This study investigates the aqueous-phase oxidation kinetics and oligomerization pathways of benzothiazoles (BTs), with notable atmospheric presence but poorly understood liquid-phase reactivity. By experimentally determining OH-radical reaction rate constants for BT, MBT, and CBT, the work provides quantitative data to constrain their atmospheric lifetimes in cloud droplets and aerosol liquid water, an underexplored aspect of BTs' environmental fate. The identification of oligomers via Orbitrap MS, coupled with nanoparticle formation observed through NTA, offers mechanistic evidence for BTs' contribution to secondary organic aerosols (SOA) through aqueous-phase processing. These findings complement existing knowledge on gas-phase BT oxidation and highlight the need to account for aqueous reactions in models predicting SOA formation in urban and industrial regions. The study advances the mechanistic understanding of heterocyclic SVOC transformations, particularly for sulfur- and nitrogen-containing species, and provides a foundation for future investigations into their role in brown carbon formation and regional haze events. These results can potentially be of interest to the ACP audiences. However, several minor issues remain in the present study and should be addressed with additional explanations and revisions. I suggest a minor revision for this manuscript.

1. In Sec. 1: The manuscript would benefit from a more narrative-driven introduction that sets up the central question: What are we missing in our current models of SOA formation? Rather than solely listing prior studies, characterizing BTs as a “missing piece” in the urban secondary aerosol puzzle could provide a sharper hook.
2. In Line 14: "aeras with heavy traffic" → Correct to "areas with heavy traffic."
3. In Line 24: The use of the term “illustration” in the abstract should be corrected. Also, the phrase “after illustration” in the Abstract is confusing – the rest of the paper implies it should be simply “after the reaction.” Ensure that such terminology is consistent and clear across several sections.
4. In Line 145: The authors briefly introduce the calculation method of OH concentrations. I think it would be nice to include a bit more information on the method to estimate OH radical concentration (direct calculation from power output or using chemical actinometry?). Publications from Anastasio group (such as Atmos. Environ., 2015, 100, 230-237) can be referred for the estimation.
5. In Line 193: "could be an significantly effective process" → Correct to "could be a significantly effective process"
6. LC-Orbitrap MS analysis identified hydroxylated products and oligomers. This piece of evidence is good, but rather qualitative. Did the authors perform LC-Orbitrap MS experiments at different time intervals, so that one can use peak areas of certain products to quantitatively/semi-quantitatively compare with the changes of long-wavelength absorption (e.g., at > 300 nm in Fig 3b)?
7. In Line 288: In this paragraph, the role of organosulfur and organonitrogen compounds deserves more attention. How might the CHONS products influence particle hygroscopicity, longevity, or climate-relevant properties? These aspects are hinted at but not explored in depth.
8. In Line 340: “BTs” in “kBTs” should be subscript and the Eq. (3) should be italic, consistent with other equations.
9. Table 3 is quite dense and might be better split or reorganized for readability.

This study employs the laboratory simulation to investigate the aqueous-phase oxidation benzothiazoles (BTs), a class of emerging environmental contaminants. The work determines the kinetics, identifies and categorizes diverse oligomers and functionalized products by LC-Orbitrap MS, which are rarely reported in prior studies on BTs. Molecular formula analyses map key pathways, advancing mechanistic insights into heterocyclic pollutant transformations. Periodic carbon-number patterns empirically confirm oligomer formation, a process in SOA generation. With its analytical depth and mechanistic contributions, this paper is recommended for acceptance after minor revisions.

