From the journal Environmental Science: Atmospheres Peer review history

11-May-2021

Dear Dr Zhao:

Manuscript ID: EA-ART-04-2021-000031
TITLE: Photo-Oxidation of cis-Pinic Acid in the Aqueous Phase: A Mechanistic Investigation Under Acidic and Basic pH Conditions

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Reviewer 1

Review of “Photo-Oxidation of cis-Pinic Acid in the Aqueous Phase: A Mechanistic Investigation Under Acidic and Basic pH Conditions”, by Jéssica Vejdani Amorim et al.
General Comments:
In this article, the authors have investigated the mechanism of aqueous-phase PA OH-oxidation using both offline high mass resolution (-)ESI–LC–MS and a novel online monitoring system (PILS–ESI–MS), then they proposed the possible reaction mechanisms for the targeted compounds, they also discussed the reaction mechanisms related to pH. The author has found the main product of the PA OH-oxidation are norpinic acid (NPA) and 3-methyl-1,2,3-butanetricarboxylic acid (MBTCA) and they also observed a discrepancy in the yield of NPA and MBTCA under pH 2 and pH 8. The results of this paper suggest the importance of further understanding the impact of pH on aqueous-phase photochemistry which add values to the study for the scientific community.
However, as outlined below, the experimental section leaves the reader with some questions on how the experiments were carried out in detail. I think that this can be fixed by going more into detail in the experimental part. What’s more, I doubt that the content of the article does not correspond well with the title.
In my view, the manuscript is of high scientific quality. This is an important work for atmospheric chemistry and hence the paper is clearly within the scope of Environmental Science: Atmospheres. The overall quality of the graphs is good and the language is fluent and precise. Overall, I can recommend the paper to be published in Environmental Science: Atmospheres after addressing the following aspects.
Detailed Comments:
1) In this article, most of it is about PA, so why do you give a name about CPA in the title?
2) There is not so much ultraviolet light in the troposphere, and the authors have used the UV-light centered at 310 nm (UVB) to oxidize, so why do you use the UVB? I suspect that it will not correspond to the real atmospheric conditions.
3) The authors have point out that they conducted an H2O2 control experiment to confirm that PA does not undergo direct photolysis in the absence of H2O2 in Section 2.2., Page 5, but I don’t find the control experiment in detail in this article, what’s more, this article just shows that the PA does not react with H2O2 under dark conditions in Section 3.3 Page 7, there are no results to confirm that PA does not undergo direct photolysis in the absence of H2O2.
4) The authors have increased the H2O2 concentration for the online analysis accordingly to 20 mM to maintain a similar [OH]ss for the online analysis. So how do you prove that [OH]ss is similar when the H2O2 concentration is 20 mM, is there some computational methods for calculating [OH]ss?
5) The authors have pointed out that the pH values of ambient cloudwater and fogs fall between 2 and 7, but you have conducted this work under pH values of 2 and 8, why don’t you do it in pH 2 and 7 or do the experiment with pH 2,3,4,5,6,7?
6) In this article, it has shown that the EICs for m/z 203 are those from the pH 8 experiments in Figure 6, why not analyze the EICs both of pH 2 and 8?
7) It has shown that the authors have performed CPA OH-oxidation and detected all the oxidation products in Section 3.2, Page 7, so suggest to show some data of it in the Supplementary Information, because you have said that “This result is in agreement with CPA’s oxidation product...” in Section 3.4, page 9, I think it would be better for readers to understand more clearly with the support of these data.
8) Suggest to unify the format of Eq.1 and Eq. (1) in Section 3.5, Page 9.

Reviewer 2

The manuscript “Photo-Oxidation of cis-Pinic Acid in the Aqueous Phase: A Mechanistic Investigation Under Acidic and Basic pH Conditions” discusses the mechanism of the photo-oxidation of pinic acid in aqueous phase under acidic and basic conditions. The manuscript is well written and contributes to the current body of research by providing mechanisms for Pinic acid oxidation in aqueous phase. Below is the list of changes required before this manuscript can be published.

Major Comment:
1. Experimental section needs to be elaborated. Several details are missing. For example, it is not clear if offline and online analysis were performed in the same experiments? If offline and online analysis were not performed in the same experiment, then why were they not? How many experiments were performed? Total time of the reaction. A detailed table of experiments should be provided, possibly expanding table 1 appropriately.

