Impact of the chemical regime on highly oxygenated molecule and secondary organic aerosol formation: Effects of varying the importance of NO, HO<sub>2</sub>&middot;, RO<sub>2</sub>&middot; reactions on &alpha;-pinene photooxidation products

Baker, Yarê; Geretti, Veronica; Kang, Sungah; Wu, Rongrong; Wang, Hui; He, Quanfu; Hohaus, Thorsten; Zanders, Annika; Bachner, Mathias; Wu, Cheng; Bannan, Thomas J.; O'Meara, Simon P.; Voliotis, Aristeides; Hallquist, Mattias; McFiggans, Gordon; Zorn, Sören R.; Mentel, Thomas F.

doi:10.5194/egusphere-2026-4788

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https://doi.org/10.5194/egusphere-2026-4788

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13 Aug 2026

| 13 Aug 2026

Status: this preprint is open for discussion and under review for Atmospheric Chemistry and Physics (ACP).

Impact of the chemical regime on highly oxygenated molecule and secondary organic aerosol formation: Effects of varying the importance of NO, HO₂·, RO₂· reactions on α-pinene photooxidation products

Yarê Baker, Veronica Geretti, Sungah Kang, Rongrong Wu, Hui Wang, Quanfu He, Thorsten Hohaus, Annika Zanders, Mathias Bachner, Cheng Wu, Thomas J. Bannan, Simon P. O'Meara, Aristeides Voliotis, Mattias Hallquist, Gordon McFiggans, Sören R. Zorn, and Thomas F. Mentel

Abstract. An important source of secondary organic aerosol (SOA) are highly oxygenated molecules (HOMs) formed by atmospheric oxidation of volatile organic compounds. HOM formation is governed by the fate of HOM peroxy radicals (RO₂·) which depends on the availability of reaction partners (RO₂·, HO₂·, NO) and on their overall lifetime – factors often insufficiently explored in laboratory studies.

We performed α-pinene photooxidation experiments systematically exploring these parameters and present the impacts on HOM and SOA formation within a generic framework explaining the changes in HOM production. Steady-state experiments were performed in the SAPHIR-STAR atmospheric simulation chamber. The reaction regime was shifted by increasing HO₂· and NO, separately and simultaneously, while keeping the α-pinene primary oxidation conditions constant. (NH₄)₂SO₄ particles were added to observe gas-phase HOM condensation and investigate product volatilities.

We find decreasing SOA formation potential when moving away from RO₂·-dominated regimes. One reason is the suppression of HOM accretion product formation from HOM-RO₂·+RO₂·. Alkoxy radicals (RO·) from RO₂·+RO₂· or RO₂·+NO play another important role. RO· are crucial intermediates in certain HOM formation pathways but also produce lower-mass, more fragmented HOM with higher volatility, decreasing SOA formation potential. Additionally, RO₂·+NO forms organic nitrates, which we show have higher volatility than other termination products. Our mechanistic considerations illustrate which factors impact the HOM product distribution and explain the reduced SOA formation through changes in HOM composition and volatility.

How to cite. Baker, Y., Geretti, V., Kang, S., Wu, R., Wang, H., He, Q., Hohaus, T., Zanders, A., Bachner, M., Wu, C., Bannan, T. J., O'Meara, S. P., Voliotis, A., Hallquist, M., McFiggans, G., Zorn, S. R., and Mentel, T. F.: Impact of the chemical regime on highly oxygenated molecule and secondary organic aerosol formation: Effects of varying the importance of NO, HO₂·, RO₂· reactions on α-pinene photooxidation products, EGUsphere [preprint], https://doi.org/10.5194/egusphere-2026-4788, 2026.

Received: 07 Aug 2026 – Discussion started: 13 Aug 2026

Competing interests: At least one of the (co-)authors is a member of the editorial board of Atmospheric Chemistry and Physics.

Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this paper. While Copernicus Publications makes every effort to include appropriate place names, the final responsibility lies with the authors. Views expressed in the text are those of the authors and do not necessarily reflect the views of the publisher.

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RC1:
'Comment on egusphere-2026-4788', Anonymous Referee #1, 11 Sep 2026 reply
This manuscript investigates how changes in the RO₂ reaction regime affect HOM composition and SOA formation from α-pinene photooxidation. The experimental design is valuable, particularly the attempt to maintain a nearly constant α-pinene–OH turnover while systematically varying the relative importance of RO₂ reactions with NO, HO₂ and RO₂. The study therefore addresses an important issue in interpreting chamber experiments, where RO₂+RO₂ chemistry can be more important than under many atmospheric conditions. The results are interesting and the manuscript contains a substantial amount of useful experimental information. I think the study is potentially suitable for publication in ACP. However, several aspects of the framework and its interpretation would benefit from further clarification. I therefore recommend that the following points be addressed before publication.
Specific comments
The framework relies on modeled HO₂ and total RO₂ concentrations, for which no direct absolute measurements are available. Could the authors provide a simple sensitivity analysis showing how plausible changes in these concentrations affect S_RO2 and the inferred reaction-partner fractions?