1. Page 1, Line 16: A grammatical issue in the abstract should be corrected, “are unclear” should be “is unclear”.
2. Page 9: Legends in Fig. 3e are too small. It should be improved for better readability.
3. Page 3, Line 70: There supposed to use “, and” before “vanillic acid”.
4. Page 3, Line 88: There supposed to use “and” before “2-chlorobenzothiazole”.
5. Page 3, Lines 85-90: The units of sodium hydroxide and perchloric acid solutions are supposed to be mol L^-1. The format of units needs to be used consistently.
6. Page 11, Line 266: The criteria used for molecular formula assignment from Orbitrap MS data should be explicitly described in this paragraph.
7. 3.3 and Sec. 3.4: Although the presence of oligomers and nanoparticles is well established, quantitative yield data (e.g., mass-based or molar-based) would strengthen the case for atmospheric relevance. If not available, this limitation should be acknowledged.
8. The Introduction establishes that BTs themselves are potentially harmful to humans but does not foreshadow any health implications of the secondary products formed by BTs in the atmosphere. There is a mild logical disconnect between the Introduction and Conclusion on the topic of health implications. The Conclusion goes beyond what the Introduction prepares the reader for: it argues that aqueous-phase oxidation of BTs can impact climate and human health. This is a significant point – it implies the secondary aerosol products of BTs contribute to fine particulate pollution that humans could inhale, thereby affecting health. To ensure logical flow, the Introduction should at least hint that studying BTs’ atmospheric oxidation is relevant not just for aerosol science but also for evaluating potential health impacts. The Introduction could include one or two sentences foreshadowing the potential health implications of BTs’ oxidation products. Revise this.
9. The authors make a compelling case for the atmospheric relevance of BT oxidation, but the manuscript would benefit from a more detailed discussion of future research directions. For instance, what are the potential impacts on human health, or how can the researchers study the potential impacts on human health?
10. Page 17, Line 380: There supposed to use “BTs concentrations are comparable” instead of “is”.

In this paper, Zhang et al. measure the reactivity in water of a specific class of aromatic organic compounds, namely benzothiazoles (BTs).

Since BTs are industrially produced, elucidating their multiphase fate in the atmosphere is important. They used the relative rate methodology to experimentally determine the reaction rates of BTs with OH. The rates they found are in line with the literature for BT, which gives trust in the new measurements for MBT and CBT  They carried out product formation experiments (IC for inorganics, Orbitrap for organics, and NTA for nanoparticles) and measured optical properties (fluorescence spectrophotometry) to propose an OH-reaction mechanism of BTs. This last part is particularly interesting for understanding the contribution of organics aqueous phase chemistry to SOA formation for instance, and organics atmospheric aging in general.

Overall the paper is good and could be published in ACP after the authors address these comments as they deem appropriate.

Major comments

 - Sect. 2.3: These types of experiments are often carried out in triplicates to account for systematic uncertainty. Maybe I missed the mention of it but if the experiments were not repeated, it would be important to explain why.

 - l. 191: Is there an explanation why previous reported rate constants are significantly lower?

 - Conclusion: The lifetime evaluation is very useful and welcomed. However, the OH aqueous concentrations used here may be criticized. The literature referenced in Table 3 is quite old and does not reflect the high uncertainty surrounding the estimation of steady-state aqueous OH concentrations. The Arakaki et al. (2013) study did a nice job revisiting estimations of aqueous OH. For instance, OH is around 10^-15 M in their maritime sample. The review of Bianco et al. (2020) also provides a more recent view on this question. I encourage the authors to revise their calculations and possibly their conclusions based on the range of reported aqueous OH concentrations.

Minor comments

 - l. 56: BT gas-phase oxidation is introduced before the presentation of Fig. 1 which shows which molecule BT specifically is

 - l. 60: the sentence is unusually formulated, it would be clearer to write something like "... could contribute to secondary organic aerosol after their oxidation into C_3-8 organic compounds which also produces sulfuric acid"

 - l. 264: this sentence is confusing, please reword it because I can't understand its meaning.

 - Fig. 4a: the histograms may be better understood if they were transposed, i.e. number of carbon on the x-axis, number of assigned formulas on the y-axis. No obligation, it's only a matter of taste.

 - l. 292: the sentence is confusing, please reword it. I guess it is supposed to mean that the CHONS fraction for BT and MBT is comparable to the CHONS+CHONSCl fraction for CBT?

References

Arakaki, Takemitsu, Cort Anastasio, Yukiko Kuroki, Hitomi Nakajima, Kouichirou Okada, Yuji Kotani, Daishi Handa, et al. ‘A General Scavenging Rate Constant for Reaction of Hydroxyl Radical with Organic Carbon in Atmospheric Waters’. Environmental Science & Technology 47, no. 15 (6 August 2013): 8196–8203. https://doi.org/10.1021/es401927b.

