From the journal Environmental Science: Atmospheres Peer review history

19-Oct-2022

Dear Dr Krieger:

Manuscript ID: EA-ART-09-2022-000127
TITLE: Oxidation pathways of linoleic acid revisited with electrodynamic balance–mass spectrometry

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Reviewer 1

General Comments: Muller et al. report the experimental investigation on ozonolysis of linoleic acid, a commonly used as a proxy for organic compounds found in atmospheric aerosol particles. By using an electrodynamic balance–mass spectrometry, they measured mass spectra from levitated droplets before and after exposure to specific ozone mixing ratios. They found almost identical reactive uptake coefficients for ozone mixing ratios from 1 to 10 ppm, while a strong increase of the uptake coefficient for lower ozone mixing ratios was observed, which they attributed to the oxidation of linoleic acid with molecular oxygen (i.e., autoxidation). The present study is important for understanding the ageing process of atmospheric aerosol particles. However, I found several fundamental issues that should be addressed before publication.

Specific Comments:
 My main concern comes from the proposed ozonolysis mechanism. A single droplet contains 10 wt% linoleic acid in pure methanol (Lines 65-66). In the presence of methanol at >M levels (where [methanol] >> [linoleic acid]), Criegee intermediates (CIs) should have reacted not only with linoleic acid but also with methanol. See Enami et al., J. Phys. Chem. A 2017, 121, 5175, Hu et al., Phys. Chem. Chem. Phys., 2021, 23, 4605 for example. I suspect that some peaks appearing at m/z < 279 would have originated from the decomposition products of alpha-alkoxyalkyl-hydroperoxides derived from CIs + methanol. Therefore, I strongly recommend to re-analyze the data by considering this secondary reaction.
 Lines 81-83, note that a discharge-type ozone generator with air should produce NOx. I suspect the observed kinetics (e.g., Fig. 3) was influenced by the presence of NOx. NOx may have slow-downed the decay of oleic acid by scavenging reactive intermediate species. The authors should evaluate [NOx] in the chamber and possible influence on the kinetics.
 Lines 219-222, note that air alone contains ambient O3. Did the authors check what happens if a droplet is exposed to pure N2 (or He, Ar)? Did the linoleic acid mass signal remain constant under inert gas? Is it possible that linoleic acid is evaporated with methanol by forming a complex (or cluster) to the gas-phase? The authors should comment on the issue.
 Lines 230-236, as mentioned above, these CIs may have been largely consumed by the reactions with methanol.
 Fig. 5c, how alkyl radical (R) is formed? Why R cannot be scavenged by O2? Is it possible metal ions at a trace level initiate the observed autoxidation? The authors should comment on these issues.

Reviewer 2

This manuscript presents experimental observations of linoleic acid oxidation by ozone, and air, in an electrodynamic balance coupled to mass spectrometry. These are challenging measurements, and have the benefit of the ability to attain long reaction times. Despite the associated uncertainties in the measurements, the authors are able to draw some meaningful kinetic conclusions, and are able to demonstrate that an autoxidation mechanism competes with ozonolysis at low (atmospherically relevant) ozone concentrations.
It appears that the new method for single droplet mass spectrometry, briefly described in Section 2, is presented in an unpublished paper that was not supplied with the manuscript under review. However, the authors do demonstrate a linear response of integrated MS signal as a function of droplet volume (within some not negligible uncertainties).
The manuscript is very clearly written and logically organized.
Minor comments follow below.

Specific Comments:

Abstract:
-state specific values that correspond to "lower ozone mixing ratios"?
-be more specific about how/why this increase in uptake coefficient is attributed to oxidation by molecular O2? Competitive adsorption to the surface?

L81-83: Type of ozone generator? Is significant NOx being produced?

Figure 3: It would be helpful to make clear in this figure caption that each data point arises from an individual droplet trapped in the EDB and exposed to ozone for a specific period of time, and it is (presumably) therefore not possible to derive error bars at every ozone exposure (though it does look like some exposures were repeated). However, in Appendix Figure 8, y-error bars represent a 30% uncertainty in droplet volume -- perhaps the authors should be consistent throughout the paper in how they treat uncertainties in decay kinetics?

L210-211: Presumably this is zero air and not e.g., room air? The methods section indicates use of an in-house supply of compressed air -- is this supply scrubbed of oxidants before use? Measured ozone concentration in this air?