Minor comments:
2. Recommend to provide quantitative results in the abstract.
3. Page 1, Para 1: “… limited by few model compounds”. Please provide few references.
4. Page 2, Para 1: “Using a chemical equilibrium ….”. This sentence appears to be grammatically incorrect. Please consider revising it.
5. Page 3, Para 1: I believe the OH concentration estimation needs to be performed with a known chemical, given the fact that authors are trying to develop the reaction mechanism for Pinic acid.
6. Page 3, section 2.3.1, Para 1: Please expand ‘FA’
7. Was a background run performed in PILS-ESI-MS system with just pinic acid to establish the signal in MS or some kind of calibration?
8. Please provide method detection limits for both offline and online analysis.
9. Please provide the spectra for offline analysis.
10. Fig. 5a: Is there a typo for m/z 171 peak? It shows 199.
11. Page 5, section 3.4, para 3: “… was similar to that of Band its isomers”. Please revise this sentence, and two more after this.
12. Page 7, section 3.5, para 5: “…MBTCA from CPA is solely from the 30% of PA reacted via OH-oxidation….”. Isn’t it should be CPA instead of PA in this sentence?
13. Page 7, section 3.5, para 5: “Overall, our results show that …”. This sentence needs revision.
14. Page 9, section 4, para 2: “We also observed a discrepancy in the …”. The word discrepancy doesn’t appear to be appropriate.

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

Review of “Photo-Oxidation of cis-Pinic Acid in the Aqueous Phase: A Mechanistic Investigation Under Acidic and Basic pH Conditions”, by Amorim et al.

Note: The following responses highlighted in red are actual modifications made to the revised manuscript.

Referee: 1
Comments to the Author
General Comments:
In this article, the authors have investigated the mechanism of aqueous-phase PA OH-oxidation using both offline high mass resolution (-)ESI–LC–MS and a novel online monitoring system (PILS–ESI–MS), then they proposed the possible reaction mechanisms for the targeted compounds, they also discussed the reaction mechanisms related to pH. The author has found the main product of the PA OH-oxidation are norpinic acid (NPA) and 3-methyl-1,2,3-butanetricarboxylic acid (MBTCA) and they also observed a discrepancy in the yield of NPA and MBTCA under pH 2 and pH 8. The results of this paper suggest the importance of further understanding the impact of pH on aqueous-phase photochemistry which add value to the study for the scientific community. However, as outlined below, the experimental section leaves the reader with some questions on how the experiments were carried out in detail. I think that this can be fixed by going more into detail in the experimental part. What’s more, I doubt that the content of the article does not correspond well with the title. In my view, the manuscript is of high scientific quality. This is an important work for atmospheric chemistry and hence the paper is clearly within the scope of Environmental Science: Atmospheres. The overall quality of the graphs is good and the language is fluent and precise. Overall, I can recommend the paper to be published in Environmental Science: Atmospheres after addressing the following aspects.
Detailed Comments:
1) In this article, most of it is about PA, so why do you give a name about CPA in the title?
The reviewer might have misread the title. It says cis-pinic acid (PA), not cis-pinonic acid (CPA). We will remove the cis- from the title to make it clearer, since in the following sections we refer to it as PA only.
2) There is not so much ultraviolet light in the troposphere, and the authors have used the UV-light centered at 310 nm (UVB) to oxidize, so why do you use the UVB? I suspect that it will not correspond to the real atmospheric conditions.
The reviewer is correct. We did not intend to simulate the real atmospheric conditions, instead, we consider our experimental set up as one that fundamentally probes the OH chemistry in the system. As we mentioned, our UV light (centered at 310 nm) is efficient and reproducible in producing OH radicals from H2O2 photolysis. We agree that UV light at 310 nm is not completely atmospherically relevant, however, we have confirmed that it does not affect the species that we need to quantify (PA, MBTCA, NPA), and the potential impact on the rest of the mechanisms has already been discussed.
The following modifications will be done for clarification in 2.2: “The UV light used in this work was chosen in order to photolyze H2O2 into OH radicals at atmospheric relevant concentrations which are estimated to be 1.4 × 10–16 to 8.0 × 10–12 M.”1,2
3) The authors have point out that they conducted an H2O2 control experiment to confirm that PA does not undergo direct photolysis in the absence of H2O2 in Section 2.2., Page 5, but I don’t find the control experiment in detail in this article, what’s more, this article just shows that the PA does not react with H2O2 under dark conditions in Section 3.3 Page 7, there are no results to confirm that PA does not undergo direct photolysis in the absence of H2O2.
We did not show the control experiments because it has already been published in our previous article. Thus, we mentioned that “the contribution of direct photolysis to the overall PA decay has been previously investigated and presented by Amorim et al.3 A dark control experiment showed no reactions between PA and H2O2 itself. This analysis was conducted in the same way as the OH-oxidation experiment, except that the UVB lamps remained off.” From our first paper, we conduct a detailed investigation of PA kinetics, and concluded that PA does not undergo direct photolysis (control) as you can see Fig.1, which we took from our previous paper.

