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: open (until 30 Nov 2025)
- RC1: 'Comment on egusphere-2025-4841', Anonymous Referee #1, 17 Nov 2025 reply
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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