L209-222: More discussion about what drives the induction period, before autoxidation takes off, would be very helpful in this portion of the paper. What is the underlying chemical reason for this delay period before autoxidation begins? What is initiating the radical chain cycling in zero air?

L267-291: Do the authors suggest that the CI decomposes to form the vinyl alkoxy and OH in the condensed phase? Is this known to occur in the condensed phase? Or would this rather arise from formation (and decomposition) of an organic hydroperoxide, via reaction with an acid group?
Some further discussion on the genesis of these radical reaction pathways would be helpful for the reader.

Appendix Fig 7: What type of linear regression is used in the calibration curve?

Dear Dr. Wang

Thank you very much for your evaluation and for sending us the reviewers reports.
We have now run additional experiments and edited our manuscript, which you will find attached.
You will find below our answers to the individual questions posed by the reviewers.

Kind regards,
Marcel Müller


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We would like to thank both referees for their valuable input. We provide answers to the questions and comments directly below the individual points. These answers include passages from the draft shown with quotation marks.


REVIEWER REPORT(S):
Referee: 1

Comments to the Author
General Comments: Muller et al. report the experimental investigation on ozonolysis of linoleic acid, a commonly used as a proxy for organic compounds found in atmospheric aerosol particles. By using an electrodynamic balance–mass spectrometry, they measured mass spectra from levitated droplets before and after exposure to specific ozone mixing ratios. They found almost identical reactive uptake coefficients for ozone mixing ratios from 1 to 10 ppm, while a strong increase of the uptake coefficient for lower ozone mixing ratios was observed, which they attributed to the oxidation of linoleic acid with molecular oxygen (i.e., autoxidation). The present study is important for understanding the ageing process of atmospheric aerosol particles. However, I found several fundamental issues that should be addressed before publication.

Specific Comments:

 My main concern comes from the proposed ozonolysis mechanism. A single droplet contains 10 wt% linoleic acid in pure methanol (Lines 65-66). In the presence of methanol at >M levels (where [methanol] >> [linoleic acid]), Criegee intermediates (CIs) should have reacted not only with linoleic acid but also with methanol. See Enami et al., J. Phys. Chem. A 2017, 121, 5175, Hu et al., Phys. Chem. Chem. Phys., 2021, 23, 4605 for example. I suspect that some peaks appearing at m/z < 279 would have originated from the decomposition products of alpha-alkoxyalkyl-hydroperoxides derived from CIs + methanol. Therefore, I strongly recommend to re-analyze the data by considering this secondary reaction.

We apologize for the misunderstanding. We assume that the methanol evaporates from the droplet long before we start the actual experiments. The statement on lines 62-64 in the manuscript has been edited as follows:
"A detailed description of the instrumentation and the typical experimental procedure can be found elsewhere.35"
(citing Müller et al. 2022). Therein, we explain that we inject droplets into a nitrogen atmosphere and maintain a flow of nitrogen through the trap until the start of the experiment, which is typically tens of minutes after the injection of the droplet. This means that all methanol should have evaporated before the experiment is started. We could not find a difference in the measured spectra when using the structurally similar oleic acid in different alcoholic solvents with high vapour pressures, indicating that the applied solvent is of negligible influence. We added the following text (lines 73-77 in the manuscript) for clarity:
"Once injected, the droplets are stored for at least 15 minutes in a nitrogen atmosphere before being exposed to oxidants. During this time, all methanol evaporates from the droplet, which is indicated by reaching a steady voltage required to counteract gravitation."


 Lines 81-83, note that a discharge-type ozone generator with air should produce NOx. I suspect the observed kinetics (e.g., Fig. 3) was influenced by the presence of NOx. NOx may have slow-downed the decay of oleic acid by scavenging reactive intermediate species. The authors should evaluate [NOx] in the chamber and possible influence on the kinetics.