<FIGURE 1 HERE>
Fig.1. OH-oxidation and control experiments for PA. This figure is from our previous paper, Amorim et al.3
4) The authors have increased the H2O2 concentration for the online analysis according to 20 mM to maintain a similar [OH]ss for the online analysis. So how do you prove that [OH]ss is similar when the H2O2 concentration is 20 mM, are there some computational methods for calculating [OH]ss?
The agreement of [OH]ss in our offline and online techniques was confirmed from the rate constant of PA (kI = 1.7x109 M-1s-1 at pH 2, and kI = 2.97x109 M-1s-1 at pH 8), assuming that it is in steady state throughout the reaction. Using the pseudo-first order, given by ln⁡(X_0/X_t )=k^(I*) 〖[OH]〗_ss the [OH]ss can be calculated by plotting ln⁡(X_0/X_t )as a function of time. For the offline analysis, the [OH]ss was calculated to be 7.99x10-13 M at pH 2 and 2.79x10-13 M at pH 8. From the online measurements, we targeted PA signal decay over time (t), with PA decay up to 20 min. The [OH]ss was determined using the same relationship, and a value of 6.69 x 1013 was found. This agreement of [OH]ss between the two methods is expected, as [OH]ss is the result of a balance between the sink (PA) and the source (H2O2). When the sink was increased, the source would have to be increased accordingly. The new [OH]ss values were added for the offline measurements at pH 2 and pH 8, and for the online analysis.
5) The authors have pointed out that the pH values of ambient cloudwater and fogs fall between 2 and 7, but you have conducted this work under pH values of 2 and 8, why don’t you do it in pH 2 and 7 or do the experiment with pH 2,3,4,5,6,7?
From our discussion in 3.1, the effective pKa of PA was measured to be 4.9. Therefore, by choosing the pH conditions of 2 and 8, we can ensure the complete ionization and protonation of PA, respectively. For the pH conditions mentioned (pH 3-7), the protonation/deprotonation oa PA, therefore, the reaction mechanisms with OH radicals will most likely be a combination of the mechanisms proposed for pH 2 and pH 8.
The last sentences in 3.1 were modified to: “Therefore, in atmospheric aqueous phases, PA can exist as a mixture of its neutral form and as a PAn anion. The ratio of which varies drastically with changing cloudwater compositions. For this reason, it is important for us to fundamentally study PA oxidation mechanisms under both acidic and basic conditions, in which PA exists only in one of the two forms.”
6) In this article, it has been shown that the EICs for m/z 203 are those from the pH 8 experiments in Figure 6, why not analyze the EICs both of pH 2 and 8?
We analyzed the oxidation products under both pH conditions, and as they are the same compound, they showed the same retention time. The EIC for m/z 203 at pH 8 was chosen as an example to be added to the manuscript due to the higher intensity of m/z 203 under this pH condition. For the reviewer’s interest, we present the EICs of m/z 203 at pH 2 and pH 8 below (Fig. 2). This figure is not included in the manuscript.
<FIGURE 2 HERE>
Fig. 2. Extracted ion chromatogram of MBTCA (m/z 203) under acidic and basic conditions.
The following sentence will be added on Fig.6’s caption for clarity: “Although we only show the EIC of m/z 203 at pH 8,the pH 2 experiments resulted in the same products, just different concentrations.”
7) It has shown that the authors have performed CPA OH-oxidation and detected all the oxidation products in Section 3.2, Page 7, so suggest to show some data of it in the Supplementary Information, because you have said that “This result is in agreement with CPA’s oxidation product...” in Section 3.4, page 9, I think it would be better for readers to understand more clearly with the support of these data.
We understand how it can be confusing, and to clarify it, the following Section will be added to the SI:
Offline (-)ESI–LC–MS for Elemental Analysis of CPA oxidation products
<TABLE S1 HERE>

