the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 12 Nov 2025
Rapid Secondary Organic Aerosol Formation at the Air–Water Interface from Methoxyphenols in Wildfire Emissions: UVA-Driven S(IV) Photooxidation to Organosulfates
Abstract. Wildfire emissions release large amounts of methoxyphenols, which serve as key precursors of aqueous-phase secondary organic aerosols (SOA). Their transformation is closely coupled with aqueous S(IV) oxidation, jointly driving the formation of sulfate and organosulfates; however, the underlying mechanisms remain poorly understood. Here, we identify a novel, metal-free mechanism for SO4•- generation under UVA light (370 nm), supported by experiments and quantum chemical calculations. Photolysis of the [SO32-+O2] complex yields a [SO3•-+O2•-] pair that forms peroxomonosulfate (SO5•-) and ultimately SO4•-. These radicals rapidly oxidize guaiacol, a biomass burning phenol, in bulk solution (k = 3.8 × 1010 M-1 s-1), producing SOA enriched in organosulfates. Microdroplet experiments show 100-fold rate enhancement due to interfacial effects. Box and global modeling indicate that this aqueous UVA pathway is a significant, previously overlooked source of sulfate. This work established a new photochemical link between S(IV) oxidation and SOA formation, with implications for aerosol composition, oxidative capacity, and climate-relevant processes.
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Status: final response (author comments only)
- RC1: 'Comment on egusphere-2025-5323', Anonymous Referee #1, 05 Jan 2026
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RC2: 'Comment on egusphere-2025-5323', Anonymous Referee #2, 05 Jan 2026
General Comments:
The manuscript investigates a proposed metal-free photochemical mechanism for generating sulfate radical anions (SO4-.) via the UVA irradiation of aqueous S(IV)-oxygen complexes. However, the work suffers from significant technical and conceptual deficiencies that undermine its primary conclusions, most notably the absence of measured photochemical quantum efficiency. Without this fundamental parameter, the use of an artificial light source with an intensity over ten times that of natural sunlight (850 W/m2) makes the atmospheric scaling and claims of global significance speculative and physically unbenchmarked. Furthermore, the assertion of a "novel" sunlight-driven pathway is contradicted by established literature (e.g., Galloway et al., 2009; Nozière et al., 2010; Cope et al., 2022) that has already documented radical-mediated sulfate chemistry under similar conditions. The reported second-order rate constants (3.78*10^10 1/M 1/s) are also at or beyond the physical diffusion limit, suggesting potential experimental artifacts or the influence of trace impurities. Additionally, the modeling fails to account for competing established pathways, such as transition metal ion (TMI) catalysis and light attenuation by black carbon, which likely results in an overestimation of this pathway’s dominance. The recommendation to the editor is to reject the manuscript in its current form. The manuscript requires a fundamental re-evaluation of its novelty, the inclusion of rigorous actinometry to determine quantum efficiency, and a balanced comparative analysis against other works before any potential reconsideration.
Major Comments:
1) The abstract claims to identify a "novel, metal-free mechanism" for generation of reactive species for sulfate formation under UVA. However, existing literature (e.g., Gong et al., 2022 and others) already suggests that UVA light promotes oxidation at the interface. The claim of absolute novelty conflicts with studies exploring S(IV) photo-excitation.
Cope et al. (2022) provided the first direct experimental evidence that UV irradiation of aqueous sulfate, whether natural sunlight or lab-based UV, generates sulfate radical anion, enabling oxidation of organic compounds even under typical tropospheric pH and ionic strength conditions.
Nozière et al. (2010) were among the first to report UV-254 nm (UV-B) irradiation of ammonium sulfate with alkenes, yielding organosulfates via radical-mediated pathways. Although the precise mechanism wasn't specified, these results imply sulfate radical involvement.
Galloway et al. (2009) observed the light-triggered production of organosulfates, such as glycolic acid sulfate, during glyoxal uptake onto ammonium sulfate aerosols under UVA, but not in the dark, supporting the presence of photogenerated sulfate-radical chemistry.
2) The text in the abstract states photolysis yields a sulfite radical anion and superoxide radical pair, but then attributes the formation of sulfate radical anion to the reaction between SO5-. and S(IV). There is a lack of clarity in the abstract regarding whether the is a direct or indirect product of the initial photolysis step.
