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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RC1: 'Comment on egusphere-2025-2331', Anonymous Referee #1, 28 Jul 2025
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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.
Citation: https://doi.org/10.5194/egusphere-2025-2331-RC1
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