Acylperoxy radicals during ozonolysis of &alpha;-pinene: composition, formation mechanism, and contribution to the production of highly oxygenated organic molecules

Zang, Han; Huang, Dandan; Zhong, Jiali; Li, Ziyue; Li, Chenxi; Xiao, Huayun; Zhao, Yue

doi:https://doi.org/10.5194/egusphere-2023-838

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https://doi.org/10.5194/egusphere-2023-838

Preprints

15 May 2023

| 15 May 2023

Acylperoxy radicals during ozonolysis of α-pinene: composition, formation mechanism, and contribution to the production of highly oxygenated organic molecules

Han Zang, Dandan Huang, Jiali Zhong, Ziyue Li, Chenxi Li, Huayun Xiao, and Yue Zhao

Abstract. Acylperoxy radicals (RO₂) are key intermediates in atmospheric oxidation of organic compounds and different from the general alkyl RO₂ radicals in reactivity. However, direct probing of the molecular identities and chemistry of acyl RO₂ remains quite limited. Here, we report a combined experimental and kinetic modelling study of the composition and formation mechanisms of acyl RO₂, as well as their contributions to the formation of highly oxygenated organic molecules (HOMs) during ozonolysis of α-pinene. We find that acyl RO₂ radicals account for 67 %, 94 %, and 32 % of the highly oxygenated C₇, C₈, and C₉ RO₂, respectively, but only a few percent of C₁₀ RO₂. The formation pathway of acyl RO₂ species depends on their oxygenation level. The highly oxygenated acyl RO₂ (oxygen atom number ≥ 6) are mainly formed by the intramolecular aldehydic H-shift (i.e., autoxidation) of RO₂, while the less oxygenated acyl RO₂ (oxygen atom number < 6) are basically derived from the C-C bond cleavage of alkoxy (RO) radicals containing an α-ketone group or the intramolecular H-shift of RO containing an aldehyde group. The acyl RO₂-involved reactions explain 50–90 % of C₇ and C₈ closed-shell HOMs and 14 % of C₁₀ HOMs, respectively. For C₉ HOMs, this contribution can be up to 30 %–60 %. In addition, acyl RO₂ contribute to 50 %–95 % of C_14–C₁₈ HOM dimer formation. Because of the generally fast reaction kinetics of acyl RO₂, the acyl RO₂ + alkyl RO₂ reactions seem to outcompete the alkyl RO₂ + alkyl RO₂ pathways, thereby affecting the fate of alkyl RO₂ and HOM formation. Our study sheds lights on the detailed formation pathways of the monoterpene-derived acyl RO₂ and their contributions to HOM formation, which will help to understand the oxidation chemistry of monoterpenes and sources of low-volatility organic compounds capable of driving particle formation and growth in the atmosphere.

Received: 26 Apr 2023 – Discussion started: 15 May 2023

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11 Oct 2023

Direct probing of acylperoxy radicals during ozonolysis of α-pinene: constraints on radical chemistry and production of highly oxygenated organic molecules

Han Zang, Dandan Huang, Jiali Zhong, Ziyue Li, Chenxi Li, Huayun Xiao, and Yue Zhao

Atmos. Chem. Phys., 23, 12691–12705, https://doi.org/10.5194/acp-23-12691-2023,https://doi.org/10.5194/acp-23-12691-2023, 2023

Short summary

Han Zang, Dandan Huang, Jiali Zhong, Ziyue Li, Chenxi Li, Huayun Xiao, and Yue Zhao

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2023-838', Anonymous Referee #1, 25 Jun 2023
General Comments
In this manuscript the authors report a combined experimental and modelling study of the formation and fate of acylperoxy radicals formed from the reaction of a-pinene with ozone in a flow reactor. The alkyl and acyl RO2 radicals and highly oxidized molecules (HOMs) were monitored using a chemical ionization mass spectrometer with nitrate ion ionization. RO2 radicals and HOMS were assigned based on elemental formulas and acyl RO2 radicals were distinguished from alkyl RO2 radicals by addition of NO2, which forms RC(O)OONO2 (acyl peroxy nitrates) that are relatively stable under the conditions of the experiments, thus removing acyl RO2 signal. Because the changes in acyl RO2 concentrations can also impact other aspects of the chemistry, a detailed F0AM model employing a modified Master Chemical Mechanism was employed to interpret the results.
Overall, the experiments and modelling were well done and the approach seems to have yielded quite useful and interesting results. The authors provide a very thorough and thoughtful discussion of the results, which is clearly written and easy to follow. Considering the high technical quality of the study and the importance of these reactions to the formation of HOMs and ROOR dimers, both of which are currently of much interest because of their potential role in secondary organic aerosol (SOA) formation, I think the paper is well suited for ACP. I have only a few minor comments.
Specific Comments
Line 137: I don’t understand the point of converting signals to “concentrations” using sulfuric acid since the actual concentrations will be highly sensitive to the structure of the RO2 radical and HOM. Presenting the results this way is misleading. Since the “concentrations” are only used to calculate contributions of various species relative to each other, normalized signals will give the same results and be a more honest presentation of the data.

