Effects of NOₓ on secondary organic aerosol yields and composition from a biogenic&ndash;anthropogenic mixture

Xie, Guangzhao; Voliotis, Aristeidis; Bannan, Thomas J.; Shao, Yunqi; Wu, Huihui; Hu, Dawei; McFiggans, Gordon

doi:10.5194/egusphere-2026-2826

Preprints

https://doi.org/10.5194/egusphere-2026-2826

Preprints

11 Jun 2026

| 11 Jun 2026

Effects of NOₓ on secondary organic aerosol yields and composition from a biogenic–anthropogenic mixture

Guangzhao Xie, Aristeidis Voliotis, Thomas J. Bannan, Yunqi Shao, Huihui Wu, Dawei Hu, and Gordon McFiggans

Abstract. Nitrogen oxides (NO_x = NO₂ + NO) play a crucial role in secondary organic aerosol (SOA) formation. The effects of NO_x on SOA formation from single precursors have been extensively studied. However, in the real atmosphere, biogenic and anthropogenic precursors often coexist, and it remains unclear whether the effects of NO_x in such mixtures can be directly extrapolated from those observed in single-precursor systems. In this study, we investigated the effects of NO_x on SOA particle mass yields and chemical composition from α-pinene, n-dodecane, and their mixture under high- and moderate-NO_x conditions in the Manchester Aerosol Chamber. The results show that SOA particle mass yields were higher under high-NO_x conditions across all systems. Enhanced oxidant levels and alkoxy radical (RO) isomerisation appear to more than compensate for the negative impacts associated with the formation of more volatile products via the reaction of organic peroxy radicals (RO₂) with NO. However, compared with the single-precursor systems, the increase in SOA particle mass yields in the mixed-precursor system was less pronounced. In the mixed-precursor system, enhancement of RO₂ + NO termination pathways was stronger, and the contribution of α-pinene-derived alkoxy-peroxy pathway may have been comparatively weaker. These changes would be expected to favour the formation of more volatile products and thus suppress SOA formation. Collectively, these observations provide evidence that the effects of NO_x in the mixed-precursor systems cannot be interpreted as a simple combination of behaviours observed in individual precursor systems.

Received: 16 May 2026 – Discussion started: 11 Jun 2026

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Guangzhao Xie, Aristeidis Voliotis, Thomas J. Bannan, Yunqi Shao, Huihui Wu, Dawei Hu, and Gordon McFiggans

Status: final response (author comments only)

RC1:
'Comment on egusphere-2026-2826', Anonymous Referee #1, 24 Jul 2026
This manuscript by Xie et al. investigates the effects of NOₓ on SOA yields and chemical composition from α-pinene, n-dodecane, and their mixture. The topic is relevant and the dataset is potentially valuable for understanding SOA formation in mixed biogenic–anthropogenic systems. However, several aspects of the experimental comparability and mechanistic interpretation require further clarification. I therefore have several comments that should be addressed before I can recommend the manuscript for publication.
Major comments:
Several high-NOₓ experiments presented in this manuscript appear to be identical to the experiments conducted without CO that were previously reported in Xie et al. (2026). For example, identical SOA mass concentrations and yields are reported for the α-pinene, n-dodecane, and mixed-precursor systems. Although the previous study is cited for certain methodological details, the reuse of these experiments and datasets is not explicitly stated. The authors should clearly identify which experiments and data have been previously published, provide a correspondence between the experiment numbers in the two studies, update the citation to the final ACP publication, and clarify which measurements, analyses, and scientific conclusions are new to the present manuscript.

The Vocus PTR-ToF-MS appears to have been used only to measure the precursor concentrations, with no gas-phase oxidation-product results presented. It would be helpful if the authors could assess whether the Vocus measurements provide additional qualitative information on gas-phase products, for example their carbon number and oxygen number distributions or the relative contributions of CHO and CHON compounds under the different NOₓ conditions. Although the H₃O⁺ mode may not capture the full range of oxidation products and may be affected by fragmentation and compound-dependent sensitivity, the available data could still provide useful supporting evidence for the proposed enhancement of volatile RO₂ + NO products under high-NOₓ conditions.