We agree with the reviewer that there is a potential influence of NOx on the reaction outcome. We now measured the NOx levels in the outflow of our ozone generators and from our in-house gas supply. Therefore, we included additional information about our ozone generators in the instrument section (lines 86-96):
"For measurements at ozone mixing ratios ≤ 1 ppm, a calibration ozone generator (Photometric O3 Calibrator - Model 401, Advanced Pollution Instrumentation, USA) was used, otherwise an ambient ozone simulator (AOS 2 with bypassed pump, BMT Messtechnik, Germany) was used. Both ozone generators use UV-light for generating ozone and, hence, we expect no significant NOx production during ozone generation. However, while the generator used for low ozone mixing ratios uses a built-in zero air generator, the one used for higher mixing ratios uses the untreated pressurised air supply."
We then also added the findings (line 98-104):
"We measured the concentrations of NOx in the air before and after ozone production using a chemiluminescence NOx sensor (Model 200a, Advanced Pollution Instrumentation, USA). For all experiments with ozone mixing ratios ≤ 1 ppm, NOx levels were below detection limit (1 ppb), whereas at all other measurements the NOx levels were those of outside air (typically below 20 ppb)."


 Lines 219-222, note that air alone contains ambient O3. Did the authors check what happens if a droplet is exposed to pure N2 (or He, Ar)? Did the linoleic acid mass signal remain constant under inert gas? Is it possible that linoleic acid is evaporated with methanol by forming a complex (or cluster) to the gas-phase? The authors should comment on the issue.

We thank the referee for this input. We now measured the ozone concentration present in the supplied air with an ozone measurement instrument (Ozone Monitor BMT 932) with 1 ppb sensitivity. No ozone could be detected in the air supply. We also added this information to the manuscript (lines 104-106):
"Ozone levels in the in-house supplied air were below detection limits (1 ppb, measured with Ozone Monitor BMT 932)."
To check for the linoleic acid signal after levitating droplets in a nitrogen atmosphere, we measured two droplets levitated for ≈ 2.75 days in a nitrogen atmosphere. The recorded two-dimensional scattering pattern does not change during this levitation period. Also, we don’t see any long-term trends in the recorded voltage data, which is required to compensate gravitation This voltage would be expected to change if the droplet size evolve. Furthermore, the recorded mass spectra of these droplets are not significantly different from the inferred initial droplet signal obtained in experiments from droplets stored only shortly in a nitrogen atmosphere. From the vapour pressure (7.98 x 10-6 Pa, UManSysProp), we do not expect linoleic acid to partition to the gas phase in the investigated timescales. We added the following statement to the manuscript for clarity (lines 246-249):
"In contrast, the composition of droplets stored in a nitrogen atmosphere for comparable times did not show any measurable deviation in the composition in comparison to unreacted droplets, nor any measurable size change."


 Lines 230-236, as mentioned above, these CIs may have been largely consumed by the reactions with methanol.

Again, we know there is no significant amount of methanol left in the droplet at the time of ozone exposure (see answer to first point). We hope the suggested changes to the draft make this clear now.


 Fig. 5c, how alkyl radical (R) is formed? Why R cannot be scavenged by O2? Is it possible metal ions at a trace level initiate the observed autoxidation? The authors should comment on these issues.

We agree with the reviewer that this question is not answered in our study. The complexity of the issue has been described for example by Pryor et al, mentioning that trace impurities can have a large effect. We estimate that impurities and traces of reactive gases in the used gas flows may lead to the formation of some initial radicals and that impurities in the droplet could potentially act as catalysts or provide pathways towards the production of radicals. Also, products from ozonolysis reactions, such as Criegee intermediates are linked with radical formation. R can of course be scavenged by O2 (see Fig. 5), however, it does not matter much, which radical initiates the autocatalytic cycle. We edited the following section (lines 373-380):
"The reason for the slow acceleration of the linoleic acid degradation reaction (i.e. the radical initiation) in our experiments is not known, but it could be caused by traces of reactive gases in our compressed gas supply or caused by impurities in the droplet phase. Pryor et al.51 stated that the measured autoxidation rates could be strongly affected by trace impurities and Frankel et al.52 emphasised that traces of metals can accelerate rates of lipid oxidation."
Later on, we added the following paragraph to put into context the shortened induction period (lines 414-419):
"It has been proposed in the literature that the induction period in autoxidation reactions may be influenced by byproducts from ozonolysis reaction.21, 51 For example, Criegee intermediates have been found to lead to OH production in gas-phase reactions,53, 54 and similar reactions have been proposed to occur in the particle phase.55"


************
Referee: 2

Comments to the Author
This manuscript presents experimental observations of linoleic acid oxidation by ozone, and air, in an electrodynamic balance coupled to mass spectrometry. These are challenging measurements, and have the benefit of the ability to attain long reaction times. Despite the associated uncertainties in the measurements, the authors are able to draw some meaningful kinetic conclusions, and are able to demonstrate that an autoxidation mechanism competes with ozonolysis at low (atmospherically relevant) ozone concentrations. It appears that the new method for single droplet mass spectrometry, briefly described in Section 2, is presented in an unpublished paper that was not supplied with the manuscript under review. However, the authors do demonstrate a linear response of integrated MS signal as a function of droplet volume (within some not negligible uncertainties). The manuscript is very clearly written and logically organized.
Minor comments follow below.