The calculated reaction-partner fractions depend directly on the generic rate coefficients adopted for RO₂+NO, RO₂+HO₂ and RO₂+RO₂. Given the uncertainty associated with applying single representative rate coefficients to a chemically diverse RO₂ population, it would be useful to show how sensitive the derived partner fractions and S_RO2 are to reasonable variation in these coefficients. This is particularly relevant for mixed regimes such as the ∼50% HO₂/45% NO case, where the distinction between the two contributions may not be robust within the uncertainty of the framework.

The manuscript frequently refers to the total bimolecular RO₂ reaction rate as a “termination” rate, although it also notes that part of the RO₂+NO and RO₂+RO₂ chemistry can proceed through RO formation and regenerate RO₂. I therefore suggest using terms such as “bimolecular RO₂ reaction rate” or “RO₂ loss frequency” for the total pool, and reserving “termination” for pathways that actually end the autoxidation chain.

In Sect. 4.6, the fraction remaining in the gas phase after seed addition is used as a proxy for product volatility, but the observed fraction may also depend on uptake kinetics, equilibration timescale, solubility and particle properties. In addition, adding a condensation sink could affect the lifetime of condensable intermediates and hence subsequent oxidation chemistry. The assumptions behind this interpretation should be stated more explicitly.

Some of the mechanistic and broader atmospheric conclusions appear stronger than can be directly established from the present experiments. The proposed importance of alkoxy–peroxy chemistry is plausible and consistent with the observed product distributions, but RO radicals or specific alkoxy–peroxy steps were not directly measured, and the authors acknowledge that persistent HOM formation may also involve fast RO₂ intramolecular H-shifts. In addition, the experiments are limited to the α-pinene+OH system, whereas the Conclusions suggest that the observed factor-of-two SOA reduction could serve as an estimate of the bias when RO₂-dominated laboratory results are extrapolated to atmospheric conditions, and that similar effects may apply to all monoterpenes. I suggest distinguishing more clearly between conclusions directly supported by the measurements and broader mechanistic or atmospheric implications.

Minor and technical comments:
The authors may consider using “highly oxygenated organic molecules” for HOM, consistent with the terminology recommended by Bianchi et al. (2019).

All experiments were conducted at 50% RH. Could the authors briefly explain the rationale for this choice? Since RH can affect aerosol water content, particle phase state and gas–particle partitioning, particularly in the ammonium-sulfate-seeded experiments, it would also be useful to comment on whether the observed HOM/SOA trends are expected to remain similar under substantially drier or more humid conditions.

A simplified overview scheme showing the competing RO₂ reactions with NO, HO₂ and RO₂, the alkoxy–peroxy pathway, and the resulting HOM-Mon, HOM-Frag, HOM-Acc and ON-HOM classes would make the manuscript easier to follow. The authors could consider expanding Fig. 1 into such an overview rather than showing only the alkoxy–peroxy step.

Figure 2a: The repeated radical dots in the axis labels make the labels visually rather cluttered.

The authors include several O₄ and O₅ compounds in the HOM classes, although HOMs are conventionally defined here as compounds containing at least six O atoms. Please make this extended definition explicit in the relevant figures/captions and sums (e.g., HOM_O_≥4, or another suitable notation to avoid ambiguity).

The agreement between the calculated partner fractions and measured product distributions is encouraging. However, the linear fit in Fig. 3 is forced through zero, while the fit in Fig. 4 is constrained through the hypothetical 1:1 point. It would be useful to also report the corresponding unconstrained fits, or briefly discuss how sensitive the apparent relationships are to these constraints.

Line 417: “HOM-Acc sum (Fig. 5c)” should be corrected to “HOM-Acc sum Fig. 5e”.

In SI Line 141: the reference to Eq. (S3)? appears to be missing. Please also report the k_wall and k_cond values actually used for the correction.

The reported steady-state NO concentrations appear inconsistent between Tables S4 and S5 and the main text.

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Short summary

Highly oxygenated molecules (HOMs) are important contributors to global organic aerosol production. The production of HOMs is based on the reaction pathways of peroxy radicals (RO₂). In this work we systematically varied available RO₂ reaction partners in a-pinene photooxidation experiments in an atmospheric simulation chamber. The results show which factors impact HOM formation and volatility and offer a mechanistic framework for interpreting laboratory and atmospheric observations.


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Impact of the chemical regime on highly oxygenated molecule and secondary organic aerosol formation: Effects of varying the importance of NO, HO2·, RO2· reactions on α-pinene photooxidation products

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Impact of the chemical regime on highly oxygenated molecule and secondary organic aerosol formation: Effects of varying the importance of NO, HO₂·, RO₂· reactions on α-pinene photooxidation products