Line 149: Have the authors considered partitioning of RO2 radicals to particles and what influence that could have on the results? The vapor pressures of the radicals should be similar to those of HOMs, so I don’t see any reason that they would not form SOA, and they likely undergo different reactions in the particles since isomerization would be restricted.

Line 342: In this section it is not clear to me what conclusions are based on measurements, modelling, or a combination of the two. Please make that more clear.

Line 530: Considering that RO2 + NO2 rate constants have been measured for a variety of alkyl and acyl RO2 radicals and are pretty consistently ~1E–11 (Orlando & Tyndall 2012), it seems unlikely that the value is as low as suggested here. Any explanation based on RO2 structure would imply that the same effects apply to the RO2 + NO rate constant, which is essentially identical to the NO2 value (Orlando & Tyndall 2012). This would have significant consequences for predictions of conditions under which autoxidation reactions are important in the atmosphere, since this usually depends on the competition between RO2 isomerization and the RO2 + NO reaction. What are other possible explanations for the apparent discrepancy?

Technical Comments
None.
Citation: https://doi.org/10.5194/egusphere-2023-838-RC1
- AC1: 'Reply on RC1', Yue Zhao, 28 Aug 2023
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-838/egusphere-2023-838-AC1-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2023-838-AC1
RC2:
'Comment on egusphere-2023-838', Anonymous Referee #2, 19 Jul 2023
General comments:
In this study, the authors investigated the fraction of acyl peroxy radicals (RO₂) formed from alpha-pinene ozonolysis. Acyl-RO₂s are of crucial atmospheric importance due to their higher reactivity and their role in the formation of aerosol precursors. In flow reactor alpha-pinene ozonolysis experiments, NO₂ was used as an acyl-RO₂ scavenger, and the reduction in RO₂ signals was used to probe the fraction of acyl-RO₂s produced. The paper is well written and makes a significant contribution towards the better understanding of a key aerosol forming system in the atmosphere. I recommend the publication of the manuscript in ACP after the authors address my minor comments below:
In addition to dimer formation and producing RO, alkyl-RO₂ cross reactions also lead to ROH + R=O products, and if the inital peroxy radical group is on a primary carbon atom, the R=O can be a source of acyl-RO₂ following a secondary OH reaction. Alkyl RO₂s can have significant yields for this reaction. Was this accounted for in the model and in the analysis of the experiments?

The more functionalized acyl-RO₂s with an -OOH group elsewhere in the molecule are known to undergo H-scrambling reactions to form peroxy acids (R-C(O)OOH) at rates of 1E3 – 1E5 s^-1 (Knap et al. 2017, J. Phys. Chem. A, 121(7), pp.1470-1479). Can the authors comment on the possible role of this reaction in their experiments? For example, the ring-opened acyl C₁₀H₁₅O₈-RO₂ that they report has a 1,6 H-scramble available that leads to a peroxy acid and an alkyl RO₂. For their model system, Knap et al. estimate a rate coefficient for the 1,6 H-shift of 1.5E5 s^-1. If the rate coefficient is comparable for the alpha-pinene derived C₁₀H₁₅O₈ acyl-RO₂ above, this could to an extent explain the low reduction in signal upon NO₂addition.

Line 468: Regarding the speculation of the formation of alkyl C₉H₁₅O₃-RO₂ to explain the discrepancy between experiments and simulations, this would compete with the formation of the ring-opened and ring-retaining C₁₀H₁₅O₄-RO₂. How did the sensitivity analysis of including the C₉H₁₅O₃-RO₂ in the model affect the yield of the C₁₀H₁₅O₄-RO₂s and the subsequent acyl-RO₂s derived from them?