We are sorry to hear that the manuscript of the back then unpublished work was not forwarded to referee 2. The paper is now published (Müller et al. 2022, https://doi.org/10.1039/D2CP03289A).


Specific Comments:

Abstract:
-state specific values that correspond to "lower ozone mixing ratios"?

We edited the corresponding sentence for more clarity:
"However, a strong increase of the uptake coefficient for ozone mixing ratios below approximately 0.2 ppm is observed."


-be more specific about how/why this increase in uptake coefficient is attributed to oxidation by molecular O2? Competitive adsorption to the surface?

The corresponding passage was edited and reads now:
"Based on measurements using an oxygen atmosphere without ozone, which also show a degradation of linoleic acid, we attribute the apparent increase of the uptake coefficient to the oxidation of linoleic acid with molecular oxygen."


L81-83: Type of ozone generator? Is significant NOx being produced?

This question was also brought up by referee 1 (second point). In brief, we measured the NOx levels in the outflow of our ozone generators and from our in-house gas supply and added the information in the instrument section.


Figure 3: It would be helpful to make clear in this figure caption that each data point arises from an individual droplet trapped in the EDB and exposed to ozone for a specific period of time, and it is (presumably) therefore not possible to derive error bars at every ozone exposure (though it does look like some exposures were repeated). However, in Appendix Figure 8, y-error bars represent a 30% uncertainty in droplet volume -- perhaps the authors should be consistent throughout the paper in how they treat uncertainties in decay kinetics?

We appreciate the hint that the figure caption was not clear enough and made the following changes:
"Each data point corresponds to an individual droplet that has been exposed to a specific gas phase for a certain time. Uncertainties on the single data points are depicted in Fig. 8 in the appendix and are left out here for clarity. The axis on the right gives the corresponding number of molecules based on an initial droplet of 25 μm radius. Exponential curves are based on point estimates from exponential fits taking into account the uncertainty in droplet size."
Consequently, we also adapted the caption of figure 9 by adding:
"For clarity, error bars from uncertainties in droplet volume estimation are left out."


L210-211: Presumably this is zero air and not e.g., room air? The methods section indicates use of an in-house supply of compressed air -- is this supply scrubbed of oxidants before use? Measured ozone concentration in this air?

This question was also brought up by referee 1 (see question 3). In brief, we measured the ozone concentration present in the supplied air and no ozone could be detected. We also added this information to the manuscript.


L209-222: More discussion about what drives the induction period, before autoxidation takes off, would be very helpful in this portion of the paper. What is the underlying chemical reason for this delay period before autoxidation begins? What is initiating the radical chain cycling in zero air?

We thank for this input and we tried to incorporate some thoughts about the mechanism. Unfortunately, with our current instrumentation, especially due to thermal fragmentation and unit mass resolution, mechanistic investigations are limited. We modified the results section (lines 232-245) as follows:
"The concentration seems to remain relatively constant for a duration of approximately 40 h, after which a degradation reaction gains traction and the linoleic acid concentration starts to decay rapidly.
After five days of exposure, no linoleic acid was detected anymore in the droplet (< 1 % compared to the intensity for unreacted linoleic acid droplets).
This behaviour could be explained with an autocatalytic reaction that is speeding up during the course of the exposure and only slows down due to the decrease in available starting material.
An approximation for the decay after 40 h was obtained by fitting an exponential curve to the data resulting in an e-folding lifetime of 29 ± 7 h, which is the same order of magnitude as in experiments with a low ozone mixing ratio (0.03 ppm) of 18 ± 5 h."
Furthermore, we adapted the discussion section by deleting the single sentence in the discussion section:
"Radical initiation may be accelerated by radicals formed as byproducts from ozonolysis reactions.21, 51"
Instead, we added the following to the discussion section when describing the linoleic acid autoxidation rate (lines 373-380):
"The reason for the slow acceleration of the linoleic acid degradation reaction (i.e. the radical initiation) in our experiments is not known, but it could be caused by traces of reactive gases in our compressed gas supply or caused by impurities in the droplet phase. Pryor et al.51 stated that the measured autoxidation rates could be strongly affected by trace impurities and Frankel et al.52 emphasised how traces of metals can accelerate rates of lipid oxidation."


