Mechanistic investigations of the formation of highly oxidized products from the multi-generation OH oxidation of styrene

Chen, Long; Huang, Yu; Xue, Yonggang; Cui, Long; Jia, Zhihui

doi:10.5194/egusphere-2025-4646

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https://doi.org/10.5194/egusphere-2025-4646

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23 Oct 2025

| 23 Oct 2025

Mechanistic investigations of the formation of highly oxidized products from the multi-generation OH oxidation of styrene

Long Chen, Yu Huang, Yonggang Xue, Long Cui, and Zhihui Jia

Abstract. Styrene is the second most efficient aromatic compound in the formation of secondary organic aerosol (SOA) after toluene. Recent experiments have identified C₇ and C₈ series compounds as the main components of SOA in the photooxidation of styrene. However, their molecular structures and formation pathways remain largely uncharacterized. Herein, the formation mechanisms of highly oxidized products from the multi-generation ·OH oxidation of styrene are studied using the quantum chemistry methods. The calculations show that the multi-generation ·OH oxidation mechanisms of styrene are modulated by RO₂ and RO radicals. For the first generation ·OH oxidation, the addition of OH radicals to the C_β-site of vinyl group is the dominant pathway, and the main C₇- and C₈-products are benzaldehyde, 1^st-ROOH (C₈H₁₀O₃) and 1^st-RONO₂ (C₈H₉NO₃). For the second generation ·OH oxidation, OH-addition reaction occurring at the ortho-position of 1^st-ROOH and 1^st-RONO₂ has a significant dominance. The peroxide bicyclic peroxy radicals (BPR) can react with HO₂·and NO to form the C₈-products 2^nd-ROOH (C₈H₁₂O₈) and 2nd-RONO2 (C₈H₁₀N₂O₁₀). The formed peroxide bicyclic alkoxy radical (BAR) can proceed either the ring-opening reactions to form the multifunctionalized dicarbonyls and ketene-enols, or the cyclization reactions to generate the highly oxidized C₆-epoxides with the branching ratios of ~30 %. For the third generation ·OH oxidation, syn-OH-addition occurring at the C=C double bond of 2^nd-ROOH and 2^nd-RONO₂ has the smallest barrier. The major C₈-products are the multifunctionalized hydroperoxides and organic nitrates. The volatility of the multi-generation ·OH oxidation products significantly decreases with increasing the number of ·OH oxidation steps.

Received: 22 Sep 2025 – Discussion started: 23 Oct 2025

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Long Chen, Yu Huang, Yonggang Xue, Long Cui, and Zhihui Jia

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RC1:
'Comment on egusphere-2025-4646', Anonymous Referee #1, 11 Nov 2025
The manuscript presents a comprehensive and theoretically sound investigation into the multi-generation ·OH oxidation mechanisms of styrene. The study employs high-level quantum chemical calculations to elucidate complex reaction pathways, including the formation of bicyclic peroxy radicals (BPR) and their subsequent reactions, which are crucial for understanding SOA formation from aromatic compounds. The topic is of significant interest to the atmospheric chemistry community. While the computational methodology is robust and the overall conclusions are supported by the data, the manuscript requires revisions to improve clarity, depth of discussion, and to address several specific technical points before it can be accepted for publication.
Comments:
please use “OH radical” and “·OH” consistently in the manuscript.

In Page 8, when introducing subscript letters (e.g., S2-1-x) to denote species for the first time, please clearly define their meaning.

In Page 8, Line 206, If ΔE_r > 59.6 kcal/mol, this is an endothermic reaction. Please verify whether this description is correct.

In Page 9, Line 238, when discussing the pseudo-first-order rate constant for the RO₂ bimolecular reaction, it is recommended that the authors provide additional clarification regarding the typical atmospheric reactant concentration corresponding to the value of ~0.01 s⁻¹. Note that the k value cited in the reference refers to indoor environments.

In Page 11, Line 288, The citation "Zhang et al., 209;" is incomplete and should be corrected to the full and correct reference, which is likely "Zhang et al., 2019".

In Page 11, Lines 279-288, authors attribute the discrepancy between this study and the results of Zhang et al. (2024) at the ·OH addition sites primarily to the latter's failure to account for pre-reaction complexes. This claim requires further evidence to substantiate. It is recommended to supplement the study with complete potential energy surfaces for the C1 and C6 sites under both methodologies. Based on the calculated energy barriers, the branching ratios for ·OH addition to the C1 and C6 sites at 298 K should be computed.