Bianco, Angelica, Monica Passananti, Marcello Brigante, and Gilles Mailhot. ‘Photochemistry of the Cloud Aqueous Phase: A Review’. Molecules 25, no. 2 (January 2020): 423. https://doi.org/10.3390/molecules25020423.

This manuscript presents an investigation into the atmospheric transformations of benzothiazoles (BTs) via OH radical-initiated oxidation in the aqueous phase. BTs are increasingly recognized as emerging environmental contaminants due to their extensive use in industrial and consumer products, and their widespread presence in urban environment, especially in air. While their gas-phase reactions have been previously studied, the aqueous-phase chemistry of BTs remains largely unexplored, despite its relevance in cloud water, fog, and deliquescent aerosols. This study addresses this knowledge gap. The authors quantified the second-order rate constants for BTs + OH reactions, characterized the reaction products via LC-Orbitrap MS, and monitored the formations of nanoparticles and light-absorbing chromophores. Key findings include the identification of multifunctional oligomers and the observation of brown carbon-like optical features among the products. These results suggest that BTs could be a previously underappreciated source of both inorganic and organic secondary aerosols in polluted environments. The integration of kinetic, molecular, and particle-level data represents a notable advance in understanding the atmospheric fate of this class of compounds. The study’s focus is within the scope of ACP and the paper is well written, and I recommend an overall minor revision with a few comments below.

Comments:

1. Both the Abstract and Conclusion highlight the formation of atmospheric brown carbon from BTs’ aqueous reactions as a key finding. However, the Introduction does not mention brown carbon or aerosol optical properties at all. The concept does not appear until the Results and Discussion section, leading to a minor narrative gap. To harmonize, the Introduction could include a brief mention that secondary aerosols can include light-absorbing organic matter (i.e., brown carbon) that affects climate.
2. The terms, “secondary aerosol” vs. “secondary organic aerosol”, are used mostly consistently, though a slight variation in usage may cause confusion. The Introduction refers to secondary organic aerosols (SOA) when giving examples of compounds that undergo aqueous processing, whereas the Abstract and Conclusion use the more general term “secondary aerosols” (which include both organic and inorganic components). The authors’ intention is to show BTs contribute to both organic and inorganic matters in aerosols. To avoid confusion, the terminology should be made consistent. The Introduction could mirror this phrasing when framing the knowledge gap. The Introduction might state that “BTs may contribute to secondary aerosol (both organic and inorganic) via aqueous-phase reactions” instead of focusing only on SOA. This would ensure the reader knows from the start that both organic and inorganic aspects are of interest.
3. 3.2: The optical data are well-presented, but their implications could be tied more explicitly to the evolving definition of atmospheric brown carbon. In other words, the authors should more directly connect their observed optical changes to the characteristics of brown carbon reported in the literature.
4. One goal of the study is to elucidate reaction mechanisms. In the Introduction it explicitly says that mechanisms would be proposed based on the products. The paper discusses these mechanisms in detail in the main text. However, the Conclusion section, while summarizing outcomes (products formed, etc.), does not explicitly summarize the mechanism that was proposed. It implies a mechanism, e.g., “suggesting radical–radical oligomerization is responsible for oligomers and brown carbon formation”, but it doesn’t plainly state something like “a mechanism involving X, Y, Z steps was deduced.” To fully close the loop, the authors could dedicate a sentence in the Conclusion to the mechanism. Including a high-level description of the reaction pathway would remind readers that the study achieved its aim of unraveling the mechanism, not just finding the end products. This would tie back to the Introduction’s promise of proposing a mechanism.
5. The Abstract provides the exact rate constant values, highlighting the kinetic data, but it does not explicitly interpret what those mean in atmospheric terms. The Conclusion, on the other hand, emphasizes the implications of those kinetics by converting them to lifetimes and comparing aqueous vs. gas-phase transformation rates. While this is not a critical inconsistency (abstracts often omit detailed interpretation due to space), it is a difference in emphasis. Readers of the abstract see numbers but might not realize that corresponds to a very short lifetime. Abstract should include a brief interpretive phrase to complement the raw rate numbers. Similarly, the Introduction might also hint at expected fast kinetics by mentioning OH’s known reactivity. Currently, the Introduction imply this generally, and the conclusion confirms this quantitatively; making sure the abstract also reflects this would complete the chain of logic across all three sections.
6. The formation of sulfate is one of the major findings of this study, highlighted in both the Abstract and Conclusion, yet the Introduction does not mention sulfate at all. The Introduction should explicitly mention the possibility of sulfate generation from BTs during aqueous-phase oxidation. Establish a clearer rationale for investigating sulfate production, ensuring logical continuity from the Introduction’s theoretical framework to the empirical results and their implications in the Conclusion.
7. L235-248: The authors state that nanoparticles formed after 5 hours of photooxidation, with sizes ranging from 50 to 400 nm and concentrations ~10^8 particles/mL. This is a striking observation, indicating significant particle formation from BT oxidation. However, this result is presented with minimal discussion. This section should be revised to include additional discussion that situates the observed nanoparticle formation within the broader literature.
8. L200-235: UV-vis and EEM data show red shifts and increased MAE values. Though the red shift is described and linked to conjugation, there is limited discussion of (1) how these light-absorbing products compare to known brown carbon chromophores, and (2) any atmospheric implications of higher MAE (e.g., warming effects).