3) The claim in the abstract that this is a "significant, previously overlooked source of sulfate" is based on box modeling. However, the model parameters (e.g., using 850 W/m2 intensity vs. 63.3 W/m2 for solar) may exaggerate the atmospheric relevance without the needed validation that is missing in this work.
4) There are inconsistencies in the introduction section (e.g., the importance of the UV Band) that require attention. The manuscript argues that UVA is the "dominant solar band" and that UVC is "negligible". Yet, the manuscript later admits that the initial S(IV) species “HSO3-” shows "nearly no UV-vis absorption" in the UVA range, creating a contradiction between the proposed driver (UVA) and the absorption properties of the reactants.
In addition, the introduction mentions that radical activity in high-ionic-strength solutions reaches picomolar concentration under UVB. The manuscript then claims similar or higher concentrations (picomolar at the interface) under UVA without sufficient comparison to why a lower-energy wavelength (UVA) would produce equivalent radical yields to a higher-energy one (UVB).
Finally, the introduction frames the research around "wildfire emissions". However, the experimental setup uses pure sodium sulfite and Guaiacol, failing to account for the complex organic/metal matrix of actual wildfire smoke that could compete for or catalyze these reactions.
5) Several issues raise concern about the validity of the study from the experimental viewpoint. First, the use of MeOH to "immediately quench the reaction" is problematic for chemistry. Methanol is a scavenger but may also react to form methyl-sulfates or other intermediates, potentially confounding the HRMS results for organosulfates (OSs).
Second, the use of a 850 W/m2 UVA lamp to simulate tropospheric chemistry is an order of magnitude higher than natural sunlight (63.3 W/m2). While a correction factor of 7.4% is applied, this linear scaling ignores non-linear radical-radical termination rates that would differ significantly between the lab and the atmosphere. More importantly, the work is missing the determination of the photochemical quantum efficiency that is key for scaling this process and for appropriate comparisons. Without quantum efficiency, the work is incomplete, lacking photochemical validity. Once they are determined a new asscoaiated subsection will be needed in the Results and Discussion.
Third, the experimental section states that pH was adjusted with sulfuric acid, but this acid introduces additional sulfate ions. This methodology risks interfering with the quantification of sulfate production and the equilibrium of S(IV) species, which the paper aims to measure.
6) Multiple scientific inconsistencies are found in the Results and Discussion section.
The first one relates to the inconsistency of reaction rates (also related to Figure 1). The reported second-order rate constant for guaicol + sulfate radical anion is 3.78×1010 1/M 1/s. This value is at or slightly above the diffusion limit for aqueous solutions, which is inconsistent with typical radical-aromatic reaction kinetics in existing literature. More importantly, the work lacks a comparison of quantum efficiencies. The absence of quantum efficiency data creates significant challenges for the scientific validity of the manuscript's claims: (a) Lack of mechanistic quantification: Without a reported quantum efficiency, it is impossible to determine the efficiency of the light-driven process; consequently, one cannot discern whether the observed sulfate production is a primary photochemical result of the (SO32- + O2) complex or an artifact of trace impurities reacting under high-intensity radiation. (b) Unverifiable atmospheric scaling: In the absence of quantum efficiency measurements, the "correction factor" used to scale laboratory results to the global atmosphere lacks a physical benchmark, making the conclusion that this pathway is "significant" or "dominant" scientifically speculative rather than empirically grounded.
Second, line 282 states has "nearly no UV-vis absorption above 250 nm," yet the mechanism relies on the photolysis of a complex at 370 nm. The manuscript fails to quantify the concentration or the molar absorptivity of this specific complex, making it difficult to verify the feasibility of the UVA-driven step.
Third, in Section 3.5, the manuscript notes that reacts "100 times faster with SO32- than with HSO3-". However, the experimental results focus on pH 4.0, where HSO3- is the dominant species. This creates a discrepancy between the highlighted mechanism (sulfite-led) and the experimental conditions (bisulfite-led).
