From the journal Environmental Science: Atmospheres Peer review history

22-Mar-2024

Dear Dr Chan:

Manuscript ID: EA-ART-02-2024-000024
TITLE: Heterogeneous Ozonolysis of Alkyl Substituted- Polycyclic Aromatic Hydrocarbons (AlkPAHs) in the Atmosphere

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Associate Editor
Environmental Science: Atmospheres
Royal Society of Chemistry

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

Reviewer 1

The authors used quartz filters as simplified reaction interface to test two PAHs and their alkyl-PAHs after exposing them to varying ozone concentration. They assessed the decay of PAHs over time and in response to ozone, and characterized the oxidation products formed from these reactions. The description of this study is very clear and the conclusion is reliable, but using quartz as the reaction interface is far from the actual situation. It is hoped that the authors can add different reaction interfaces in the future to obtain more comprehensive results. It is recommended to accept this manuscript to publish in this Journal after minor revisions.

The third highlight of this study is a bit too general, please make the modifications more specific and concise.

Please briefly demonstrate the reasons why quartz filter membranes can be used to simulate the heterogeneous reaction of alkyl-PAHs on the surface of particulate matter, and the possible uncertainties that may exist with the actual situation. The actual particles in the air are in a three-D shape, while the surface of the quartz filter membrane is artificially processed.

Lines 101-104: Why did this study choose of these PAHs as the objects?

Line 109-110: Please specify the monomers of PAHs.

Why is the concentration of PAC before the reaction analyzed using GCMS, and the oxidation products of its heterogeneous reaction analyzed using LCMS? Besides, how to ensure the comparability of results between the two types of instruments?

Line 255: Please modify x to a multiplication sign. And address this issue throughout the entire text.

Why did this study choose four testing times for pyrene, while six testing times for 1-methylpyrene?

The organic compounds in particulate matter is mixed. What is the difference between the test experiment based on the reaction rate of a relatively single species and the actual environment situation in this study?

What does the red font mean in Table 3?

Why did the research on oxidation products not involve the following three substances (Chrysene, 6-Methylchrysene and 6-Ethylchrysene)?

Please include the material and source of the surface in the abstract summary.

Reviewer 2

This manuscript by Zaherdinne et al. provides important experimental results on the heterogeneous oxidation of polycyclic aromatic hydrocarbons (PAH). They find that alkyl-substituted PAHs react significantly faster with ozone than PAHs. In addition, with pyrene and 1-methylpyrene as examples, they show that alkyl-PAHs form more toxic oxygenated products in higher yields than PAHs, which is an extremely important find for improving air quality models. The manuscript is technically sound and the clearly presented results are an important step towards improved understanding of urban air quality. I recommend that the manuscript be accepted for publication. Some minor comments below:

1. The faster ozonolysis rates of the alkyl-PAHs compared to regular PAHs is interesting. I would imagine that this is more likely due to a better stabilization of the transition state of the ozonolysis reaction than the intermediates as the authors speculate…

2. Was crust formation observed for any of the other alkyl-PAC, such as methylchrysene and ethylchrysene?

3. The ring-opening mechanisms in Figure S5 and in Cochran et al. 2016 that the authors cite involve water reactions. Was the relative humidity measured during the experiments? Can the RH be modified to constrain the formation of ring opened products?

REVIEWER REPORT(S):
Referee: 1

Comments to the Author
The authors used quartz filters as simplified reaction interface to test two PAHs and their alkyl-PAHs after exposing them to varying ozone concentration. They assessed the decay of PAHs over time and in response to ozone, and characterized the oxidation products formed from these reactions. The description of this study is very clear and the conclusion is reliable, but using quartz as the reaction interface is far from the actual situation. It is hoped that the authors can add different reaction interfaces in the future to obtain more comprehensive results. It is recommended to accept this manuscript to publish in this Journal after minor revisions.
We thank the reviewers for their comments. The line-by-line edits and comments are addressed below.

