the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 17 Oct 2025
The impact of CO on secondary organic aerosols formed from the mixture of α-pinene and n-dodecane
Abstract. Atmospheric simulation chambers are powerful tools for investigating atmospheric processes and form the basis for model parameterisations. Ensuring the atmospheric relevance of experimental conditions is crucial for understanding and predicting the impacts of secondary organic aerosols (SOA) on air quality and climate. However, chamber studies are often conducted under simplified conditions, which may limit their applicability to real-world scenarios. Here, we investigated the impact of CO on the mass yields and chemical composition of SOA particles formed from a biogenic volatile organic compound (VOC, α-pinene), an anthropogenic intermediate-volatility organic compound (IVOC, n-dodecane), and their mixture in the presence of nitrogen oxides (NOx = NO2 + NO) in the Manchester Aerosol Chamber (MAC). This photochemical system better represents polluted atmospheric conditions. The results show that the influence of CO differed between single- and mixed-precursor systems. In the single-precursor systems, CO significantly suppressed SOA particle mass yields, whereas no such suppression was observed in the mixture. Moreover, compared with the single-precursor systems, CO exerted a diminished impact on the organic peroxy (RO2) radical reaction pathways in the mixture, with the extent of this change differing between α-pinene and n-dodecane. These findings demonstrate that variations in reaction conditions can lead to different responses in SOA particle properties between the single- and mixed-precursor systems, highlighting the importance of conducting laboratory experiments under atmospherically relevant conditions.
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Status: final response (author comments only)
- RC1: 'Comment on egusphere-2025-4841', Anonymous Referee #1, 17 Nov 2025
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RC2: 'Comment on egusphere-2025-4841', Anonymous Referee #2, 01 Dec 2025
The paper by Xie et al. investigates the SOA formed from aPinene and dodecane, alone, and then in mixtures. The radical budget was altered by addition of CO, which increased the HO2 concentrations. The experiments utilized the FIGAERO-CIMS to determine the chemical composition of the SOA formed under the different conditions. The authors present their results clearly, and there are some questions about determination of OH concentration and their ability to achieve iso-reactivity. Though, there are serious questions about the interpretation of their data with respect to radical chemistry. The discussion then attempts to relate the chemical composition observed in the SOA to understand the radical chemistry taking place within the chamber. The discussion of the radical chemistry suffers greatly by only considering RO2 + RO2 chemistry. The authors state that the formation of specific molecules exclusively forms via RO2 + RO2 reactions, when they do not consider other radical pathways (e.g. alkoxy radicals). The other aspects of the paper are relatively well put together, but the leg on which the paper stands is being able to connect their FIGAERO-MS data to the radical chemistry in the chamber. At the moment, I don't see that clear connection because of my concerns about the radical chemistry discussion.
Major Comments:
Lines 539 – 562: The discussion here focuses purely on the RO2 + RO2 reaction pathway, and does not present a holistic understanding the radical pathways present in the reactions of α-pinene + O3 or OH. This involves the alkoxy radical pathway, which is important part of both the RO2 + RO2 and RO2 + HO2 reaction schemes. This limitation is serious with this paper specifically because on lines 551-553 the authors state that the C10H14Ox can only be formed via RO2 + RO2. Molteni et al (2019) presents clear pathways to the same proposed products that do not invoke RO2 + RO2 (see R2 and R3a/R3B + R5). Because of the weight on these specific molecules (C10H14Ox) and their corresponding products from dodecane being used as the specific proof of the change of the RO2 + RO2 radical reaction pathways, it is crucial for the authors to change their discussion.
The discussion begins to diverge on lines 525-537: I do not understand what the authors mean by “fragment derived RO2 radicals”. My understanding of fragmentation is associated with the alkoxy radical pathway. (Molteni et al. 2019).
More related with the gas-phase reaction pathways:
Section 4.3.1: It appears that the discussion here focuses on RO2 reactions, with the 3 pathways being RO2 + RO2, RO2 + HO2, RO2 + NO, or RO2 (autoxidation). What do the authors expect for the lifetimes of RO2 radicals in the chamber for the different experiments toward the 4 different pathways? (or 3 different pathways if it is not possible to discuss autoxidation) The general increase in N-containing products is surprising in the CO containing experiments. Lines 517-520: What would specifically cause the increase in the RO2 + NO pathway? It seems counter intuitive based on the lower NO levels when CO is present.
Considering the FIGAERO-CIMS can result in the degradation of molecules on the filter, here the authors only present molecular formula measurements (D’Ambro et al, 2017). Do the authors believe there is any serious degradation taking place? No thermograms have been presented, so it is difficult to understand if these are likely intact molecules or fragments.
Minor comments:
Lines 50-62: There is also rich literature about mixtures of VOCs and their impact on new particle formation.