Are any of the acyl peroxy nitrates detected by the NO₃-CIMS? Alpha-pinene derived APNs with 8 oxygen atoms or more should have at least 2 -OOH functional groups and will presumably cluster well with NO₃^-. Do the decrease in e.g. C₇ and C₈ RO₂ signals when NO₂ is added show an increase in the corresponding APN signals? I think a spectrum figure maybe in the supplementary showing the acyl-RO₂ and acyl-ROONO₂ peaks would be useful.

Figure 4. In pathway 3, the final H-shift of the acyl-oxy is unlikely to compete with CO₂ See reaction r12 and description therein in Vereecken et al. 2009, Phys. Chem. Chem. Phys. 11(40), pp.9062-9074.
Citation: https://doi.org/10.5194/egusphere-2023-838-RC2
- AC2: 'Reply on RC2', Yue Zhao, 28 Aug 2023
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-838/egusphere-2023-838-AC2-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2023-838-AC2

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2023-838', Anonymous Referee #1, 25 Jun 2023
General Comments
In this manuscript the authors report a combined experimental and modelling study of the formation and fate of acylperoxy radicals formed from the reaction of a-pinene with ozone in a flow reactor. The alkyl and acyl RO2 radicals and highly oxidized molecules (HOMs) were monitored using a chemical ionization mass spectrometer with nitrate ion ionization. RO2 radicals and HOMS were assigned based on elemental formulas and acyl RO2 radicals were distinguished from alkyl RO2 radicals by addition of NO2, which forms RC(O)OONO2 (acyl peroxy nitrates) that are relatively stable under the conditions of the experiments, thus removing acyl RO2 signal. Because the changes in acyl RO2 concentrations can also impact other aspects of the chemistry, a detailed F0AM model employing a modified Master Chemical Mechanism was employed to interpret the results.
Overall, the experiments and modelling were well done and the approach seems to have yielded quite useful and interesting results. The authors provide a very thorough and thoughtful discussion of the results, which is clearly written and easy to follow. Considering the high technical quality of the study and the importance of these reactions to the formation of HOMs and ROOR dimers, both of which are currently of much interest because of their potential role in secondary organic aerosol (SOA) formation, I think the paper is well suited for ACP. I have only a few minor comments.
Specific Comments
Line 137: I don’t understand the point of converting signals to “concentrations” using sulfuric acid since the actual concentrations will be highly sensitive to the structure of the RO2 radical and HOM. Presenting the results this way is misleading. Since the “concentrations” are only used to calculate contributions of various species relative to each other, normalized signals will give the same results and be a more honest presentation of the data.

Line 149: Have the authors considered partitioning of RO2 radicals to particles and what influence that could have on the results? The vapor pressures of the radicals should be similar to those of HOMs, so I don’t see any reason that they would not form SOA, and they likely undergo different reactions in the particles since isomerization would be restricted.

Line 342: In this section it is not clear to me what conclusions are based on measurements, modelling, or a combination of the two. Please make that more clear.

Line 530: Considering that RO2 + NO2 rate constants have been measured for a variety of alkyl and acyl RO2 radicals and are pretty consistently ~1E–11 (Orlando & Tyndall 2012), it seems unlikely that the value is as low as suggested here. Any explanation based on RO2 structure would imply that the same effects apply to the RO2 + NO rate constant, which is essentially identical to the NO2 value (Orlando & Tyndall 2012). This would have significant consequences for predictions of conditions under which autoxidation reactions are important in the atmosphere, since this usually depends on the competition between RO2 isomerization and the RO2 + NO reaction. What are other possible explanations for the apparent discrepancy?

Technical Comments
None.
Citation: https://doi.org/10.5194/egusphere-2023-838-RC1
- AC1: 'Reply on RC1', Yue Zhao, 28 Aug 2023
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-838/egusphere-2023-838-AC1-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2023-838-AC1
RC2:
'Comment on egusphere-2023-838', Anonymous Referee #2, 19 Jul 2023
General comments:
In this study, the authors investigated the fraction of acyl peroxy radicals (RO₂) formed from alpha-pinene ozonolysis. Acyl-RO₂s are of crucial atmospheric importance due to their higher reactivity and their role in the formation of aerosol precursors. In flow reactor alpha-pinene ozonolysis experiments, NO₂ was used as an acyl-RO₂ scavenger, and the reduction in RO₂ signals was used to probe the fraction of acyl-RO₂s produced. The paper is well written and makes a significant contribution towards the better understanding of a key aerosol forming system in the atmosphere. I recommend the publication of the manuscript in ACP after the authors address my minor comments below:
In addition to dimer formation and producing RO, alkyl-RO₂ cross reactions also lead to ROH + R=O products, and if the inital peroxy radical group is on a primary carbon atom, the R=O can be a source of acyl-RO₂ following a secondary OH reaction. Alkyl RO₂s can have significant yields for this reaction. Was this accounted for in the model and in the analysis of the experiments?