Why did the author consider only the addition pathway and not the H-abstraction pathway when studying the ·OH oxidation of 2nd-ROOH (S6) and 2nd-RONO₂ (S26) in Section 3.3?
Citation: https://doi.org/10.5194/egusphere-2025-4646-RC1
RC2:
'Comment on egusphere-2025-4646', Anonymous Referee #2, 02 Dec 2025
This study employs theoretical calculations to investigate the reaction mechanism and kinetic properties of the styrene–OH radical reaction. The proposed mechanism is detailed, and the potential link to highly oxygenated molecules (HOMs) is discussed. The work is scientifically sound and contributes valuable insights to the field. However, numerous theoretical studies have previously examined initiation and subsequent reactions (e.g., H-shift, NO reactivity) of benzene-series compounds with OH radicals. The authors should clearly articulate the novelty and advantages of this work compared to existing literature. In addition, the following issues require careful attention and revision.
Major comments
The manuscript emphasizes HOM formation from the styrene-·OH reaction. However, the discussion and conclusions do not clearly identify which products qualify as HOMs. In the mechanism discussion and conclusions, the relationship between the reaction pathway and HOM formation should be more explicitly elaborated, with clear identification of which products qualify as HOMs.

Explain how low-NOₓ and high-NOₓ conditions are represented in the computational workflow.

Current kinetic interpretation relies heavily on potential energy barriers without considering other factors such as pre-exponential terms and tunneling effects. To substantiate claims, calculate rate constants for key competing steps. Statements like “Based on ΔEa, H-shift is the rate-determining step” are speculative without quantitative kinetic data. Re-examine all sections where competitiveness is inferred without rate constant calculations.

While individual product generations are discussed, the manuscript lacks an integrated view of the overall mechanism and product yields. It is strongly recommended that the authors leverage their MESMER 6.0 kinetic model to simulate and present the branching ratios (or fractional yields) of the major products. Present fractional yields for major products and illustrate competition between RO2 + NO and RO2 + HO2 channels, highlighting how pathway dominance shifts with NOₓ levels.

The discussion regarding the reaction pathways for certain intermediates, such as S9 in Figure 3 and S29 in Figure 5… appears to be incomplete. Specifically, these radical intermediates are theoretically capable of undergoing additional reactions, such as H-atom shifts or O2 addition, which are not depicted. The authors should either clarify the rationale for excluding these pathways (e.g., based on kinetic insignificance) or include them in the mechanism for a more comprehensive analysis.

Other specific comments:
Methodology (Lines 135–139): Given the relatively small reaction system, CCSD(T) calculations are computationally feasible. Benchmarking the adopted methods for this specific system is essential to validate accuracy.

Section 2.2, Please clarify why conformation searches were specifically performed for RO2· and RO· species. How were the initial conformer samples selected?

Lines 285–288: The authors use the consistency of the styrene-·OH reaction with the toluene-·OH reaction to justify discrepancies in calculated addition sites compared to previous literature. However, the distinct effects of methyl (-CH₃) and hydroperoxyl (-OOH) functional groups on the benzene ring should be explicitly discussed.

The manuscript contains multiple normative errors: (i) Hydroxyl radicals should be denoted as "OH radicals" or "·OH"; (ii) RO₂· and RO· should be consistently expressed as "RO₂·/RO·" or "RO₂/RO radicals". The accuracy of scientific notation and writing requires improvement. Additionally, ΔE (e.g., in Fig. 5 and Supporting Information) should be presented in italics.

Manuscript nomenclature for intermediates/products is complex (such as PS4-add1'-OO-a/-s and PS4-add1''-OO-a-1) and poorly correlated with corresponding figures/structures, significantly hindering readability.

In Lines 238-239 and 467-468, the author also quoted the literature when describing the rate constant. Is this rate constant calculated by the author or given in the literature?

Some figures are difficult to read due to formatting issues, e.g. In Figure 2, the font size is far too small. The energies and species labels are illegible at standard viewing sizes.
Citation: https://doi.org/10.5194/egusphere-2025-4646-RC2

Long Chen, Yu Huang, Yonggang Xue, Long Cui, and Zhihui Jia

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https://doi.org/10.5194/egusphere-2025-4646-supplement

Long Chen, Yu Huang, Yonggang Xue, Long Cui, and Zhihui Jia

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Short summary

The mechanisms and kinetics of the formation of highly oxidized products from the multi-generation ·OH oxidation of styrene in the absence and presence of NO_x are studied using the quantum chemistry methods. The calculations show that the volatility of the multi-generation ·OH oxidation products significantly decreases with increasing the number of ·OH oxidation steps.


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