7) Specific comments about figures and associated text:
Figure 1: The reported second-order rate constant is at the diffusion limit for aqueous systems, yet the figure lacks a comparison with established radical quenchers to rule out artifacts from the high-intensity lamp (850 W/m2). Error bars in Panel B need clarification, are they 95% confidence intervals or standard deviations? The caption should clearly define “pseudo-first-order” conditions, as the linear trend in Panel B seems overly simplified given the complexity of radical chain reactions.
Figure 2: The spectra clearly identify organosulfates and TEMPO adducts, but isotopic pattern analysis is missing to confirm sulfur in peaks like m/z 188.9858 and 203.0014. The comparison between dark (Panel C) and UVA (Panel D) suggests sulfate radical formation, yet the TEMPO-SO4-. adduct peak is small compared to the S(IV) adduct, raising questions about radical efficiency. The caption should clarify whether these are single scans or averaged spectra, as the uniform baseline noise across panels may indicate heavy smoothing or processing.
Figure 3: Panel A conflicts with the proposed mechanism: the text claims Na2SO3 has almost no absorption at 370 nm, yet the mechanism depends on photoexcitation of an [SO32- + O2] complex. The figure only shows calculated transitions, not an experimental spectrum for this complex. Include a zoomed-in absorbance plot of the SO32-/O2 mixture to confirm its presence. Without this, the link between 370 nm irradiation and the T1→T2 transition remains unverified.
Figure 4: Panel B shows a claimed 200-fold rate enhancement, but uses 2 mm droplets, about 100× larger than real cloud droplets. The log-scale Y-axis hides possible early non-linearities; raw intensity plots should be provided instead of normalized log plots. The caption must specify the droplet equilibration period, as surface enrichment is time-dependent and may not reflect real atmospheric dynamics.
Figure 5: The UVA pathway estimates use lamp intensities far above natural sunlight, weakening scientific consistency. The shaded uncertainty range appears arbitrary rather than statistically derived. The legend should clearly separate literature-based rates (H2O2, O3) from new experimental values. The caption must note that “Beijing haze” conditions may ignore light attenuation by black carbon, which would likely reduce UVA significance. A new figure with quantum efficiency is needed.
Figure 6: The figure applies a lab-derived “interfacial enhancement” factor globally without accounting for variations in liquid water content, risking over-extrapolation. High rates in Asia and South Africa lack sensitivity analysis for transition metal catalysis (TMI). Critically, the comparison should only be made after scaling sunlight photon flux with quantum efficiencies, not raw lamp-based assumptions. The caption should state these are estimated, not measured, rates and include the GEOS-Chem model version and year for reproducibility.
8) The Atmospheric Implications section provides a global assessment based on the lab-derived model estimated sulfate production rates (kobs). This assumes the "interfacial enhancement" (200-fold) observed in isolated 2 mm droplets applies uniformly to global cloud and aerosol water, which likely overestimates the contribution in bulk cloud systems. This is problematic.
While this UVA-driven pathway is said to provide a new route for sulfate formation, its global significance must be weighed against established iron-catalyzed dark reactions that efficiently produce secondary organic aerosol in acidic and viscous systems. The manuscript should explain (e.g., after line 372) the work of Al-Abadleh et al. (2021 and 2022) while indicating that future modeling should integrate these competing interfacial and TMI-catalyzed mechanisms to accurately reflect the complexity of tropospheric oxidation."
The manuscript also states the UVA-pathway "dominates" in Beijing Haze. This contradicts literature citing NO2 and transition metals (TMI) as the primary drivers of haze-sulfate. The model used for comparison may not fully account for the suppressing effects of high ionic strength or light attenuation by soot in haze.
Finally, while SO4-. is proposed as the key oxidant, the final implications focus on "sunlight-accessible S(IV) oxidation" without adequately addressing if other solar-generated oxidants (like HO or other species) would render this specific pathway minor in a real atmospheric multi-pollutant mix.