Zhang et al. investigated the aqueous-phase OH reaction of benzothiazoles. The reaction kinetics, transformation products, reaction mechanism, and optical properties were reported. The atmospheric fate of benzothiazoles is not well illustrated at this point, this study highlights that the aqueous-phase oxidation of BTs can contribute to the secondary aerosol mass in the atmosphere, and have the potential to change the optical properties of ambient particles. I recommend this paper to be published after some revisions.

1. The focus of this study is aqueous-phase reaction of benzothiazoles. The atmospheric significance of this process depends on the concentrations of benzothiazoles in atmospheric droplets. The authors mention that the highest concentration of benzothiazoles in PM2.5 is at the level of ng/m3. I wonder what is concentration in cloud water or aerosol liquid water? Not all benzothiazoles in PM2.5 will be present in aerosol liquid water, so the concentration in aqueous-phase is likely to be lower than ng/m3. If the concentration of benzothiazoles in aerosol liquid water is low, this will limit the potential for aqueous-phase reactions to change the optical properties of atmospheric particles. The authors need to further discuss this point in Implication section. 

2. Experiments were conducted at two different pH conditions (pH 2 and pH 10). The authors need to provide the rationale for selecting these two pH to perform experiments. Are they relevant to atmospheric conditions?

 

3. Some reference compounds such as suberic acid and toluic acid are organic acids. Will the pH of solution be changed when adding these acids into the solution? 

4. L185, the authors state that the rate constants of selected BTs reacted with OH radicals under the highly acidic condition were slightly lower than those under the weakly alkaline condition. Why the reaction rate constants at low pH are lower than those under high pH conditions? Please clarify. 

5. It is interesting that nanoparticles were formed after OH reactions. Did you observe different patterns for the formation of nanoparticles for different BTs. I would expect to see different patterns given that their structures are different and the product composition prolife in Figure 4 is also different. 

6. L274, The authors state that more products can be identified using ESI+ mode because organic products are rich in basic functionalities and slightly less rich in acidic functionalities. I am not sure this is correct for all organics or this is because BTs contain N, and N-containing compounds are more easily to be detected with ESI+ mode. 

7. The proposed mechanism for BT reactions is convincing. I recommend the authors to describe the electron transfer process in Figure 5 such as ring-opening process, so that readers can better understand the chemical mechanism. 

8. When discussing the atmospheric lifetime of BTs, the authors need to consider the impact of reaction conditions. The current calculation is based on an assumption that the pH of cloud droplets and deliquescent particles are similar to those using in the current experiments (pH 2 and pH 10). The authors need to point out the uncertainty associated with this assumption.