From the journal Environmental Science: Atmospheres Peer review history

12-Jun-2023

Dear Professor Li:

Manuscript ID: EA-ART-05-2023-000073
TITLE: Co-photolysis of Mixed Chromophores Affects Atmospheric Lifetimes of Brown Carbon

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Associate Editor, Environmental Sciences: Atmospheres

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Reviewer 1

This manuscript describes aqueous phase brown carbon photobleaching experiments focused on binary mixtures involving four species, two methoxyphenols and two nitrophenols. The experiments are carefully designed and clearly presented. The authors find that a slowing of photochemistry due to “shading” between absorbers could only be significant in super-micron, highly concentrated aerosol, whereas photosensitization can be very significant in any size particle and at a wide range of concentrations. The methoxyphenol compounds are much stronger photosensitizers than the nitrophenol compounds, even though the nitrophenols absorb light more strongly. This paper convincingly demonstrates complexity in brown carbon photochemistry, and should be of interest to those studying or modeling aerosol optical properties. The manuscript is ready for publication after a few minor revisions to address the following points.

Line 264: More explanation should be given for the reason for a 2-orders-of-magnitude difference in quantum yields for VL and 4NC between this and previous studies. Can the authors pinpoint a difference in the conditions that is the most likely cause of the discrepancy?

The manuscript makes the important point that at high concentrations typical of aqueous aerosol particles, secondary reactions such as radical-initiated oligomerization can consume more BrC chromophores than direct photolysis (leading to quantum yields that were occasionally larger than one). In my opinion, this interesting result should be highlighted in the abstract.

Technical corrections:
Line 117: The authors should give the value of the correction factor here, but leave the explanation of its derivation in the SI.

Lines 206 and 213: “We refer this method to” should be changed to “We refer to this method as”

Reviewer 2

This study investigates the shadowing and sensitizing effects observed during the co-photolysis of different BrC chromophores in a bulk photochemical reactor, focusing on elevated concentrations in attempt to mimic ambient aerosol particles. Specifically, the co-photolysis of methoxyphenols and nitrophenols was investigated in terms of shadowing and sensitizing. Factors such as particle size, concentration, and the sensitization ability of the chromophores are discussed. The experimental setups primarily utilized air-saturated aqueous solutions, but also included selected experiments in an organic solvent or under N2-saturated conditions to explore matrix dependence and the role of reactive oxygen species. This is a quality work and its findings have important implications for a better grasp of the atmospheric lifetime of BrC aerosols, thus their effects on climate and atmospheric chemistry. I recommend accepting the submission after the following comments are addressed.

The concentration of a species in aerosol liquid water is influenced by various factors such as solubility, volatility, and gas phase concentration. In the context of the study focusing on methoxyphenols and nitrophenols, it is important for the authors to provide a more comprehensive explanation of the atmospheric relevance of the high concentration regime investigated. This is necessary to better assess the significance and implications of this research within the broader atmospheric context.

It would be beneficial for the authors to provide more detailed mechanistic explanations for the observed sensitization and shadowing effects. This will enhance the understanding of the underlying reasons behind these processes.

Describing the appearance of the figures, such as the color of the symbols, in the manuscript texts is distracting and redundant. It would be more appropriate to include this information in the figure caption.

Line 44, defining “optically thin solution” using concentration “<1mM” can be misleading, as the optical property of a solution is not just dependent on the its concentration but also the solute’s absorptivity.

Page 11, details on how N2-saturated experiments are performed should be provided. It is also important to justify the usage of isopropanol and effects on photochemical reactions.

Page 14, 2nd paragraph (line 231- 240), the discussions on the quenching effect appear to be overly simplistic and lacking in specific details. A more comprehensive explanation of the quenching effect in the context of reaction pathways would be useful. The fact that isopropanol can scavenge OH radicals should be addressed and whether it is considered as a form of quenching effect.

Line 281 – 283, this speculation, while intriguing, should be validated through studies on ambient particles, including measurement of oxidant concentrations and degradation of BrC precursors. It is important to assess whether the observed phenomenon aligns with the actual atmospheric conditions.

Line 323, when discussing “the ground-state BrC tends to react with reactive intermediates generated from photosensitized reactions…”, it is crucial to acknowledge the fact that the reactive intermediates are not in direct contact with ground-state BrC species that are shielded from light exposure.

Figure 2, consider to use consistent colors for added BrC species. This will make the figure easier to read.

Figure 4, please clarify the measurement method used in the figure caption, as the formulas from different instruments may have different interpretations.

In Fig. S2, it is evident that the light source utilized in this study emits UVB photons with a wavelength below 300 nm. It is crucial to address and consider any potential artifacts that may be associated with this specific light source.

Fig. S3, explain the error bars

Dear Editor,

Thank you for handling our manuscript.

We have now addressed the comments by the reviewers, and made changes in the revised manuscript accordingly. We also provide here with point-by-point response to those comments, together with the revised (tracked and clean) manuscript and supporting information.

Thanks again for your time.

Best
YJ Li
University of Macau

25-Jun-2023

Dear Professor Li:

Manuscript ID: EA-ART-05-2023-000073.R1
TITLE: Co-photolysis of Mixed Chromophores Affects Atmospheric Lifetimes of Brown Carbon

Thank you for submitting your revised manuscript to Environmental Science: Atmospheres. I am pleased to accept your manuscript for publication in its current form. I have copied any final comments from the reviewer(s) below.

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Prof. Nønne Prisle
Associate Editor, Environmental Sciences: Atmospheres

Reviewer 2

The authors have addressed the review comments to satisfaction. However, the sentence on Line 246-249: “This observation indicates that in the commonly present organic matrix in
atmospheric aerosols might enhance the nitrophenol degradation.” is not grammatically correct. This sentence should be corrected