Although the experiments were designed to have similar initial OH reactivities, Fig. S2 shows clear differences in OH concentrations and their temporal evolution. It would be helpful to clarify whether differences in oxidation age may also contribute to the observed SOA yields and product distributions.

Minor comments:
Table 1: SOA particle mass yields are unavailable for Experiments 5, 8, and 9, although SOA mass concentrations are reported. Please clarify why the yields could not be calculated, including whether precursor-consumption data from the Vocus PTR-ToF-MS were unavailable or considered unreliable for these experiments.

Particle wall losses were corrected, but vapor wall losses were not discussed. The authors are encouraged to consider whether vapor wall losses could affect the reported SOA yields and product distributions, particularly for low-volatility compounds.

Specific typos:
Figure 5 caption: “accretion products (C > 10)” should be corrected to “accretion products (C > 12)”.

Page 10, Line 219: “Measurement were conducted” should be corrected to: “Measurements were conducted”.

Page 13, Lines 277: “Exp. 5” should be corrected to “Exp. 6”, as the reported SOA mass concentration and yield correspond to Exp. 5 in Table 1.

Page 19, Lines 383–386: “high- NOₓ conditions” contains an unnecessary space and should be corrected to: “high-NOₓ conditions”.

Several references contain formatting errors, including Chen et al. (2022), Clapp and Jenkin (2001), Ehn et al. (2014), Nozière et al. (1999), Pospisilova et al. (2020), Pye et al. (2019) and Zhang et al. (2018).
Citation: https://doi.org/10.5194/egusphere-2026-2826-RC1
RC2:
'Comment on egusphere-2026-2826', Anonymous Referee #2, 29 Aug 2026
This manuscript reports results from an atmospheric simulation chamber-based investigation into the effects of nitrogen oxides (NO_x) on the formation of secondary organic aerosol (SOA) from the hydroxyl-initiated oxidation of α-pinene and n-dodecane. The mass yields and chemical composition of the SOA were compared in experiments performed on the precursors both separately and when combined.
The manuscript is well-written; the results are presented and interpreted in a clear and concise manner. Overall, this is an interesting and relevant piece of research which fits very well within the scope of ACP.
The following minor comments should be addressed prior to publication.
It would be useful for the authors to provide more information on the relative amounts of NO and NO₂ during the different experiments. Initial mixing ratios could be added to Table 1 and the time series for NO could also be added to Figure S1. This information would help support the discussion around the influence of NOx on oxidant levels.

Lines 40-41: Surely RO_x = OH + HO₂ + RO₂ + RO?

Line 99: Table 1 appears too early and would be better placed within Section 2.3. What is the assumed SOA density? Also, are the authors confident about reporting ozone mixing ratios to the first decimal place?

Line 126: the phrase “regenerating RO₂ radicals” should be changed to avoid the reader misinterpreting this to mean that the same peroxy radicals are formed again.

Lines 266-271: Are the authors confident about reporting ozone mixing ratios to the first decimal place?

Lines 293-294: Join this two-sentence paragraph with the previous one. In fact, there are several other instances in the manuscript where very short paragraphs could be combined with the preceding or succeeding text.

Line 300: “C₉H₁₂,₁₄O₆” is not clear. Suggest changing this.

Line 304: “Significantly” is an overstatement. There are only single digit changes in %.

Line 352: Detection of products with the formula C₁₂H₂₆O_x is an interesting observation as it implies no loss of hydrogen despite the initial mechanism being a H-atom abstraction reaction by the hydroxyl radical. What RO₂ reaction pathways could explain the formation of these species? Does the abundance of C₁₂H₂₆O_x species change with the precursor/NO_x ratio?

