the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 13 Aug 2026
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
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.
Competing interests: At least one of the (co-)authors is a member of the editorial board of Atmospheric Chemistry and Physics.
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Status: final response (author comments only)
- RC1: 'Comment on egusphere-2026-4788', Anonymous Referee #1, 11 Sep 2026
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RC2: 'Comment on egusphere-2026-4788', Anonymous Referee #2, 28 Sep 2026
This manuscript presents a well-designed experimental study investigating how the bimolecular reactions of RO₂ radicals affect HOM formation and subsequent SOA formation under different reaction regimes. The authors systematically vary the overall bimolecular RO₂ reaction rate and the relative contributions of RO₂ + HO₂, RO₂ + NO, and RO₂ + RO₂ reactions while keeping the α-pinene OH turnover constant. The combination of HOM product characterization, targeted seeding experiments, and mechanistic interpretation provides an interesting framework for connecting laboratory observations of HOM chemistry with atmospheric SOA formation.
I am primarily an aerosol guy with a background in bulk aerosol chemistry rather than a specialist in detailed RO₂/HOM chemistry. Therefore, I may not be in the best position to critically assess all of the mechanistic interpretations and reaction pathways discussed in the manuscript. From an aerosol perspective, however, I find the experimental design and the attempt to link changes in HOM composition and volatility to SOA formation particularly interesting. The results appear to provide useful insights into how reaction conditions commonly used in laboratory studies may influence the inferred SOA formation potential.
I have only a few comments, mainly concerning the description and characterization of the experimental measurements and some details of presentation. Addressing these points would improve the reproducibility and clarity of the study.
In Sect. 3.2, more information on the calibration procedures and measurement characterization of the PTR-TOF-MS and CIMS would be helpful. In particular, the authors might describe the calibration methods, calibration standards or compounds used, and, where relevant, the sensitivity/response factors and detection limits.
L54: “formation pathways a common concern” should be revised to “formation pathways, a common concern.” Similar missing commas should be checked throughout the manuscript. For example, in L280, “As explained in Sec. 3.1 two different…” should be revised to “As explained in Sec. 3.1, two different…”.
L242: Please provide the size range of the SMPS and the flow rates used for the relevant instruments. These details are important for assessing the aerosol characterization and would improve the reproducibility of the experiments.
Citation: https://doi.org/10.5194/egusphere-2026-4788-RC2 -
RC3: 'Comment on egusphere-2026-4788', Anonymous Referee #3, 28 Sep 2026
The authors present a systematic laboratory study of α-pinene photooxidation in the SAPHIR-STAR chamber, focusing on how the chemical regime affects HOM and SOA formation. The experiments vary NO and HO2 separately and simultaneously while keeping the α-pinene + OH turnover approximately constant. The authors introduce a generic framework based on two parameters: the total pseudo-first-order bimolecular RO2 reaction rate and the fractional contributions of NO, HO2, and RO2 to that sink. They analyze changes in HOM product families, organic nitrates (ON), accretion products (HOM-Acc), monomers (HOM-Mon), and fragments (HOM-Frag). This is a well-designed and timely study that addresses an important gap in understanding how NOx and HO2 availability control HOM and SOA formation. The manuscript is generally clear and the mechanistic discussion is thoughtful. However, several conclusions rest on modeled radical concentrations, generic rate coefficients, and product-class assignments that require additional validation and uncertainty analysis. The atmospheric implications are interesting but need to be more carefully reconciled with the observed reductions in SOA yield.
Major Comments
The central framework depends on box-modeled steady-state concentrations of HO2 and the sum of RO2 radicals. The authors acknowledge that no direct HO2 or RO2 measurements were available. Given that the key parameters are derived from these modeled concentrations, the robustness of the entire regime classification should be quantified. I recommend a sensitivity analysis in which the modeled HO2 and RO2 concentrations are perturbed within reasonable uncertainties and the resulting changes in the regime parameters and conclusions are reported.
The framework uses generic rate coefficients for HOM-RO2 + NO, HOM-RO2 + HO2, and HOM-RO2 + RO2, and assumes an average autoxidation rate of 0.1 s-1. Real HOM-RO2 chemistry involves a wide distribution of H-shift rates, and the fate of a given RO2 depends on its specific structure. The conclusion that a bimolecular rate exceeding 0.1 s-1 implies earlier termination is therefore an oversimplification. The authors should either (a) provide a sensitivity study using a range of autoxidation rates (e.g., 0.01–1 s-1), or (b) more clearly frame the framework as a qualitative/tendency indicator rather than a quantitative predictor.