Line 71-73: I understand this is a statement from the Baker paper, but it is simply not true as it stands. The SOA yield of a mixture with a dominant RO2 + RO2 pathway is the SOA yield for those specific conditions. The unspoken aspect of this sentence is that the HO2/RO2 ratio is not environmentally relevant for RO2 + RO2 dominant studies, meaning using a RO2 + RO2 dominant yield when the reality is that the RO2 + HO2 pathway is dominant would create an overestimate of the yields in whatever model you choose to use.
Lines 108 – 110: What is the total spectrum of UV light look like? What is the jNO2? Who is the supplier for the UVC lamp? (since the lights are slightly different with the addition of the 254nm lights compared to the Shao et al. publication)
Line 110-112: were the injections performed with a syringe?
Lines 122 – 125: What was the order of the seed injection and humidification? At the moment it is unclear to me what the phase state of the seed is.
Lines 169-181: Is it wise to heat the PTFE filter over 260 °C? There can be degradation of PTFE and the release of fumes from the filter above that temperature. (Sajid et al. 2017)
Section 2.2: how was OH radical concentration determined in Figure S5? I don’t see something in the methods section that describes this, and with the presence of O3 does this complicate the determination of O3 when using aPinene as an OH tracer? I see this is mentioned briefly in section 4.1, but it warrants a clear explanation in the methods section. Since this is a batch mode experiment, how does dilution in the chamber impact the depletion of CO? Why is dodecane not included in mixture of Figure S5?
Section 2.3.2: Is the VOCUS run with a GC column? If so please provide the relevant details. I suspect that there is a GC column because of the mention of a chromatography cycle on line 198.
Section 2, what types of blank measurements were performed with the chamber?
Lines 202-204: how did you verify that the injected concentrations are what you think they were?
Lines 204-205, what fragments were used with this method?
Lines 205 – 206: were calibration performed similar to Figure S2 to verify the robustness of using C10H21+
Section 2.3.3: was a dryer used with the AMS? If not how was it verified that the collection efficiency was the same between the experiment and the calibration? I ask because there was likely different RH conditions between the experiment and the calibration.
Figure 1: because of the presence of O3 what is the difference in the OH vs O3 reactivity in the different experiments? The caption should provide information about what fragments mean. Also, how does the OH produced by aPinene ozonolysis impact the iso-reactivity calculations?
(Continuing with Figure S12) In Figure S12, it is not clear if each bar corresponds to the integrated OH/O3 reactivity or is it for that specific unit time? The y-axis label should be changed, at the moment it appears to indicate a ratio of OH / O3, which isn’t what the figure is showing.
Line 465-467: This doesn’t appear to be true for the dodecane case because the OH never ‘recovered’.
Line 470 – 475: I do not understand this discussion. It would appear to be true at face value if isoreactivity was achieved, but it clearly wasn’t perfectly achieved in Figure S5. So aren’t the changes in OH concentrations purely able to describe these results?
Figure 5 and section 4.2: I am a bit confused by the purported ~50% difference in the yield with vs. without CO for aPinene. Based on Figure 5 (left panel) the yield should be effectively the same with vs. without CO. Can the authors comment on the apparent discrepancy in the text and Table 1 with the Figure?
Lines 543 -545: the way the percentages are talked about are misleading. Perhaps the authors should talk about the percentage reduction of specific molecular cases e.g. 2% reduction for C10H14Ox is a reduction from ~11% à 9% (Figure 6), which is a reduction of ~20%
References:
Sajid, M., Ilyas, M. PTFE-coated non-stick cookware and toxicity concerns: a perspective. Environ Sci Pollut Res 24, 23436–23440 (2017). https://doi.org/10.1007/s11356-017-0095-y
Ugo Molteni, Mario Simon, Martin Heinritzi, Christopher R. Hoyle, Anne-Kathrin Bernhammer, Federico Bianchi, Martin Breitenlechner, Sophia Brilke, António Dias, Jonathan Duplissy, Carla Frege, Hamish Gordon, Claudia Heyn, Tuija Jokinen, Andreas Kürten, Katrianne Lehtipalo, Vladimir Makhmutov, Tuukka Petäjä, Simone M. Pieber, Arnaud P. Praplan, Siegfried Schobesberger, Gerhard Steiner, Yuri Stozhkov, António Tomé, Jasmin Tröstl, Andrea C. Wagner, Robert Wagner, Christina Williamson, Chao Yan, Urs Baltensperger, Joachim Curtius, Neil M. Donahue, Armin Hansel, Jasper Kirkby, Markku Kulmala, Douglas R. Worsnop, and Josef Dommen, ACS Earth and Space Chemistry 2019 3 (5), 873-883, DOI: 10.1021/acsearthspacechem.9b00035
D'Ambro, E. L., Lee, B. H., Liu, J., Shilling, J. E., Gaston, C. J., Lopez-Hilfiker, F. D., Schobesberger, S., Zaveri, R. A., Mohr, C., Lutz, A., Zhang, Z., Gold, A., Surratt, J. D., Rivera-Rios, J. C., Keutsch, F. N., and Thornton, J. A.: Molecular composition and volatility of isoprene photochemical oxidation secondary organic aerosol under low- and high-NOx conditions, Atmos. Chem. Phys., 17, 159–174, https://doi.org/10.5194/acp-17-159-2017, 2017.
