| 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.
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