speculate that the partition
coefficient of hydroxyacetone precursors may change for surface films containing different concentrations
of water, leading to hydroxyacetone being formed at different overall rates and increasing its gas-phase
concentration at higher RH. Hydroxyacetone molecules may also be displaced by water molecules on
surfaces leading to it having lower surface concentrations and higher gas-phase concentrations at higher
relative RH.”
“Similarly, the measurements show that 4-oxobutanoic acid, 5-hydroxy-4- oxopentanal and levulinic acid
decreased with decreasing RH, which is the opposite of the expected trend based on the chemical
mechanism”
Can the authors discuss why these processes do not include or consider in their model simulations in order
to better explain the results?
We did not include any of these processes in the model as we are uncertain as to which process is causing
the measured trend and there are also too many uncertainties associated with each of the processes. Further
studies are required to identify the most important processes and to determine parameters that could be used
in the model.
The following text has been added to the manuscript:
“We have not tried to explicitly test any of these hypotheses in the model due to too many uncertainties
being associated with them. Further studies are required to identify important processes and to determine
parameters that could be used in the model.”
Page 17, “These differences in parameters may indicate missing processes in the model and may also be
related to calibration issues with instrumentation as discussed earlier. It should also be noted that people
emit many compounds in their breath, such as acetone and isoprene, which has not been treated in the model.
Hydroxyacetone may form from the OH initiated oxidation of isoprene which should be considered in future
models.” Also, the question is how the importance of these missing processes determine or affect the
performance of the model simulations given the models have already well captured the results at current
settings?
Slightly different parameters have been used to fit the measurements in the different figures. At this stage
it is unclear as to whether the different parameters were due to calibration issues or to missing processes in
the model. If we determine an important missing process in the future, then we would also require it to be
constrained using experimental data.
Acetone, hydroxyacetone and isoprene were species which were not measured in Figure S5, which is also
the only figure where experiments were performed in the presence of people and so we were unable to
determine the potential impact of breath emissions on their concentrations. There may be physiological
reasons that we are unaware of which could cause breath emissions to be dependent on relative humidity.
However, this is beyond the scope of this work and should be investigated in the future. The impact of
relative humidity on OH concentrations is currently unknown and should be investigated as it may impact
the formation of species such as hydroxyacetone from its reaction with isoprene.
The following text has been added to the manuscript:
“It is currently unclear how the OH radical concentrations will vary as a function of RH and this should
be investigated in future work.”
Referee: 2
The authors developed a chemical mechanism for squalene ozonolysis and implemented it into several
models to improve our mechanistic understanding of the formation of products under different RHs. They
then included the mechanism in kinetic models for ozone reaction system of squalene and skin oil. They
found that RH has an influence on both the chemistry and partitioning processes. Overall, this paper is well
written and contains thorough discussion. However, it may be a bit long and has 6 multi-panel large figures.
The authors should check if there is word count limit. They should make the paper more succinct with only
a few straight-to-point major findings.
We thank the reviewer for the positive evaluation of our manuscript. There is no word count limit for this
journal and we are keen to show the range of species that were modeled in this work, which requires figures
with many panels with thorough discussions. We address other comments below:
Further, there are many optimizations, adjustment, and simplification for the model parameters, and it is
unclear why and how the authors choose the parameters. It reads like they chose these parameters only
because it fits better to the experimental data. For example: What are “relatively consistent parameters” in
the abstract?
All parameters are shown in Table S3 and all rate coefficients are within two orders of magnitude for the
different modeled scenarios, with most being much closer or identical. Except for the partitioning
coefficient of geranyl acetone into clothing and the rate coefficients, all other parameters are the same
between scenarios. We believe that this makes the parameters “relatively consistent”. We agree with the
reviewer that we have changed parameters because it fits better to the experimental data. As stated in the
manuscript, fitted parameters were determined by varying their values over a range which incorporated any
published values. We have made sure to show the impact of using the same parameter set for all
measurements in Figures S4 and S6 and we have discussed potential uncertainties that could be leading to
these differences, such as losses to non-human surfaces or uncertainties in the measurements caused by
calibration issues. This study is a first attempt at modeling and better understanding the complex processes
controlling gas-phase concentrations of squalene ozonolysis products as a function of relative humidity.
Additional processes and reactions may need to be added in the future which should hopefully make
parameters more consistent between different scenarios.
The following text has been added to the manuscript:
“Our study is a first attempt at modeling and better understanding the complex processes controlling gasphase concentrations of squalene ozonolysis products as a function of relative humidity. All rate coefficients
were within two orders of magnitude among different scenarios with most being much closer or identical