Citation: https://doi.org/10.5194/egusphere-2025-4841-RC2
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- 1
In "The impact of CO on secondary organic aerosols formed from the mixture of α-pinene and n-dodecane" Xie et al. present results from smog chamber experiments investigating the formation of secondary organic aerosol (SOA) from 3 different systems of precursors (two single-precursor systems and the mixed system) and each with and without CO added. Each added level complexity represents slightly more realism. NOx, ozone and UV illumination facilitate photochemical oxidation of the precursors. Concentration ratios of precursors to NOx and total precursor OH reactivities are chosen to be more or less constant in the initial mixtures. That approach plus an appropriate set of instrumentation (most importantly mass spectrometers to investigate SOA composition) allow the authors to hypothesize how differences in the mixtures modify radical chemistry as well as SOA yield.
Commendable features/highlights of the paper are the nice figures (including good use of distinctive colors for the 3 different aerosol precursor systems), and the candid discussion of the challenges in attempting to obtain similar conditions across experiments, in particular in terms of oxidant and radical concentrations when changing precursor mixtures, even if certain initial ratios can be controlled.
All in all, I judge the paper of high quality and good interest for the readership of Atmospheric Chemistry & Physics. I recommend its acceptance subject to minor revisions in consideration of my comments below. The comments generally call for a bit more clarity or slightly extended discussion (adding a few details, considering minor restructuring).
Line numbers refer to the preprint PDF.
Main comments:
1)
I wonder if the authors could briefly hypothesize, how changes in RO2 pathways (or other chemistry) between the different systems could relate to the observed changes in SOA yields?
2)
The DMPS is presented as part of the instrument line-up. But I do not recall any of its measurement results being presented or even discussed. How were its data used? Would it be worth discussing its results?
3)
Section 2.2: Precursor mixture ratios were chosen according to OH reactivity. Is it possible to assess, how relevant the resulting mixtures then are to atmospheric conditions?
4)
If Table 1 reports mean values over several experiments for each "experiment number", that should be somehow communicated within Table 1 (or its caption). And standard deviations shown.
Related to that, for Fig. 1:
- It should be clarified how many repeat experiments were done for each system.
- I believe Fig. 1 would work better if the (d) plots were incorporated into panel (c), either as a combined 3rd panel, or as purple lines into the existing (c)-panel plots.
- I would also more explicitly state that time 0 is the start of step iii (lights on, I guess)
5)
Section 4, L534: What instrumental limitations specifically? Figs. 2-4 suggest that accretion product concentrations do indeed decrease in the CO-added cases. Wouldn't the data shown there directly allow for making quantitative assessments?
6)
Sections 5 + 6: The last two sections confused me a bit. Section 6 ("Conclusions") is rather a summary (minus the last short paragraph), whereas Section 5 ("Implications") seems more like the conclusions I would have expected from Section 6.
To improve flow and readability, I suggest swapping those two sections (probably making that last paragraph in the current Section 6 superfluous) and rename them as appropriate.
Minor comments:
Abstract: A quick summary of employed methodology could be added. Presumably measurement methods, though when reading only the abstract, the paper kind-of could be a pure modeling study too.
L22: "better" than what else?
L52: "precursors" of what?
L60: The key findings of those more recent studies should be briefly summarized as well.
L66: Only older studies are cited here, though newer ones have contributed substantially to our understanding of the role of RO2 chemistry in SOA formation (e.g., autoxidation). I suggest somewhat expanding that discussion here accordingly.
L109: (major) wavelengths of those lamps?
L113: NOx cylinder specs?
L118: what kind of aerosol generator?
L122 (and 134): what is "cyclic flushing"?
L128: how was step iii initiated?
L167: DMPS specs?
2.3.1: There must be some mistake with the temperatures, as 310 °C would probably destroy a PTFE filter rather quickly.
L183: What is that weekly "instrument background procedure"? Please explain.
L185: Similarly, why was data only analyzed for a specific section of the mass spectrum?
L198: what is the "4 min chromatography cycle"? Judging from the timings, I guess that is mistake? (L188 even implied that chromatography was not required for the Vocus PTR-MS, but if some chromatography step was included nonetheless, that should of course be described.)
L203: does "set values" refer to calculated concentrations based on what was injected into the glass bulb?
L213-214: are these values to be expected based on previous studies?
Eq. 2: what does the superscript "SUS" refer to?
L216-221: unclear what the correction is trying to achieve (correct for; or "calibrate"?)
L225: "per unit of precursor" could be confusing. I assume DeltaHC is also in units of mass (like DeltaSOA)?
L277: "170-280 Da" ... From Section 2 I had assumed that data below 200 Da was not analyzed (L185)?
... Likewise, Figs. 2 etc...
L288: "the two systems" ... please clarify what the "systems" refer to.
Technical comments:
L224: typo (measured)
L297: missing "the"
L529: check grammar