the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 13 Nov 2025
A Novel Short-Pathlength Photoreactor to Study Aqueous-phase Photochemistry: Application to Biomass-Burning Phenols
Abstract. Aqueous-phase oxidation of biomass-burning phenols is a significant but poorly constrained source of secondary organic aerosol (SOA) in the atmosphere. Laboratory studies replicating aerosol liquid water (ALW) – characterized by high solute and chromophore concentrations – remain scarce due to strong light attenuation and thermal gradients in conventional photoreactors. To address these limitations, we developed a short-pathlength photoreactor (SPP) that minimizes optical screening and provides precise control of temperature, humidity, and illumination conditions. Using guaiacyl acetone (GA) as a model phenol compound and 3,4-dimethoxybenzaldehyde (DMB) as a triplet precursor (3C*), the SPP successfully reproduced SOA yields from established photoreactors under dilute conditions and further enabled experiments under strongly light-absorbing ALW regimes. The system maintained stable temperature and relative humidity, consistent photon flux, and reproducible photochemical performance. Positive matrix factorization (PMF) of high-resolution aerosol mass spectra resolved distinct stages of GA-derived aqueous SOA (aqSOA) evolution across a wide range of ionic strengths. The analysis further revealed the formation of GA dimers through photosensitized coupling pathways, with dimer formation rates increasing significantly with ionic strength. Overall, the SPP provides a validated and versatile platform for investigating aqSOA formation and transformation processes under atmospherically relevant droplet and ALW conditions.
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Status: final response (author comments only)
- RC1: 'Comment on egusphere-2025-5414', Anonymous Referee #1, 11 Dec 2025
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RC2: 'Comment on egusphere-2025-5414', Anonymous Referee #2, 25 Dec 2025
The key technical innovation of this work lies in the development of a short-pathlength photoreactor that overcomes optical screening and thermal artifacts, enabling controlled aqueous-phase photochemistry under strongly light-absorbing, high-ionic-strength ALW conditions that were previously inaccessible. The experimental concept and reactor design are well motivated and technically sound. The discussion part could be strengthened to better emphasize the broader relevance of the new reactor beyond the GA/DMB system and to more clearly extract its general advantages from the experimental results. In addition, the comparison with previously used reactors (e.g., RPR-200 and the small tower reactor) would benefit from a brief introduction of their key features and limitations, as well as a clearer explanation of the specific optimizations implemented in the short-pathlength photoreactor. Details comments as below:
Linear 39, any range information for that “higher ionic strength.”
Line 59, Please add a short line, reminding the difficulty in controlling ionic strength, to connect context.
Line 174, please add a short description of the methods for internal standard, SOA yield calculation and PMF solution selection, instead of using citation here only.
Line 179, it is not clear whether the authors intend to state that the whole PMF results, including factor number and spectrum, are internally consistent with Jiang 2021 et al, or only that one factor primarily represents the unreacted GA fit. Clarification is needed.
Line206-214, The description of RH control is potentially misleading. The manuscript implies that liquid water mass is controlled “via salt deliquescence,” which suggests that salt behavior plays an active role in RH regulation. In reality, salt deliquescence appears to be used as a passive validation of equilibrium water uptake under externally controlled RH. This distinction should be clarified.
Line 215, In Figure 3, the agreement between measurements and predictions varies among different salts. For LiCl in particular, the measurements do not show a clear trend with increasing predicted values. Here need to add some discussion to clarify before the statement of agreement.
Line 225, maybe use log scale for x-axis for better display of all data
Line 240, does this thermal control refer to the temperature curve in Figure 2? In this four-day test, the temperature does not stop increasing at the right end of x-axis. Do author has longer data set to show the stability of the SPP system in temperature control? So far the data set is not convincing enough. Also in this chapter, the major discussion is around topic of RH/ liquid water mass and optical control. Please modify this conclusion.
Line 266, Figure 5(b), the sulfate difference was marked twice, by the color of Exp ID and the size. The author needs to rethink the way to separate the experiment marker. Using color to differentiate Exp ID is less helpful; better use color, transparency, and size of marker to indicate the key parameters, GA, GA/DMB, and SO4. Same for Figure 7.
Line 275, for PMF analysis, the final source factor selection criteria are very important, especially here different analysis methods are applied.
Line 282, is there cross comparison of factor mass spectra between this study and previous studies?
Line 289-294, The paragraph is somewhat contradictory. The authors aim to highlight the SPP’s improvements in simulating real aqueous chemistry instead of surface reactions in previous studies, yet use the similarity to previous results as evidence of its effectiveness. Need to be clarified.
Line 363-370, the description of experiments 11-13 changes between ionic strength and salt effect, which is misleading and needs to be unified.
Citation: https://doi.org/10.5194/egusphere-2025-5414-RC2
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General comments
The authors present comparative results of a new photoreactor designed to enable an understudied regime of aqueous phase aerosol chemistry. Overall the manuscript is both clear and thorough, and the results indicate a promising revision to previous approaches using similar setups. Given the number of labs interested in this kind of work, increasing access to such a technique will be impactful.
A few of the figures could use improvements for clarity and there are small technical corrections.
Specific comments
Section 2.4 Chemical Analyses - the authors have used an aerosol mass spectrometer to analyze the cloud water (and ALW) mimics, by aerosolizing the liquid samples so they can be sampled by the AMS. Likely, the reason for this is because the only HRMS available is an AMS and not because it is a particularly good method of sample prep. Or, possibly, it is to enable a direct comparison to ambient samples collected behind a CVI. Given that using an AMS to analyze an aqueous solution is not the obvious choice, it would be helpful to indicate just briefly why the authors selected this method (e.g. “to leverage our HRMS, which is an aerosol mass spectrometer, samples were…). This way, those unfamiliar will not assume this method is beneficial or confers certain advantages.
The color scale of Figure S5 need not go below 0.5 based on the correlation coefficients, it seems, because the current use of the full range compresses the color-based comparison of coefficients to the point of ineffectiveness.
Figure 5 would be clearer if there was a shaded block behind the first 3 bars to indicate they are replicants and should be directly compared, perhaps with an extra space before experiment 2.
The results of the PMF analysis showing the disappearance of the 1st generation product, the appearance and then disappearance of the second generation product, and the slow appearance of the third generation product/s was really impressive.
Lines 338-340 seem to be the same sentence, twice, but rephrased.
Technical corrections
The Supplemental Information contains an abstract that is actually the journal’s instructions to authors, not an abstract.
Figure S4 Caption reads “error! Reference source not found”
Also Figure S7 and Figure S8