The more functionalized acyl-RO₂s with an -OOH group elsewhere in the molecule are known to undergo H-scrambling reactions to form peroxy acids (R-C(O)OOH) at rates of 1E3 – 1E5 s^-1 (Knap et al. 2017, J. Phys. Chem. A, 121(7), pp.1470-1479). Can the authors comment on the possible role of this reaction in their experiments? For example, the ring-opened acyl C₁₀H₁₅O₈-RO₂ that they report has a 1,6 H-scramble available that leads to a peroxy acid and an alkyl RO₂. For their model system, Knap et al. estimate a rate coefficient for the 1,6 H-shift of 1.5E5 s^-1. If the rate coefficient is comparable for the alpha-pinene derived C₁₀H₁₅O₈ acyl-RO₂ above, this could to an extent explain the low reduction in signal upon NO₂addition.

Line 468: Regarding the speculation of the formation of alkyl C₉H₁₅O₃-RO₂ to explain the discrepancy between experiments and simulations, this would compete with the formation of the ring-opened and ring-retaining C₁₀H₁₅O₄-RO₂. How did the sensitivity analysis of including the C₉H₁₅O₃-RO₂ in the model affect the yield of the C₁₀H₁₅O₄-RO₂s and the subsequent acyl-RO₂s derived from them?

Are any of the acyl peroxy nitrates detected by the NO₃-CIMS? Alpha-pinene derived APNs with 8 oxygen atoms or more should have at least 2 -OOH functional groups and will presumably cluster well with NO₃^-. Do the decrease in e.g. C₇ and C₈ RO₂ signals when NO₂ is added show an increase in the corresponding APN signals? I think a spectrum figure maybe in the supplementary showing the acyl-RO₂ and acyl-ROONO₂ peaks would be useful.

Figure 4. In pathway 3, the final H-shift of the acyl-oxy is unlikely to compete with CO₂ See reaction r12 and description therein in Vereecken et al. 2009, Phys. Chem. Chem. Phys. 11(40), pp.9062-9074.
Citation: https://doi.org/10.5194/egusphere-2023-838-RC2
- AC2: 'Reply on RC2', Yue Zhao, 28 Aug 2023
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-838/egusphere-2023-838-AC2-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2023-838-AC2

Peer review completion

AR: Author's response | RR: Referee report | ED: Editor decision | EF: Editorial file upload

AR by Yue Zhao on behalf of the Authors (28 Aug 2023) Author's response Author's tracked changes Manuscript

ED: Publish as is (31 Aug 2023) by John Liggio

AR by Yue Zhao on behalf of the Authors (02 Sep 2023) Manuscript

Journal article(s) based on this preprint

11 Oct 2023

Direct probing of acylperoxy radicals during ozonolysis of α-pinene: constraints on radical chemistry and production of highly oxygenated organic molecules

Han Zang, Dandan Huang, Jiali Zhong, Ziyue Li, Chenxi Li, Huayun Xiao, and Yue Zhao

Atmos. Chem. Phys., 23, 12691–12705, https://doi.org/10.5194/acp-23-12691-2023,https://doi.org/10.5194/acp-23-12691-2023, 2023

Short summary

Han Zang, Dandan Huang, Jiali Zhong, Ziyue Li, Chenxi Li, Huayun Xiao, and Yue Zhao

Supplement

https://doi.org/10.5194/egusphere-2023-838-supplement

Han Zang, Dandan Huang, Jiali Zhong, Ziyue Li, Chenxi Li, Huayun Xiao, and Yue Zhao

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Short summary

Acylperoxy radicals (RO₂) are key intermediates in atmospheric oxidation of organic compounds, yet our knowledge of their identities and chemistry remains poor. Here, using direct measurements and kinetic modeling, we identify the composition and formation pathways of acyl RO₂ and quantify their contribution to highly oxygenated organic molecules during α-pinene ozonolysis, which will help to understand oxidation chemistry of monoterpenes and sources of low-volatility organics in the atmosphere.


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