making them relatively consistent among scenarios. Additional processes and reactions could be added in
the future as they become better understood, which would ideally make parameters fully consistent among
different scenarios.
At the end of page 4, why is this number (k1 = 2.2 ×10-17 cm3 s-1 in the bulk) and used? Are there any
references?
There are several reasons that a value of 2.2 ´ 10-17 cm3 s
-1 has been used in this model. Firstly, this value
was previously obtained when optimizing KM-SUB-Skin and KM-SUB-Skin-Clothing by reproducing
experimental data. Secondly, as noted in Table S2, this value is a factor of 10 lower than the value used in
Heine et al.1 which is based on work reported in Razumovskii and Lisitsyn2 and is the only measured rate
coefficient of squalene with ozone as far as we are aware. However, this rate coefficient was measured in
chloroform, and it is possible that it would be different in other substrates such as pure squalene and skin
oil. Finally, as noted in Table S2, this rate coefficient and the partitioning coefficient of ozone were
codependent,so the rate coefficient could have been increased by a factor of 10 if the partitioning coefficient
had been decreased by a factor of 10. However, this would have caused a larger discrepancy between the
value of KO3 used in the model and the value determined by molecular dynamics simulations. We tried to
remain within approximately a factor of 5 of the calculated KO3 value. Note that as more measurements or
theoretical calculations of the rate coefficient of squalene with ozone and the partitioning coefficient of
ozone into squalene and skin oil become available, we will be able to update the model and ideally resolve
the apparent discrepancy between this rate coefficient and the partitioning coefficient of ozone.
The text in Table S2 has been modified as follows:
“Same as Lakey et al.3, 4 where measurements were able to be reproduced reasonably well using this value.
Note that this value is about a factor of 10 smaller than used in Heine et al.1 which is based on work
reported in Razumovskii and Lisitsyn2 and is the only measured rate coefficient of squalene with ozone to
our knowledge. However, the rate coefficient was measured in chloroform, and it is possible that it would
be different in other substrates such as pure squalene and skin oil. Increasing the rate coefficient caused a
larger discrepancy between the value of KO3 used in the model and the value determined by molecular
dynamics simulations (see below). k1,bulk and KO3 were codependent. Note that as more measurements or
theoretical calculations of the rate coefficient of squalene with ozone and the partitioning coefficient of
ozone into squalene and skin oil become available, we will be able to update the model and ideally resolve
the apparent discrepancy between this rate coefficient and the partitioning coefficient of ozone.”
On page 5, the authors made a lot of assumptions on the model. They should give clearer reasons.
Unless specifically stated in the text, most assumptions and simplifications in the model are made to not
overcomplicate the model that would make it too computationally expensive to run or because we are
lacking information regarding certain reactions or processes and are unable to constrain them. For example,
we assume that all carbon-carbon double bonds react at the same rate as we do not have information
regarding the specific rate coefficient of each individual double bond. We also do not treat potential reaction
pathways if we do not see clear evidence of species being formed from them and to reduce the number of
reactions in the model, as it would be too computationally expensive to treat all possible reactions.
Additionally, we do not treat interfacial reactions as we want to keep the model as simple as possible by
reducing the number of unconstrained parameters and to make sure that the model is not too
computationally expensive to run.
We have added the following text to the manuscript:
“Assumptions and simplifications were applied to increase computational efficiency and also because some
mechanisms are so poorly understood that they cannot be constrained with confidence.”
Page 6 beginning, what is the difference between simplified and non-simplified mechanism? and the
outcome.
The simplified mechanism is shown in Table S1 and was used in the KM-SUB-Skin-Clothing model as it
was too computationally expensive to include the entire mechanism in this more complex model. The
simplified mechanism was obtained by systematically testing the impact of each reaction on the gas-phase
concentrations of volatile and semi-volatile compounds. Any reactions that had only a negligible or very
small impact on these concentrations were removed. Therefore, we don’t expect that using the simplified
mechanism will impact our conclusions, as the main focus of this manuscript is on gas-phase species. Note
that the simplified mechanism would not be able to predict the concentrations of species that remain in the
bulk.
The following text was added to the manuscript:
“Any reactions that only had a negligible or very small impact on gas-phase products were removed from
the mechanism.”
“We do not expect that using the simplified mechanism will impact any of our conclusions as the focus of
this study is simulating gas-phase species.”
What is the uncertainty of the model? And how can the authors evaluate the errors?
It is difficult to assess the uncertainty of the models as there may be uncertainties in parameters in the
models as well as missing reactions and processes. It is difficult at this point to identify missing processes
and reactions that may be important without experimental evidence. Even if potential missing processes are
identified, it is difficult to constrain parameters that are used to describe them in the model. In addition,
there may be uncertainties in the measurements due to calibration issues. We have presented some
sensitivity tests showing the impact of using the same parameter sets for the different data sets (Figures S4
and S6) which should give a sense of some of the uncertainty in the model. However, we have made sure