Table S1. List of Major CPA OH-oxidation Peaks Detected by (-)ESI‒LC‒MS.

In addition, a reference to SI will be added to the sentence: “We have also performed CPA OH-oxidation and detected all the oxidation products previously identified by Witkowski et al. (Table S2).”
8) Suggest to unify the format of Eq.1 and Eq. (1) in Section 3.5, Page 9.
Done.
Referee: 2
Comments to the Author
The manuscript “Photo-Oxidation of cis-Pinic Acid in the Aqueous Phase: A Mechanistic Investigation Under Acidic and Basic pH Conditions” discusses the mechanism of the photo-oxidation of pinic acid in aqueous phase under acidic and basic conditions. The manuscript is well written and contributes to the current body of research by providing mechanisms for Pinic acid oxidation in aqueous phase. Below is the list of changes required before this manuscript can be published.
Major Comment:
1. Experimental section needs to be elaborated. Several details are missing. For example, it is not clear if offline and online analysis were performed in the same experiments? If offline and online analysis were not performed in the same experiment, then why were they not? How many experiments were performed? Total time of the reaction. A detailed table of experiments should be provided, possibly expanding table 1 appropriately.
We appreciate the suggestion. Offline and online measurements were done independently. In Table 1 we note that the parameters are different, such as the vessel volume, PA concentration, reaction time, etc. There are a few reasons why we had to do them separately: (1) either H2SO4 or NaOH was added to the offline samples for pH adjustment. These salts are incompatible with the online ESI-MS, as salts cause signal suppression and contamination of the ESI source; (2) the online method required a large amount of PA, which we couldn’t afford due to the limited amount of synthesized PA. In particular, the atomizer required a large volume of solution (100 mL) to sustain atomization, and the online ESI instrument required a higher PA concentration (330 µM) to detect the products forming. Therefore, we could only perform the online experiment twice to confirm the reproducibility (see below).
For the offline analysis, in Section 2.2 we mention that “All OH-oxidation experiments were conducted in triplicates to ensure reproducibility and to enable error analysis for quantitative information.” We agree with the reviewer and have added the total length of reaction to Table 1. In terms of online measurements, we performed the online PA experiment in duplicates, and the reproducibility of the products was confirmed. We further performed an online experiment for CPA, but this experiment was not discussed in the manuscript. The following additions will be done in Table 1:
Table 1. Experimental Parameters Used for Offline LC–MS and Online MS.
<TABLE 1 HERE>

The following sentence was added to Section 2.2: “A duplicated online analysis was conducted to ensure the reproducibility. Given that this experiment required a higher concentration and a larger volume of PA solution, we could not afford more replicates with a limited amount of PA synthesized.”
Minor comments:
2. Recommend to provide quantitative results in the abstract.
We thank the reviewer for the suggestion, and the yields of NPA and MBTCA have been added to the abstract.

