the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 25 Jun 2025
Exploring biogenic secondary organic aerosol using a PTRMS-CHARON in laboratory experiments: characterization and fingerprint analysis
Abstract. Volatile Organic Compounds (VOC), particularly of biogenic origin emitted by vegetation and soils, play an important role in the global Organic Aerosol (OA) budget. The introduction of field-deployable Aerosol Mass Spectrometers in the early 2000s, combined with statistical analysis of their mass spectra, has significantly improved our understanding of the impact of secondary processes on fine-mode aerosol concentrations. While delivering innovative and significant insights, those analyses usually fail to explicitly identify precursors/mechanisms. In this context, this work focuses on laboratory-generated secondary OA (SOA) of biogenic VOC and its spectral analysis through a new generation of aerosol mass spectrometers, notably a Proton Transfer Reaction Mass Spectrometer coupled to a Chemical Analysis of AeRosol Online (PTRMS-CHARON) inlet. Aerosol particles were formed in the DouAir atmospheric chamber via isoprene (ISOP) OH oxidation, monoterpene O3 (limonene, MT), and sesquiterpene O3 (β-caryophyllene, SQT) oxidation. ISOP experiments targeted "low-NO" environments, typically remote forested tropical areas, via epoxidiols formation (ISOP-IEPOX-SOA), or through an alternative branching favored in the absence of acidic seed particles (ISOP-Non-IEPOX-SOA) and "high NO" environments, representative in urban and polluted regions (ISOP-NO-SOA). Experiments showed that those five SOA formation pathways (ISOP-IEPOX-SOA, ISOP-Non-IEPOX-SOA, ISOP-NO-SOA, and the ozonolysis reactions of MT and SQT) exhibited distinguishable spectra, with identifiable tracer ions, such as m/z 83.049 (C5H6O), m/z 119.07 (C5H10O3), m/z 137.081 (C5H12O4) for ISOP-IEPOX-SOA, C5H10O4 (m/z 135.070), C5H10O6 (m/z 167.055) for ISOP-Non-IEPOX-SOA, and m/z 85.028 (C4H4O2) for NO-SOA pathways, as well as molecules with C7-C10 and C7-C15 structures identified during MT and SQT oxidation experiments, respectively. These laboratory findings depict promising results for ambient near-real-time biogenic SOA source apportionment, notably in forested and/or urbanized areas.
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Status: open (until 27 Oct 2025)
- RC1: 'Comment on egusphere-2025-2331', Anonymous Referee #1, 28 Jul 2025 reply
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RC2: 'Comment on egusphere-2025-2331', Anonymous Referee #2, 24 Oct 2025
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The paper of Ramírez-Romero et al., “Exploring biogenic secondary organic aerosol using a PTRMS-CHARON in laboratory experiments: characterization and fingerprint analysis” describes 5 SOA experiments with precursors isoprene, limonene and b-caryophyllene in a smog chamber. The particle phase was measured by the newly developed inlet CHARON coupled to a PTRMS. Limonene SOA has also been measured by Gkatzelis et al. (2018), but isoprene and b-caryophyllene SOA have not been characterized by the CHARON-PTRMS system. However, the manuscript is quite weak as it is: a lot of details have not been discussed thoroughly; the experiment section needs to be reorganized. The results should be rewritten. It seems that the paper has a lot of information hidden but it is fast and informally written. The conclusion section is quite weak, and the reader feels confused. I believe these results deserve publication but only after a major revision.
Comments:
Page 2, lines 40-43: please use more recent literature
Page 3, line 108: “artificial UV lamp” you need to explain what kind of lamps were used (range of radiation), number of lamps, JNOx etc..
Page 5, lines155-156: “To ensure consistency across experiments, an EF value of 21 was applied, as specified by the manufacturer's certified documentation”. Why didn’t you apply the EF that you found for levoglucosan (24.1)? An EF of 21 is what is suggested and has been measured for a specific instrument. It doesn’t mean that EF is the same across all CHARON systems. How much do the results change applying an EF of 21? Shouldn’t NH4NO3 EF be near 21 as well? Please explain.
Page 5, lines 159-162: “Other mass spectrometers were used to complement PTRMS-CHARON observations, such as a second PTRMS (second generation, Kore Technology Inc.) for VOC measurements (Michoud et al., 2017).” Are there any data from the second PTRMS presented in the paper? If not, then there is reason to mention it. Please explain.