L267-291: Do the authors suggest that the CI decomposes to form the vinyl alkoxy and OH in the condensed phase? Is this known to occur in the condensed phase? Or would this rather arise from formation (and decomposition) of an organic hydroperoxide, via reaction with an acid group?
Some further discussion on the genesis of these radical reaction pathways would be helpful for the reader.

Again, mechanistic considerations are limited for instrumentation reasons. In principle, the production of radicals via ozonolysis could explain the apparent increase in the uptake coefficient for low ozone mixing ratios (<= 200 ppb) and indeed, there is evidence for the mentioned decomposition reaction to take place in the condensed form (Zeng et al.). We included this discussion on lines 414-419 in our revised draft:
"It has been proposed in the literature that the induction period in autoxidation reactions may be influenced by byproducts from ozonolysis reactions.21,51 For example, Criegee intermediates have been found to lead to OH production in gas-phase reactions,53,54 and similar reactions have been proposed to occur in the particle phase."


Appendix Fig 7: What type of linear regression is used in the calibration curve?

A linear regression through the unweighted data points and through zero was applied. We edited the manuscript to specify this:
"Calibration curve with 95 % confidence interval from linoleic acid droplets with different sizes. A linear regression through unweighted data points was applied and forced through the origin."


References:

Müller, A. Mishra, T. Berkemeier, E. Hausammann, T. Peter and U. K. Krieger, Electrodynamic balance–mass spectrometry reveals impact of oxidant concentration on product composition in the ozonolysis of oleic acid, Physical Chemistry Chemical Physics, 2022.

D. Topping, M. Barley, M. K. Bane, N. Higham, B. Aumont, N. Dingle and G. McFiggans, UManSysProp v1.0: an online and open-source facility for molecular property prediction and atmospheric aerosol calculations, Geoscientific Model Development, 2016, 9, 899–914.

W. A. Pryor, Mechanisms of radical formation from reactions of ozone with target molecules in the lung, Free Radical Biology and Medicine, 1994, 17, 451–465.

W. A. Pryor, J. P. Stanley, E. Blair and G. B. Cullen, Autoxidation of Polyunsaturated Fatty Acids, Archives of Environmental Health: An International Journal, 1976, 31, 201–210.

E. N. Frankel, Lipid Oxidation, Elsevier, 2012, pp. 15–24.

J. H. Kroll, N. M. Donahue, V. J. Cee, K. L. Demerjian and J. G. Anderson, Gas-Phase Ozonolysis of Alkenes: Formation of OH from Anti Carbonyl Oxides, Journal of the American Chemical Society, 2002, 124, 8518–8519.

F. Herrmann, R. Winterhalter, G. K. Moortgat and J. Williams, Hydroxyl radical (OH) yields from the ozonolysis of both double bonds for five monoterpenes, Atmospheric Environment, 2010, 44, 3458–3464.

M. Zeng, N. Heine and K. R. Wilson, Evidence that Criegee intermediates drive autoxidation in unsaturated lipids, Proceedings of the National Academy of Sciences, 2020, 117, 4486–4490.

E. N. Frankel, W. E. Neff and E. Selke, Analysis of autoxidized fats by gas chromatography-mass spectrometry: VII. Volatile thermal decomposition products of pure hydroperoxides from autoxidized and photosensitized oxidized methyl oleate, linoleate and linolenate, Lipids, 1981, 16, 279–285.

18-Nov-2022

Dear Dr Krieger:

Manuscript ID: EA-ART-09-2022-000127.R1
TITLE: Oxidation pathways of linoleic acid revisited with electrodynamic balance–mass spectrometry

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Reviewer 1

I found the ms was well revised according to my comments.
Although I still think that the mechanism how initial radical oxidation starts in their system should be revealed, it may be beyond the scope of the present work.
Therefore, I am looking forward to seeing the next paper that deals with the issue for more details.