General comments:

This study reports the formation of oxygenated and small organosulfates from aqueous-phase oxidation of an a-pinene-derived organosulfate (apOS-249), some of which have been detected in ambient aerosols but with unclassified precursors or formation pathways. The authors systematically investigated the kinetics and the reaction products and demonstrated that aqueous OH oxidation can serve as an important sink for the model organosulfate (apOS-249) under relevant atmospheric cloud conditions. Overall, this manuscript is well-written, and the conclusions are clearly supported by the results. Some questions and suggestions are listed below for the authors’ consideration.

1. pH has been shown to greatly impact reactivity in particles and droplets, while the pH of cloud droplets may range somewhere from 3 to 7. Could the authors provide some explanations about why a buffer solution of pH 4.5 was selected for this study and how would the range of pH observed in the atmosphere affect the breakdown of apOS-249? The authors also mentioned that the pH of the reaction mixtures were monitored (page 5, Lines 15-17). Did the pH values change at all or remain constant throughout the experiments, given the formation of inorganic sulfate and many different organosulfates that potentially have different pKa values?
2. Similar to the last comment, ionic strength effects reactivity within aqueous solutions. Cloud water in urban areas is dominated by ammonium sulfate and ammonium nitrate, while marine NaCl. Would the presence of these salts at their atmospheric concentrations affect the rate of OH production and subsequent OS oxidation? Similar to the last comment, ionic strength affects reactivity within aqueous solutions. Cloud water in urban areas has high concentrations of ammonium sulfate and ammonium nitrate, while in marine environments, sodium chloride is the dominant salt. Would the presence of these salts at their atmospheric concentrations affect the rate of OH production and subsequent OS oxidation?
3. Within this experiment the authors used a solution of apOS-249, benzoic acid (BA), and H₂O₂ to track the oxidation of apOS-249. The authors investigated the formation of fragmentation and functionalized (more oxygenated) products over time with lower carbon counts due to C-C bond cleavage during oxidation processes. Did authors observe any reaction products with higher carbon count numbers than C10 at early phases of the experiment that were potentially resulting from reactions between apOS-249 and BA?
4. The authors investigated the formation of lower chain OS and inorganic sulfate due to oxidation by aqueous OH radicals. Did the authors observe any “sulfide” or “sulfonate” formation throughout their oxidation experiments? Figure S5 shows a peak at ~3.5 minutes. Is this corresponding to sulfite? Reporting these findings would be beneficial due to the potential environmental impacts of sulfite/sulfonates and sulfate/OS. This may also help close the sulfur mass balance when considering the breakdown of apOS-249.

Minor comments:

1. Page 3, Lines 7-9 – The authors noted that OS has profound effects on various physiochemical properties, such as surface tension and hygroscopicity. Due to the extensive discussion of OS sizes later in their findings, it will be helpful to provide more information in the introduction regarding how variations in carbon chain length can cause completely different aqueous properties of OS compounds. Reference: Bain, Alison, Man Nin Chan, and Bryan R. Bzdek. "Physical properties of short chain aqueous organosulfate aerosol." Environmental Science: Atmospheres 3.9 (2023): 1365-1373.
2. Page 3, Line 32 – The phrase "retain or release inorganic sulfates" is somewhat confusing. I suggest changing it to "retain the sulfate moiety or release inorganic sulfates."
3. Page 6, Line 24 – Use of NaOH as an eluent is typically done under an inert atmosphere due to the ability of carbon dioxide to partition into carbonic acid and then bicarbonate. These reactions will cause peak broadening within the system. Did the authors use any inert headspace over eluent or check to see if broadening of peaks changed over time in subsequent runs, which may have impacted peak area integration? If the same standard concentration showed a broader peak after multiple runs, concentrations of sulfate calculated by peak area might be inaccurate.
4. Page 6, Line 25 – It might be helpful to include the exact retention time observed. Although the IC curves are shown in the SI, readers can compare literature and simulated elution times to those that are observed by the authors.
5. Page 9, Figure 2 – Were total intensities normalized to something, such as a reference compound, to account for instrument variations between runs at different time periods?
6. Page 11, Lines 4-10 –As a precaution for readers, it would be helpful if the authors could also discuss how, if this assumption was found to not be true, it would affect results.
7. Page 13, Lines 10-12 – Did the MS2 fragmentation patterns for any of these potentially isomeric compounds show significant differences? If significant differences are found, it would be helpful to include this information in supplemental materials.
8. Page 14, Lines 22-23 – The authors noted the minor presence of inorganic sulfate due to hydrolysis and accounted for this in the quantification formed by oxidation. Was any kinetics data collected on this hydrolysis, as hydrolysis would likely continue alongside OH oxidation throughout the experiment? If the rate is low enough, this amount might be negligible compared to the aqueous OH oxidation. However, it is currently unclear if hydrolysis is contributing to the increased inorganic sulfate concentration during the irradiation process, even after adjustment for the initial SO₄ concentration.
9. Page 14, Lines 29-30 – Were any control experiments performed in which UV lights remained active until no change was seen in SO₄ concentration? It would be interesting to see if a stagnation of inorganic sulfate concentration consumed all OS compounds in solution at these relevant concentrations of OH radicals.
10. Pages 16-17, Atmospheric Implications – It would be helpful to comment on the potential influence of this finding on acidity levels due to inorganic sulfate formation, as well as the impact of trending towards smaller OS due to their different physicochemical properties than large OS, as noted in the comments above.

Lai et al. investigate the aqueous phase oxidation kinetics of an α-pinene-derived organosulfate by ^•OH radicals, reporting a second-order rate constant of (2.2 ± 0.2) × 10⁹ L mol^–1 s^–1 and corresponding atmospheric lifetimes ranging from minutes to two days. The study identifies a series of new OS products, including more oxygenated C10 OS compounds, smaller OS fragments, and significant amounts of inorganic sulfate. Based on the detected products and proposed mechanisms, the authors suggest that these OS species form through functionalization and fragmentation processes. Finally, a comparison with literature data reveals that most of the observed OS products have been previously detected in field studies, and some of these products exhibit previously unknown sources and formation pathways. The authors thus propose that aqueous-phase oxidation of larger OSs could explain the atmospheric presence of these OS compounds. This is a timely, well-executed study, clearly written and well-supported by the data. The findings will be of interest to ACP readers, and I think the manuscript is suitable for publication after addressing the following minor comments:

Page 3 Line 15: The statement “Various mechanisms have been proposed for OS formation” would benefit from including specific examples of these mechanisms to enhance clarity and support for the reader.

Page 5 Line 15: The authors need to clarify if pH affects the reaction rate. For example, please provide commentary on how the experimental results may differ under more acidic conditions.

Page 17 Line 10: In Table 1, the use of colored cells should be avoided or removed.

Finally, I wonder if the authors can comment on how their findings might extend to other smaller organosulfates, particularly isoprene-derived OS. Given that isoprene-derived OS are often more abundant in the atmosphere, would you expect similar formation pathways (e.g., functionalization and fragmentation) to generate new OS products through aqueous-phase ^•OH oxidation? Some additional discussion on this point would help place these important results in a broader atmospheric context.