Page 5: Experimental protocol: This part is very confusing. It should contain only the experimental procedure. If needed you may add a methodology section, but in my opinion it should not be named experimental protocol. Most of it consists of theory mentioning previous literature. Please remove the theory as it is not the appropriate place. If you need the theory in this part either transfer it to the theory part (with the corresponding modifications) or use it later to explain your observations.
In the experimental part you need to be clear in which experiments you used seeds, how OH was produced and how did you calculate the concentrations of OH in Table 1. Also please add information about the precursors used (purity, company).
Page 5, Lines 230-231: “All the measurements were corrected for dilution and wall losses as detailed in the SI.” In the SI there is an equation that corresponds to dilution correction. But there are no details about the wall losses appart from the phrase “First order wall loss rates were determined for gases, O3, and particles and are reported in Table S2.” What kind of particles did you use? (organics, ammonium sulfate?) Were they polydisperse (please give the diameter range), monodispersed (please give the diameter)? Between each experiment? At the beginning of the campaign? How long did the wall losses measurements last (e.g. 4 hours?). Did you use a protocol (may refer to previous literature?). Were these chamber wall losses measured by the CHARON-PTRMS, by the AMS, by an SMPS? If there were from AMS or SMPS how did you apply them on CHARON-PTRMA data?
Page 7, Table 1: Please add RH, NO kai NO2 and seeds concentrations for each experiment. Is the maximum SOA formed, obtained by the CHARON?AMS?SMPS? Please explain.
Page 8, Figure 2a: At the beginning of the experiment there is organic phase appearing along with the ammonium sulfate. Is this organic contamination from seeds? (it has the same trend). As it is quite high (~1/3) of the total mass concentration it can affect the data quality. Did you correct the time series and the mass spectra for this contamination? How? Please explain.
In Figure 2 is the gas phase measured by the second PTRMS? Please explain.
How did you convert the signal of the PTRMS/CHARON in ug/m3 (left y axis)? What k rate did you use? Please explain in the text.
Why SOA formation increase is not continuous in Figures 2a and 2b? The are 3 and 2 “bumps” but there is no precursor added just before. Please explain.
How well do the AMS organic mass concentrations match with the CHARON concentrations for each experiment? What was the CE of the AMS (or what was assumed?)
Page 8, lines 268-276: “The HR-AMS……Morgan et al., 2020)”. This part does not belong to the results section. It is again theory. You may use it to explain your results but as it is right now it looks irrelevant with the rest of the text.
Page 8, lines 279-281: Here the reader does not understand much. What are you referring to? What is the result shown in Fig 3? What does Fig 3 mean? Please explain Fig 3 and then using the theory (maybe in the previous paragraph) support your results. However, I don’t understand why Figure 3 is important or why it should be shown in the paper. What is it relationship with RH or acidic particles?
Page 9, Figure 4: The time series of m/z 83.049, m/z 119.07 and m/z 137.081 (Figure 4a) have completely different trends from the AMS organic phase (Figure 4b). They also do not agree with the time series of CHARON organics (Figure 2a). All these 3 figures refer to the same experiment but they are different between each other. Please explain.
Why is the number concentration of the SMPS necessary in Figure 4b? It is not even discussed in the text.
Page 10, lines: 299-300: “Figure 5 depicts the signature spectrum for the experiments studied here. For ISOP-Non-IEPOX-, MT-, and SQT-SOA, the signature spectra were chosen for the period when OA peaked.” Do you mean when the uncorrected OA mass concentration peak? Did you apply the wall-losses and dilution corrections to check when is the real maximum? Did you compare the mass spectra with the final ones? Moreover, did you compare between the initial and final mass spectra for the rest of the experiments? Please explain.
Page 10, lines: 300-301: “strongly dependent on seed particle acidity” what does this mean?
Page11, Figure 5: Did you correct for any organic contamination in the first (Figure 5a) mass spectrum?
Page11, lines 337-352: “Similar…. Jaoui et al., 2013). Here you need to identify these ions: you need to look for previous literature (which used analytic instrumentation) and link the measured characteristic m/zs to possible compounds. Right now, this paragraph, it is just describing what was observed.
Page 12, lines 367-374: “Non-sulfated dimers….. ISOP-Non-IEPOX-SOA route.” What are you trying to support in the paragraph? I can’t understand the meaning here. Please rewrite.
Page 13, line 394: “mass loadings from 3-5 μg m-3”. Is these concentrations with or without wall loss and dilution corrections?
Page 13, line 408: “under varying NO conditions”. But you didn’t try various NO conditions.
