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 (SO₃^2- + 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 SO₃^2- 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 [SO₃^2- + O₂] complex. The figure only shows calculated transitions, not an experimental spectrum for this complex. Include a zoomed-in absorbance plot of the SO₃^2-/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 (k_obs). 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