throughout the manuscript to include notes concerning uncertainties in the model. We hope that in the future,
as missing processes and parameters associated with them become clear, our current models will be able to
be adapted to incorporate these.
The authors mention that “Levulinic acid and succinic acid are relatively well reproduced by the model
(note that these were treated as non-volatile, which may indicate that they are binding with bases present in
skin oil)”. What type of base is in skin oil? What is the pH of skin oil?
Although skin oil tends to be slightly acidic, studies have shown that amines can be present in skin.5 We
can speculate that if amines are present then they could potentially bind with levulinic acid and succinic
acid. However, as mentioned in the text there may also be other explanations for the observed levulinic and
succinic acid signals. For example, skin-surface pH is reported to be in the range of 4.1-5.8 with a mean of
4.9.6 The pKa values of levulinic and succinic acids are below the mean (4.6 and 4.2)7, 8 pH suggesting that,
if enough water is present, then much or most of these acids can be present in their non-volatile, conjugate
base form.
The following text has been modified as follows in the manuscript:
“Levulinic acid and succinic acid are relatively well reproduced by the model (note that these were treated
as non-volatile, which may indicate that they are binding to glass surfaces in the experimental setup or with
bases such as amines which may be present in skin oil5 or that there may be a small amount of water present
in the system leading to the formation of the levulinate and succinate conjugate bases) (Fig. 4b-c). Skinsurface pH is reported to be in the range of 4.1-5.8 with a mean of 4.9.6 The pKa values of levulinic and
succinic acids are below the mean (4.6 and 4.2)7, 8 pH suggesting that, if enough water is present, then
much or most of these acids can be present in their non-volatile, conjugate base form.”
“Hydroxyacetone molecules may also be displaced by water molecules on surfaces leading to it having
lower surface concentrations and higher gas-phase concentrations at higher relative RH. Although this has
not yet been investigated experimentally or theoretically for hydroxyacetone, measurements have shown
that limonene can be displaced by water from hydroxylated SiO2,” At higher RH, the aqueous film on
surfaces is thicker, and polar/water soluble species like hydroxyacetone will partition more into aqueous
phase and has lower gas phase concentration. This is different from the model results. While, limonene is
nonpolar and the effect of RH is less significant, and the change of the impact of aqueous film thickness is
different from polar compounds.
For hydroxyacetone the experimental measurements show that at higher relative humidity there is more
hydroxyacetone in the gas phase. If the dominant process that was happening was partitioning of
hydroxyacetone into water films on surfaces at high relative humidity, then we would expect more
hydroxyacetone on the surfaces and less hydroxyacetone in the gas phase at high relative humidities, which
is the opposite of what was observed in the measurements. Therefore, there must be a mechanism which
reduces hydroxyacetone on surfaces or alternatively leads to more hydroxyacetone formation or less
destruction at high relative humidities. We currently don’t have such a process in the model, and we are
only able to speculate regarding the cause of this behavior. Systematic experiments on different types of
surfaces may help elucidate the cause of this behavior.
The following text has been added to the manuscript:
“Hydroxyacetone is polar and water soluble, so we may expect more partitioning to surfaces covered in
aqueous films under high RH conditions. However, this would increase hydroxyacetone surface
concentrations and reduce gas-phase concentrations upon an increase of RH. The measurements show the
opposite trend, suggesting that this is not the dominant process that occurs.”
The following text has also been modified:
“Future studies should investigate whether hydroxyacetone can be displaced by water molecules on
surfaces leading to it having lower surface concentrations and higher gas-phase concentrations at higher
relative RH on different indoor relevant surfaces.”
References
1. N. Heine, F. A. Houle and K. R. Wilson, Connecting the elementary reaction pathways of criegee
intermediates to the chemical erosion of squalene interfaces during ozonolysis, Environ. Sci.
Tech., 2017, 51, 13740-13748.
2. S. Razumovskii and D. Lisitsyn, Reactions of ozone with double bonds in polymer and biosystem
chemistry, Polymer Science Series A, 2008, 50, 1187-1197.
3. P. S. J. Lakey, G. C. Morrison, Y. Won, K. M. Parry, M. von Domaros, D. J. Tobias, D. Rim and
M. Shiraiwa, The impact of clothing on ozone and squalene ozonolysis products in indoor
environments, Commun. Chem., 2019, 2, 56.
4. P. S. J. Lakey, A. Wisthaler, T. Berkemeier, T. Mikoviny, U. Pöschl and M. Shiraiwa, Chemical
kinetics of multiphase reactions between ozone and human skin lipids: implications for indoor air
quality and health effects, Indoor Air, 2017, 27, 816-828.
5. A. T. Slominski, M. A. Zmijewski, C. Skobowiat, B. Zbytek, R. M. Slominski and J. D. Steketee,
in Sensing the Environment: Regulation of Local and Global Homeostasis by the Skin's
Neuroendocrine System, Springer, 2012, pp. 7-26.
6. E. Proksch, pH in nature, humans and skin, The Journal of dermatology, 2018, 45, 1044-1052.
7. G. Kortüm, W. Vogel and Andrussow, Disssociation constants of organic acids in aqueous
solution, Pure Appl. Chem., 1960, 1, 187-536.
8. J. A. Dean, Handbook of organic chemistry, McGraw-Hill New York, New York, NY, 1987.

21-Nov-2022

Dear Dr Shiraiwa:

Manuscript ID: EA-ART-08-2022-000112.R1
TITLE: Emerging investigator series: Quantifying the impact of relative humidity on human exposure to gas phase squalene ozonolysis products

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Reviewer 1

The authors have thoroughly addressed the reviewers' comments. I strongly support the publication of this work. The revision is ready for publication in its current form.

Reviewer 2

The authors have carefully addressed the previous comments. The paper is much improved and can be accepted.