The third highlight of this study is a bit too general, please make the modifications more specific and concise.
Noted.
Revised text :”1-MethylPyrene yields a greater variety of oxidation products compared to Pyrene, with certain products being shared between them”

Please briefly demonstrate the reasons why quartz filter membranes can be used to simulate the heterogeneous reaction of alkyl-PAHs on the surface of particulate matter, and the possible uncertainties that may exist with the actual situation. The actual particles in the air are in a three-D shape, while the surface of the quartz filter membrane is artificially processed.
Quartz filters are made of silica fibers interweaved into each other. Each fiber has a diameter on the order of microns. Given our experimental conditions (more specifically, the amount of PAH relative to the surface area), the deposited PAH is expected to be present on the fiber surface in a monolayer. Therefore, this experimental model represents PAHs adsorbed to the surface of silica, and mimics PAHs adsorbed onto the surface of mineral-rich atmospheric particles. In previous literature, this experimental provides a simplified and reliable surface for the preliminary assessment of the kinetics of alkPAHs. Further investigation on other surfaces is required.

Lines 101-104: Why did this study choose of these PAHs as the objects?
The majority of heterogeneous oxidation studies have centered around Benzo[a]pyrene and pyrene for example, Cochran et al., 2016; Miet et al., 2009 and Zhou et al., 2019. We focused on pyrene because it is abundant in the atmosphere and in the middle of the atmospheric PAH volatility range. This choice was further supported by the easy availability of standards for 1-methylpyrene, 6-methylchrysene, and 6-ethylchrysene while analogues for other PAHs are not as readily available.

Line 109-110: Please specify the monomers of PAHs.
Edited.
Revised text:” Each of the compounds, including pyrene, chrysene, 1-methylpyrene, 6-methylchrysene, and 6-ethylchrysene, was dissolved in DCM and spiked onto a filter.”

Why is the concentration of PAC before the reaction analyzed using GCMS, and the oxidation products of its heterogeneous reaction analyzed using LCMS? Besides, how to ensure the comparability of results between the two types of instruments?
The concentration of the PACs during oxidation was analyzed quantitatively using a GCMS whereas the LCMS was used only for semi-quantitative identification of the oxidation products. Oxidized organic compounds are not amenable to GC analysis as they are more likely to be irreversibly lost in the GC column, so liquid chromatography is often preferred for separation of more polar compounds. Furthermore, electrospray ionization coupled with LC separation limits fragmentation during MS analysis and allows for identification of molecular formulas, but only ionizable (i.e. polar) compounds are amenable to ESI.

Line 255: Please modify x to a multiplication sign. And address this issue throughout the entire text.
Edited.

Why did this study choose four testing times for pyrene, while six testing times for 1-methylpyrene?
This study opted for four testing times for pyrene and six testing times for 1-methylpyrene due to differences in their oxidation rates. The faster oxidation rate of 1-methylpyrene led to the need for more testing points to capture its double-exponential decay pattern, particularly within the initial 0–30-minute reaction period, ensuring a thorough analysis.

The organic compounds in particulate matter is mixed. What is the difference between the test experiment based on the reaction rate of a relatively single species and the actual environment situation in this study?
In the 2012 study by Ringuet et al., aerosol samples collected on filters were exposed to ozone to assess reaction rates. Despite the experimental simplicity compared to the mixed composition of real aerosols, the observed reaction rates were consistent with those measured on model particles like silica and graphite. This suggests that while the simplified experiments may not fully replicate environmental complexity, they still provide valuable insights into the reactivity of organic compounds in atmospheric samples.

What does the red font mean in Table 3?
These compounds have chemical formulas corresponding to 13 or 14 carbons, and were excluded from the analysis due to the complexity of the mechanisms involved in their formation. In the revised manuscript, we have removed it from Table 3. Instead we note this exclusion in the Methods:

Line 225: Elemental formulas with fewer than 15 carbons were excluded from this analysis.

Why did the research on oxidation products not involve the following three substances (Chrysene, 6-Methylchrysene and 6-Ethylchrysene)?
As this study serves as a starting point for investigation, it is important to note that the research on oxidation products did not encompass Chrysene, 6-Methylchrysene, and 6-Ethylchrysene due to the limited knowledge regarding their oxidation products.. Conversely, studies on pyrene have identified some of its oxidation products, providing a basis for further investigation. Furthermore, we chose to focus on pyrene and 1-methylpyrene as they are the most reactive, which makes it easier to understand the evolution of these compounds upon oxidation.