Line 455 – 459: Why is there increased fragmentation of RO under high-NO_x conditions?
Citation: https://doi.org/10.5194/egusphere-2026-2826-RC2
RC3:
'Comment on egusphere-2026-2826', Anonymous Referee #3, 02 Sep 2026
This paper presents experimental results on the influence of NOx on SOA mass yields and chemical composition. a-pinene, n-dodecane and mixtures thereof were oxidized in a state of the art atmospheric simulation chamber. Overall, the results presented in the paper are timely and of high quality, and the research broadens the knowledge on the impact of mixed precursors on SOA formation and the impact of varying levels of NOx concentrations on the system of both single precursor and mixed precursor SOA formation. There are, however, some comments that should be addressed before publication can be recommended.
Main comments:
There is a thorough discussion on the impact of RO2 fate with NO, understandably as this is the focus of the paper. However, the large differences in ozone concentration and precursor concentration between experiments, brings into question how this has impacted the RO2 fate in terms of HO2 and isomerization. What is the expected impact of the changing RO2/HO2 and RO2/OH ratios on the resulting aerosol formation pathway?

The variability in [OH] provided in figure S.2b is ground for slight concerns regarding the dodecane experiments. The [OH] concentration seems to be varying significantly from measurement point to measurement point, why is this the case? An additional explanation on this would be beneficial.

Whilst the data is given for the loss of precursor VOC in the chamber as % total decline over 5 hours, it would be useful to see the full timeseries of the PTR-MS data for all precursors. Is the loss rate of precursors similar initially or changing significantly between the six experiments? For alpha pinene does the loss trend increase as ozone increases and if so, does this follow the expected loss as a result of increasing ozone?

Regarding the C-AMS data used to determine the SOA yield, no collection efficiency is mentioned and no comparison in mass is made with other instrumentation (e.g. SMPS). A more thorough description on the treatment of the AMS data would therefore be in place to justify the confidence in the SOA yield.

Further, a particle size distribution at the start (Seed) and the end (Seed+SOA) would be a nice addition to show if there is any nucleation or if all the SOA is strictly from condensation.

Regarding SOA mass yields, it is concluded that also in the mixed system higher NOx leads to higher SOA yield, however for the mixed precursor systems the two values reported are 0.11 and 0.08 and there is only one value available for each NOx condition– what are the uncertainties on the SOA yields?

As pointed out by reviewer 1 it should be clarified if/how the high NOx experiments relate to the low CO experiments in Xie et al. 2026.

There should be more data available from the AMS such as H/C, O/C and N/C ratios which could be useful tools to better understand the changes in oxidation on mixed precursors. This could be of general interest for the AMS community and help support the data already presented in the paper. An AMS Van krevelen plot in the supplement would be a good place to start.

Please provide uncertainties on the values given (in percentages) on the contributions from different groups to the total mass, e.g. page 13 – values are given as 55%, 30% and 15 % - are these from one experiment only?

Suggestions to figures:
I would suggest that the colour scheme of figure S4 and S5 is changed to the same colours as fig 1b making them easier to compare.

Further I believe that line 273’s statement of a n-dodecane/Nox =0,8 is not consistent with the data provided in figure 1c.

Line specific changes:
Line 268: what % is referred to with “nearly” and why “approximately” for the SOA mass, you provide this data when referring to all other experiments but the one in this line.

Line 302: “overall contribution was relatively low” was it not just stated that 15% was accretion products? Or is this referring to the normalised signal for each different compound?
Citation: https://doi.org/10.5194/egusphere-2026-2826-RC3

Guangzhao Xie, Aristeidis Voliotis, Thomas J. Bannan, Yunqi Shao, Huihui Wu, Dawei Hu, and Gordon McFiggans

Supplement

https://doi.org/10.5194/egusphere-2026-2826-supplement

Guangzhao Xie, Aristeidis Voliotis, Thomas J. Bannan, Yunqi Shao, Huihui Wu, Dawei Hu, and Gordon McFiggans

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Short summary

NO_x plays a crucial role in secondary organic aerosol (SOA) formation. However, most studies have focused on its effects on individual SOA precursors. This study investigated the effects of NO_x on SOA particle mass yields and chemical composition from a biogenic-anthropogenic mixture. The results reveal that the effects of NO_x in the mixed-precursor systems cannot be interpreted as a simple combination of behaviours observed in individual precursor systems.


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