General comment:
Since you have nice particle phase (CHARON/PTRMS) and gas phase (PTRMS) data for the b-caryophyllene experiment as shown in Figure 2e, I suggest you should calculate the volatility of the measured species using for example the approach of Gkatzelis et al. (2018) or Kostenidou et al. (2024). I suggest you to focus only to major m/z’s.
Citation: https://doi.org/10.5194/egusphere-2025-2331-RC2
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- 1
The paper "Exploring biogenic secondary organic aerosol using a PTRMS-CHARON in laboratory experiments: characterization and fingerprint analysis" applies the PTRMS-CHARON instrument to investigate the chemical composition of SOA obtained from the oxidation of well-known
biogenic reactive organic precursors i.e. isoprene, limonene and beta-caryophyllene. The study employed the DouAir atmospheric simulation chamber to simulate ambient aerosol formation. The CHARON data is also supported by additional gas- and particle-phase
measurements. The paper is well-written and research discussion is appropriately supported with citations of relevant previous measurements. However, I felt the study is a bit lacking in casting a proper scope to justify the work. It can be considered for publication after the following concerns are resolved:
1. The scope of the study is not clear. The introduction section is quite large and the authors use only the last paragraph of the section to show that PTRMS-CHARON has been in existence for nearly 10 years already and used in ambient as well as chamber/lab measurements. Several studies are cited to support this. However, the nuances of its use in these studies are not properly laid out. It'd be good to provide some more detail on how was the CHARON instrument used in previous work that leaves open a gap for a systematic investigation of BVOC oxidation products. If the paper is about the application of PTRMS-CHARON, then it should somehow be the focus element of the introduction, especially since the oxidation of BVOCs in itself is not a new thought.
2. Lines 104-115: No citation is provided for previous characterization tests of the DouAir chamber in section 2.1. Is this a new chamber? If so, it should be stated as such since this may introduce uncertainties in measurements. Is the chamber mixed mechanically? The schematic in figure 1 does not show the mixer.
3. Line 123: The thermodesorption unit of the CHARON was operated at 140C. How was this operating temperature set for your instrument and up to what volatility range is evaporated from the particle-phase at this setting?
The more recent Fusion-CHARON instrument from Ionicon Analytik operates at around 170C to evaporate up to ELVOCs.
4. Line 144-145; 152-153: EF was determined as a ratio of the flows before and after the ADL. It is not clear what the term "flow" here means. Is it the particle count or the volumetric flow rate? In line 152, it is unclear what the authors mean by "PTRMS-CHARON measurements". The CHARON provides chemical speciation of the incoming aerosol sample. CPC on the other hand provides particle number/ mass count for monodispersed particles. The authors should specify what CHARON measurement is being ratioed with the CPC data. Citations are provided in lines 145-146 but there should be a brief description to help the reader.
5. I could not find information about the accuracy of mass calibrations in this study. It should be stated to ascertain confidence in peak identifications/molecular formulas.
6. Line 242: A maximum of 5 ppb for a total injection of 60 ppb is interesting. The half life of SQT with ozone would be a few seconds following pseudo first order kinetics. In order to say that the loss is primarily due to high reactivity, the timescale of mixing of the precursor inside the chamber should also be stated.
7. Figure 2: Since the paper is CHARON focused, the PTR-CHARON data in this figure should be made clearer visually. I also raise the following points:
(a) Why the OA exhibits two modes while the isoprene injection occurred only once.
(b) Isoprene injection occurred twice but three modes appear in OA.
(d) Three injections of monoterpene but a smooth enhancement in the OA signal.
(d) SQT-SOA and the precursor SQT signal are positively correlated. Should the precursor not reduce over time as OA grows? Or am I not understanding this correctly.
8. Is C4H8O a real peak or a fragment in figure 5c? Similarly for the C5H6O trace in the CHARON measurements in figure 4a. In AMS measurements, C5H6O is a fragment produced from electron ionization of parent species, which should be interpreted differently than a C5H6O trace signal in PTRMS-CHARON measurements. These compounds appear prominently in the CHARON mass spectra in figures 5a and c and therefore should be carefully discussed. I am not sure whether such oxidation products partition enough to the particle phase to appear so strongly in the aerosol spectra.
9. Figure 3 caption should clearly note whether this is AMS data. Add units/ (e.g. # or fraction) to the y-axis label in figure 3 if there is one on the x-axis. The x-axis unit %o is a bit confusing. Is it a percentage?
10. Figure 4: (b) "AMS/SMPS" can be confused as a ratio. A comma or "and" would be more appropriate.