3. Page 1, Para 1: “… limited by few model compounds”. Please provide a few references.
We thank the reviewer for the suggestion. As we are focused on the oxidation products arisig from ⍺-pinene, exclusively, the current model compounds are limited to cis-pinonic acid and limononic acid mainly. References will be added and the following addition will be done: “… limited by few model compounds such as cis-pinonic acid (CPA) and limononic acid.”4–6
4. Page 2, Para 1: “Using a chemical equilibrium ….”. This sentence appears to be grammatically incorrect. Please consider revising it.
The following modification was done: “When cloudwater is present in an air mass, PA and other OAs from α-pinene will be present exclusively in the aqueous phase.”7
5. Page 3, Para 1: I believe the OH concentration estimation needs to be performed with a known chemical, given the fact that authors are trying to develop the reaction mechanism for Pinic acid.
The reviewer is correct. A known chemical is needed when one wants to use the relative rate method to determine the second order reactivity rate with OH radicals of a compound with unknown reactivity. The kinetics of PA has already been published in our previous paper.3 In this work, our focus is the reaction mechanism and products of PA. [OH]ss is needed as a rough confirmation of our reaction condition, thus we do not think the use of a reference compound will be needed. In fact, the presence of a reference compound will complicate the product identification.
6. Page 3, section 2.3.1, Para 1: Please expand ‘FA’
Expanded to formic acid.
7. Was a background run performed in the PILS-ESI-MS system with just pinic acid to establish the signal in MS or some kind of calibration?
Yes, for the PILS-ESI-MS the background was performed before the light was turned on. The PILS-ESI-MS monitored PA alone in the solution until a stable signal was obtained before the light was on. That is how we determined the trace amount of impurities in the solution, as pointed out in Section 3.3. We did not do any quantitative assessment of the online PILS-ESI, as discussed in the manuscript, as we do not think the PILS-ESI is a good system for quantitative analysis.
We have added the following sentence to clarify the background of PILS-ESI-MS: “At the beginning of the experiment, a background signal was obtained by sampling the solution containing PA and H2O2 but without light. The background signal for the PA solution shows that some products were already present before the start of the experiment…”

8. Please provide method detection limits for both offline and online analysis.
Following this comment, we have estimated our LOD’s for TCA and PA, surrogates for MBTCA and NPA, respectively, using an external calibration curve. The detection limit was calculated by the equation: LOD = 3.3 x /S, where S is the slope of the calibration curve, and is the standard deviation of the intercept. As a consequence the LOD’s of NPA and MBTCA are: 0.60 µM and 0.21 µM, respectively. This means that MBTCA detected from PA OH-oxidation is above the LOD. However, MBTCA detected from CPA OH is below LOD for both, pH 2 and pH 8. Based on our LOD, the maximum yield of PA from MBTCA is 0.1%. Following this analysis, we have modified Table 3 in our manuscript, and LOD discussion will be added. This reanalysis offered us an opportunity to re-assess the yield of MBTCA and NPA by using the new calibration curve. The reanalysis led us to a readjustment of the yield values that we report on the manuscript. We believe that the new values are more consistent and reliable, while the overall conclusion of our manuscript remains the same.
The following modifications were made in Section 3.5:
Parag. 3: “In this work, a PA and TCA standard solutions were prepared and analyzed using (-)ESI–LC–MS as described in Section 2.3.1. The signal of MBTCA was related to that of TCA for quantification, while the signal of NPA was related to that of PA.”
Old Table 3:
<OLD TABLE 3 HERE>

New Table 3:
<NEW TABLE 3 HERE>

aSignal was below the limit of detection. The value represents an upper-limit estimation.
Parag. 4: Assuming that MBTCA from CPA is solely from the 30% of CPA reacted via OH-oxidation, MBTCA’s Yaq from CPA was calculated to be below the limit of detection of our ESI–LC–MS method. Based on the detection limit of MBTCA (0.21 µM), the upper-limit estimate for MBTCA’s Yaq from CPA is 0.1 %.
We have also revised the paragraph explaining how we obtained the Yaq: In this work, we found that the Yaq values were relatively constant until 60% of PA was consumed; therefore, we have decided to report Yaq by averaging those from all the samples with at least 40% of PA remaining.
9. Please provide the spectra for offline analysis.
The spectra for offline analysis of PA OH-oxidation at pH 2 and pH 8 were no included in the manuscript because the raw base peak chromatogram (bpc) does not give enough information on the formation of products arising from PA OH-oxidation, as it can be seen in Fig. 3 for the experimental conditions under pH 2. Therefore, extracted ion chromatograms (EIC) were provided instead.
<FIGURE 3 HERE>