Please include the material and source of the surface in the abstract summary.
Noted.

Reference:
R. E. Cochran, H. Jeong, S. Fisseha Derseh, A. Gowan, J. Beránek, A. Kubátová, Atmos. Environ., 2016, 128, 92-103. DOI: 10.1016/j.atmosenv.2015.12.036
K. Miet, K. Le Menach, P. Flaud, H. Budzinski, E. Villenave, Atmos. Environ., 2009, 43 (24), 3699-3707. DOI: 10.1016/j.atmosenv.2009.04.032
S. Zhou, B. C. Hwang, P. S. Lakey, A. Zuend, J. P. Abbatt, M. Shiraiwa, Proc. Natl. Acad. Sci. U.S.A., 2019, 116 (24), 11658–11663. DOI: 10.1073/pnas.1902517116
J. Ringuet, A. Albinet, E. Leoz-Garziandia, H. Budzinski, E. Villenave, Atmos. Environ., 2012, 61, 15-22. DOI: 10.1016/j.atmosenv.2012.07.025


Referee: 2

Comments to the Author
This manuscript by Zaherdinne et al. provides important experimental results on the heterogeneous oxidation of polycyclic aromatic hydrocarbons (PAH). They find that alkyl-substituted PAHs react significantly faster with ozone than PAHs. In addition, with pyrene and 1-methylpyrene as examples, they show that alkyl-PAHs form more toxic oxygenated products in higher yields than PAHs, which is an extremely important find for improving air quality models. The manuscript is technically sound and the clearly presented results are an important step towards improved understanding of urban air quality. I recommend that the manuscript be accepted for publication. Some minor comments below:
We thank the reviewers for their comments. The line-by-line edits and comments are addressed below.

1. The faster ozonolysis rates of the alkyl-PAHs compared to regular PAHs is interesting. I would imagine that this is more likely due to a better stabilization of the transition state of the ozonolysis reaction than the intermediates as the authors speculate…
Thank you for your insightful comment. Indeed, the hypothesis of improved stabilization of the transition state in alkyl-PAHs is compelling and aligns with our speculation. It suggests that structural modifications introduced by alkyl groups may enhance the reactivity of these compounds. Further investigations into the mechanistic aspects of ozonolysis reactions in PAHs would be valuable to confirm and deepen our understanding of this phenomenon.

2. Was crust formation observed for any of the other alkyl-PAC, such as methylchrysene and ethylchrysene?
The oxidation of 6-methylchrysene and 6-ethylchrysene exhibits slow kinetics, with only 20% reacted by 300 minutes, which may not be sufficient for us to observe crust formation within this timeframe.

3. The ring-opening mechanisms in Figure S5 and in Cochran et al. 2016 that the authors cite involve water reactions. Was the relative humidity measured during the experiments? Can the RH be modified to constrain the formation of ring opened products?
The relative humidity (RH) during the experiments was not measured but is expected to be around 35-40% The specific details regarding RH measurements and potential modifications to constrain the formation of ring-opened products were not provided in the manuscript. Further information on RH measurements and adjustments would be valuable for understanding the experimental conditions and their impact on the formation of ring-opened products.
Revised text:” Additionally, it is important to explore and comprehend the mechanism of oxidation, along with factors such as relative humidity.” (line 412-413)

25-Apr-2024

Dear Dr Chan:

Manuscript ID: EA-ART-02-2024-000024.R1
TITLE: Heterogeneous Ozonolysis of Alkyl Substituted- Polycyclic Aromatic Hydrocarbons (AlkPAHs) in the Atmosphere

Thank you for submitting your revised manuscript to Environmental Science: Atmospheres. 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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Dr Tzung-May Fu
Associate Editor
Environmental Science: Atmospheres
Royal Society of Chemistry

Reviewer 2

I'm satisfied by the authors' responses to my comments and I recommend that the manuscript be accepted for publication.

Reviewer 1

According to the review comments, the authors have made appropriate improvements to the manuscript, and I have no further questions.