References
Al-Abadleh, H. A.; Rana, M. S.; Mohammed, W.; Guzman, M. I. Dark iron-catalyzed reactions in acidic and viscous aerosol systems efficiently form secondary brown carbon. Environ. Sci. Technol. 2021, 55, 209–219. https://doi.org/10.1021/acs.est.0c05678
Al-Abadleh, H. A.; Motaghedi, F.; Mohammed, W.; Rana, M. S.; Malek, K. A.; Rastogi, D.; Asa-Awuku, A. A.; Guzman, M. I. Reactivity of aminophenols in forming nitrogen-containing brown carbon from iron-catalyzed reactions. Commun. Chem. 2022, 5, 112. https://doi.org/10.1038/s42004-022-00732-1
Cope, J. D.; Bates, K. H.; Tran, L. N.; Abellar, K. A.; Nguyen, T. B. Sulfur Radical Formation from the Tropospheric Irradiation of Aqueous Sulfate Aerosols. Proc. Natl. Acad. Sci. U. S. A. 2022, 119 (36), e2202857119. https://doi.org/10.1073/pnas.2202857119
Galloway, M. M.; Chhabra, P. S.; Chan, A. W. H.; Surratt, J. D.; Flagan, R. C.; Seinfeld, J. H.; Keutsch, F. N. Glyoxal Uptake on Ammonium Sulfate Seed Aerosol: Reaction Products and Reversibility of Uptake under Dark and Irradiated Conditions. Atmos. Chem. Phys. 2009, 9 (10), 3331–3345. https://doi.org/10.5194/acp-9-3331-2009
Nozière, B.; Ekström, S.; Alsberg, T.; Holmström, S. Radical‑Initiated Formation of Organosulfates and Surfactants in Atmospheric Aerosols. Geophys. Res. Lett. 2010, 37, L05806. https://doi.org/10.1029/2009GL041683
Citation: https://doi.org/10.5194/egusphere-2025-5323-RC2
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- 1
Line 122:
Please define “FIDI-MS.” Additionally, for the FIDI-MS experiments, what was the ambient relative humidity (RH) in the laboratory? What were the composition and concentrations of species in the suspended droplets? Would they be different from that of bulk stock solutions?
Line 122:
“Droplets approximately 2 mm in diameter (~4 μL volume) were suspended from the tip of a stainless-steel capillary, positioned equidistantly between two parallel plate electrodes separated by 6.3 mm.” Does droplet size affect reaction rates? Furthermore, how can these findings be extrapolated to submicron-sized droplets?
Line 152:
For Section 2.3 (theoretical calculations), were droplet size or curvature effectsconsidered in the DFT, TDDFT, and MD calculations?
Line 179:
For the box model conditions, since experimental measurements were conducted with 2 mm droplets, should potential size effects on reaction rates be considered if they are significant? Additionally, should other factors—such as concentration variations and the presence of inorganic and organic species in atmospheric droplets—be incorporated to better represent atmospheric conditions in the simulations?
Line 232:
“At pH 4.0, GUA (0.1 mM) was added to 2.0 mM Na₂SO₃ solution under continuous zero-air bubbling.” Can the authors justify the chosen GUA concentration? Is it atmospherically relevant?
Line 239:
“We further investigated how reagent concentrations influence degradation kinetics. At high Na₂SO₃:GUA molar ratios (≥ 20), …” What would typical atmospheric ratios be?
Line 254:
“These signals indicate OS formation from GUA reacting with SO₄•⁻ radicals photochemically generated from SO₃²⁻ and O₂ under UVA.” What are their formation mechanisms for the detected OS species?
Line 256:
“To verify SO₄•⁻ involvement, we introduced 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO; C₉H₁₈NO) as a radical scavenger (Bai et al., 2016).” Would the presence of OH radicals in the aqueous phase affect the results?
Line 280:
“Photochemical Pathway of SO₄•⁻ formation from Na₂SO₃ under UVA irradiation.” Were droplet size and curvature effect considered?
Line 323:
“Microdroplets also facilitate gas exchange, boosting [SO₃²⁻ + O₂] complex formation and SO₄•⁻ production under UVA. Thus, GUA photodegradation is expected to be far greater in microdroplets than in bulk water—potentially by several orders of magnitude.” Is the enhancement primarily attributed to gas exchange due to change in droplet size?
Line 326:
“To test this, we used field-induced droplet ionization mass spectrometry (FIDI-MS) (Huang et al., 2018; Gong et al., 2022; Zhang et al., 2023) to monitor UVA-induced photodegradation of 0.1 mM GUA in microdroplets, with and without 3.0 mM Na₂SO₃ (see Methods).” Were the compositions identical in both cases?