Fig. 3. Bpc for PA sample at pH 2 taken at time t = 0 min (red) and t = 5.0 min.
10. Fig. 5a: Is there a typo for m/z 171 peak? It shows 199.
Yes, the reviewer is correct. I will change it to m/z 171.
11. Page 5, section 3.4, para 3: “… was similar to that of Band its isomers”. Please revise this sentence, and two more after this.
The sentences were revised to the following: “The fragmentation pattern of the product detected as m/z 203 was similar to that of MBTCA and its isomers. Offline analysis with ESI–LC–MS detected a dominant isomer of m/z 203 at Rt = 0.67 min and a few minor ones. To confirm that the dominant isomer is attributable to MBTCA, we compare the extracted ion chromatogram (EIC) of m/z 203 to that of TCA, which exhibits a peak at Rt = 0.59 min (Figure 6).”
12. Page 7, section 3.5, para 5: “…MBTCA from CPA is solely from the 30% of PA reacted via OH-oxidation….”. Shouldn't it be CPA instead of PA in this sentence?
Yes, It should be CPA instead of PA. The necessary change was made in Section 3.5.
13. Page 7, section 3.5, para 5: “Overall, our results show that …”. This sentence needs revision.
We thank the reviewer for the suggestion. We will add a clarification that the MBTCA’s Yaq we are referring to is from PA OH-oxidation. The new sentence will be “Overall, our results show that MBTCA’s Yaq from PA OH-oxidation is much greater than that of CPA, which is the currently known precursor of MBTCA. It is also noteworthy that MBTCA’s Yaq from CPA is also observed to be pH-dependent, indicating that the impact of pH can be seen in many other OAs. “

14. Page 9, section 4, para 2: “We also observed a discrepancy in the …”. The word discrepancy doesn’t appear to be appropriate.
The word discrepancy was substituted for difference in Section 4
References
(1) Ervens, B.; Turpin, B. J.; Weber, R. J. Secondary Organic Aerosol Formation in Cloud Droplets and Aqueous Particles (AqSOA): A Review of Laboratory, Field and Model Studies. Atmos. Chem. Phys. 2011, 11 (21), 11069–11102.
(2) Herrmann, H.; Schaefer, T.; Tilgner, A.; Styler, S. A.; Weller, C.; Teich, M.; Otto, T. Tropospheric Aqueous-Phase Chemistry: Kinetics, Mechanisms, and Its Coupling to a Changing Gas Phase. Chem. Rev. 2015, 115 (10), 4259–4334.
(3) Amorim, J. V.; Wu, S.; Klimchuk, K.; Lau, C.; Williams, F. J.; Huang, Y.; Zhao, R. PH Dependence of the OH Reactivity of Organic Acids in the Aqueous Phase. Environ. Sci. Technol. 2020, 54 (19), 12484–12492.
(4) Witkowski, B.; Gierczak, T. Cis-Pinonic Acid Oxidation by Hydroxyl Radicals in the Aqueous Phase under Acidic and Basic Conditions: Kinetics and Mechanism. Environ. Sci. Technol. 2017, 51 (17), 9765–9773.
(5) Witkowski, B.; Al-sharafi, M.; Gierczak, T. Kinetics of Limonene Secondary Organic Aerosol Oxidation in the Aqueous Phase. Environ. Sci. Technol. 2018, 52 (20), 11583–11590.
(6) Lignell, H.; Epstein, S. A.; Marvin, M. R.; Shemesh, D.; Gerber, B.; Nizkorodov, S. Experimental and Theoretical Study of Aqueous Cis-Pinonic Acid Photolysis. J. Phys. Chem. A 2013, 117 (48), 12930–12945.
(7) Wania, F.; Lei, Y. D.; Wang, C.; Abbatt, J. P. D.; Goss, K.-U. Using the Chemical Equilibrium Partitioning Space to Explore Factors Influencing the Phase Distribution of Compounds Involved in Secondary Organic Aerosol Formation. Atmos. Chem. Phys. 2015, 15 (6), 3395–3412.

01-Jul-2021

Dear Dr Zhao:

Manuscript ID: EA-ART-04-2021-000031.R1
TITLE: Photo-Oxidation of cis-Pinic Acid in the Aqueous Phase: A Mechanistic Investigation Under Acidic and Basic pH Conditions

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Reviewer 2

I would like to thank authors for addressing reviewer's comments adequately. The manuscript is acceptable for the publication. However, I encourage authors to revise and edit the supplementary information document to be consistent with journal's style and recommendations. A few comments are -

1. Title needs to be updated. 'cis' needs to be removed.
2. A few copy editing issues in the text.
3. The font size is small at size 9.