The attribution of HOM families to specific termination pathways is central to the interpretation. Some assignments are convincing, but others are ambiguous. For example, C10H18O2 can arise from HO2 termination or from RO2 + RO2 alcohol formation; C10H14O2 can be formed by multiple pathways, including internal termination and alkoxy radical chemistry. The authors acknowledge some of these issues, but the manuscript would benefit from a more systematic uncertainty table for each key family. In particular, the claim that ON-HOM are uniquely from RO2 + NO is supported by the UV-A-off experiment, but PAN-like formation from acyl peroxy + NO2 cannot be fully excluded. The interference of ON-HOM with HOM-RO2 detection in the CIMS is also acknowledged, but its impact on the trends in Fig. 5b and 5e is not quantified. How much of the apparent decrease in C10-HOM-RO2 or HOM-Acc could be due to overlapping ON peaks or reduced sensitivity? This needs a more detailed error analysis.
The conclusion that ON-HOM have higher volatility than non-ON termination products relies on the fraction-remaining approach with ammonium sulfate seed. This method is useful but sensitive to seed surface area, particle mass loading, equilibration time, wall losses, and condensed-phase diffusion. The authors should report the seed surface area and mass concentrations, the time to reach steady state, and whether gas-phase wall losses differ between ON and non-ON compounds. Subtracting the nitrate group mass is an interesting approach, but it assumes that the volatility difference is mainly due to molar mass and not to functional-group-specific interactions such as hydrogen bonding or solubility. The discussion of Sagawa and Shikata (2014) and Suda et al. (2014) is helpful, but additional support—e.g., comparison with literature volatility estimates or a thermodynamic model—would strengthen the conclusion.
The wall-loss-corrected SOA yields show a maximum reduction of about a factor of two when moving away from RO2-dominated conditions. The authors argue that this is due to both lower HOM production and higher HOM volatility. However, SOA yield also depends on the absorptive partitioning of semivolatile products, which in turn depends on the total organic mass concentration. Since the experiments were designed to have similar α-pinene + OH turnover but different HOM concentrations, the organic mass loading may differ between regimes. The authors should either (a) show that the organic mass concentrations were sufficiently similar, or (b) use a partitioning model to separate changes in production from changes in partitioning. Without this, the statement that the observed yield reduction is an estimator of the bias of RO2-dominated laboratory systems may be premature.
The conclusion states that “more ‘urban’ atmospheric conditions with higher availability of NO might be more efficient in SOA formation than previously assumed.” This appears to conflict with the observed decrease in SOA yield with increasing NO. The authors seem to mean that HOM-RO2 + NO can continue autoxidation and open new formation pathways, partially offsetting termination. This nuance should be stated more carefully. In addition, the experiments use j(NO2) ≈ 1.0 × 10-3 s-1, which is lower than typical summertime atmospheric values (~4 × 10-3 s-1), and NO steady states are generally <1 ppbv. Real urban environments can have much higher NO and different HO2/NO ratios. The authors should discuss how the framework could be extrapolated to higher NOx conditions and what additional experiments would be needed to test the “urban” implication.
Citation: https://doi.org/10.5194/egusphere-2026-4788-RC3 -
RC4: 'Comment on egusphere-2026-4788', Anonymous Referee #4, 29 Sep 2026
General Comments
This study investigates how systematic changes in both the magnitude and composition of the bimolecular RO₂· reaction sink, through varying contributions from NO, HO₂·, and RO₂·, affect HOM composition and SOA formation while maintaining an approximately constant α pinene + OH turnover rate in the SAPHIR STAR chamber. The work builds directly on Baker et al. (2024), extending the previously explored HO₂·/RO₂· dimension to include NO containing and mixed NO/HO₂· chemical regimes.
Several aspects of the study are particularly noteworthy. First, the experimental design provides strong control over the entry flux into α pinene oxidation while systematically modifying the downstream RO₂· reaction environment, allowing chemical regime effects to be investigated more cleanly than in many conventional chamber experiments. Second, the analysis in Sect. 4.4 is particularly interesting. The inferred shift between the C10H15Ox· and C10H17Ox· precursor families is examined using three complementary molecular signatures, including non ON monomers, C₂₀ accretion products, and organic nitrates. Together, these observations provide a persuasive mechanistic picture. Third, the authors are generally transparent about the limitations of their framework and appropriately report observations that cannot be fully explained by the proposed mechanisms, such as the persistence of the C10H14Oz family.
Overall, the qualitative conclusions are well supported by multiple lines of evidence. In particular, the data consistently indicate substantial changes in HOM composition and a decrease in SOA formation potential as the system moves away from the RO₂· dominated regime. The seed addition experiments also support the conclusion that ON HOMs partition less efficiently to the particle phase than non ON products of similar molar mass under the experimental conditions employed here. However, several of the more quantitative and mechanistic conclusions require further refinement, because the central chemical regime parameters rely on modeled radical concentrations and generic reaction rate coefficients. Most of the concerns raised below appear addressable through additional analysis of existing data, clearer quantification of uncertainties, and expanded discussion rather than through new experiments.
Please see attached file for specific comments.
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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
Minor and technical comments: