From the journal Environmental Science: Atmospheres Peer review history

16-Jul-2023

Dear Dr Li:

Manuscript ID: EA-ART-06-2023-000093
TITLE: Investigation of HO₂ uptake onto Cu(II)- and Fe(II)-doped aqueous inorganic aerosols and seawater aerosols using laser spectroscopic techniques

Thank you for your submission to Environmental Science: Atmospheres, published by the Royal Society of Chemistry. I sent your manuscript to reviewers and I have now received their reports which are copied below.

I have carefully evaluated your manuscript and the reviewers’ reports, and the reports indicate that major revisions are necessary. In particular, one reviewer pointed out the need to better emphasize the innovation of this study and differentiate your methodology with those previous used by groups in this field.

Please submit a revised manuscript which addresses all of the reviewers’ comments. Further peer review of your revised manuscript may be needed. When you submit your revised manuscript please include a point by point response to the reviewers’ comments and highlight the changes you have made. Full details of the files you need to submit are listed at the end of this email.

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Environmental Science: Atmospheres strongly encourages authors of research articles to include an ‘Author contributions’ section in their manuscript, for publication in the final article. This should appear immediately above the ‘Conflict of interest’ and ‘Acknowledgement’ sections. I strongly recommend you use CRediT (the Contributor Roles Taxonomy, https://credit.niso.org/) for standardised contribution descriptions. All authors should have agreed to their individual contributions ahead of submission and these should accurately reflect contributions to the work. Please refer to our general author guidelines https://www.rsc.org/journals-books-databases/author-and-reviewer-hub/authors-information/responsibilities/ for more information.

I look forward to receiving your revised manuscript.

Yours sincerely,
Dr Tzung-May Fu
Associate Editor
Environmental Science: Atmospheres
Royal Society of Chemistry

************

Reviewer 1

See the attached file.

Reviewer 2

In this paper the authors measure the rate of heterogeneous uptake of HO2 on different inorganic aerosols doped with transition metals. The work is important to improve models since these processes have been shown to be very important in determining oxidizing capacity of the atmosphere but still there are quite a lot of uncertainties.

In the abstract the authors claim: "Here, we established an approach for detecting the loss rate of HO2 uptake by inorganic aerosols derived from NaCl, (NH4)2SO4, Na2SO4, or diluted seawater using laser-pump and laser-induced fluorescence techniques", this gives the impression that laser-induced fluoresce technique is used for first time to do that and this is not the case, there are already another group in University of Leeds doing that and some of the references are given in the paper.
- What is the difference between the experimental approach in this paper and those in papers like the ones included in the references: George et al 2013 and Lakey et al 2015? In both cases an aerosol flow tube is used and HO2 is detected after being converted in OH by laser induced fluorescence. What are the advantages of this new set up in relation what is have been already published?
In the paper they claim the following advantages:
"The CC/LP-LIF offered the possibility to detect the loss rate of HO2 radicals by controlling the sampling of air, either with or without aerosols, to separate heterogeneous HO2 loss from gas-phase loss. Furthermore, no additional calibration was required. Thus, we established a direct measurement strategy for determining HO2 uptake by applying the CC/LP-LIF method with an atomizer for the first time."
In the set up described in the references is possible as well to introduce aerosol or not as this is necessary to determine kloss homogeneous that is necessary to determine the uptake.
Another advantage highlighted is that does not require a calibration. In the references they do not claim that they need calibration, they just determine it to know the HO2 initial concentration but for determining the uptake is not a data required.
In the references by Leeds group they are using an atomizer in the same way they are using in this paper.
It is not clear how they change the contact time to determine the HO2 heterogeneous rate loss.... Or do they do the experiments at fixed time? This has been done as well by Leeds group for example in the thesis: https://core.ac.uk/download/pdf/30267581.pdf (papers already published, too). It is equivalent to fixed injector experiments.
Further discussion about the atmospheric implications of their results need to be included.
Moreover, the discussion comparing their results with previous results need to be more in depth.

Dear Dr. Editor:

Subject: submission of revised paper “Investigation of HO2 uptake onto Cu(II)- and Fe(II)-doped aqueous inorganic aerosols and seawater aerosols using laser spectroscopic techniques” (Manuscript ID: EA-ART-06-2023-000093).

Thank you so much for your emails dated 16 July, 2023 enclosing the reviewers’ comments. We also thank reviewers for taking time and giving us valuable remarks. We have carefully reviewed the comments and have revised the manuscript accordingly.

We hope the revised version is now suitable for re-review and look forward to hearing from you in due course.

Sincerely,
************
Reviewer 1:
Li et al. developed and validate a novel method to measured HO2 uptake, and used it to examine HO2 uptake onto Fe(II)- and Cu(II)-doped aerosol particles. HO2 uptake can be very important for tropospheric chemistry, but the uptake coefficients are still under debate. This work is significant and well conducted, and it can be accepted after the following comments are addressed.

>>We greatly appreciate the valuable comments from the reviewer. We have carefully addressed each point of view and revised accordingly as explained in detail hereinafter.

Line 58-84, Sections 2.1-2.3: although I am very familiar with aerosol flow tubes, it took me a long time to understand the difference between the apparatus used in the present work and those used by Lakey et al. (2015) and Zou et al. (2019). I have a few suggestions: 1) The authors may need to explain concisely in the instruction (line 71-84) and in detail in Sections 2.1-2.3 why the apparatus they used did not require a movable injector to change the reaction time; 2) It will help if Figure A2 is moved into the main text, as this figure shows the critical raw data.

>>Many thanks for addressing this important issue. Firstly, we think that the results measured from AFT approaches are reliable, and our result was validated by comparing with AFT-result. Secondly, AFT-approach in most cases uses a movable injector to vary the residence time of HO2 interacting with aerosols, and measure corresponding radical concentration, thus both concentration and residence time is necessary to obtain the uptake coefficient. We fixed the interaction time of HO2 with aerosols and measured the HO2 reactivity rather than concentration. HO2 reactivity onto varied volumes of aerosols was measured assuming it is a pseudo-first-order kinetic process. We have specified the difference between our method compared with previous literatures as described hereinafter.
In the introduction (line 79–83), we have added more information to explain briefly as follows:
"Also, it is flexible to detect the HO2 loss rate onto varied total aerosol surface area concentrations by changing the flow rate of zero gas passing the atomizer. Note that the residence time of HO2 mixing with aerosols in the reaction cell is short as 1 s, allowing high time resolutions and also there is no need to change the interaction time using our apparatus. AFT-reactor approach requires a movable injector to change the reaction time of radicals with aerosols, and detect different radical concentrations at varied time to determine its loss rate. However, we directly measure the loss rate of radicals onto increased/decreased volume of aerosols to obtain its uptake coefficient. Furthermore, no additional calibration was required because we measured HO2 reactivity rather than its concentration."
In the section 2.1–2.3, we added the detail as follows. Figure A2 was kept in the appendix considering it is from our simulation with different uptake coefficients. We added a Table (Table 2) to compare the characteristics of AFT-reactor and our apparatus regarding the radical generation, time resolution, reaction cell conditions (Pressure & Temperature), calibration requirement, respective advantages, and so on.

The total surface area concentrations of aerosols generated by atomizer can be varied using different flow rates as introduced in Section 2.2. Consequently, larger total surface area concentrations result in faster decay rates. Different to the AFT-reactor approach, we fixed the residence time of radicals and aerosols in the reaction cell as 1 s, and then regenerate radicals every second. The determined decay rate of HO2 radicals was averaged every 240 circles. Each experiment was repeated three–five times to ensure reproducibility and reduce uncertainty. Then we measured with a varied volume of aerosols to extrapolate the relationship between decay rates and total surfaced area concentrations in order to quantify the effective uptake coefficient.

Line 241-248: it is not clear to me how the authors used the two thermodynamic models to calculate Cu(II) or Fe(II) concentrations in aqueous aerosol particles they obtained, as the two models do not consider Cu or Fe. I assume the authors used the model to calculate Na+ concentrations in aerosol particles (use the NaCl+Fe mixture as an example), and assumed that the Na+/Fe ratio in aerosol particles was equal to that in the bulk mixture used to generate aerosol particles. More details are needed here.

>>Yes, we used a similar approach to assess the Cu(II) or Fe(II) concentrations in the aerosol phase by calculating the change rate of liquid water content in aerosol phase to aqueous phase. We apologize for the unclear explanations. The thermodynamic models can estimate the aerosol water content from the liquid under a known RH and temperature. We then used the same ratio to calculate the condensed concentration of TMIs in the aerosol phase.
"We assessed the TMI concentrations indirectly by using the change rate of liquid water content in aerosol phase compared to the liquid phase. A same change ratio of the condensed TMIs was assumed to simulate the concentrations in the aerosol phase.

Line 392-414: Although the author used Fe(II), Fe(III) is more stable than Fe(II) in the presence of O2. As a result, some Fe(II) in aqueous solutions/particles could be oxidized to Fe(III). This chemistry is complicated and slightly beyond the scope of this work; however, the author may need to mention this possibility/uncertainty."

>>We agree and appreciate this viewpoint. It is highly possible that the Fe(III) exists in our system which is not considered in our work. We have added the statement of uncertainties regarding Fe(III) in the conclusion, as follows.
"Fe(II) in the aqueous/aerosol phase could be oxidized to Fe(III), as indicated in Table 4, which should be more stable than sole Fe(II). The co-existence of Fe(II) and Fe(III) may bring uncertainties to unconsidered side-reactions, which is beyond the scope of the current study."

Line 155: change “eliminate uncertainty” to “reduce uncertainty”, as no one can really eliminate uncertainty.

>>Thanks for pointing this out, we have revised as suggested.

Reviewer 2:

Comments to the Author
In this paper the authors measure the rate of heterogeneous uptake of HO2 on different inorganic aerosols doped with transition metals. The work is important to improve models since these processes have been shown to be very important in determining oxidizing capacity of the atmosphere but still there are quite a lot of uncertainties.

>>Many thanks for spending time on our work and giving us valuable comments. We have carefully revised our manuscript based on each suggestion. The significance to model studies has been emphasized in the atmospheric amplification (conclusion) section.

In the abstract the authors claim: "Here, we established an approach for detecting the loss rate of HO2 uptake by inorganic aerosols derived from NaCl, (NH4)2SO4, Na2SO4, or diluted seawater using laser-pump and laser-induced fluorescence techniques", this gives the impression that laser-induced fluoresce technique is used for first time to do that and this is not the case, there are already another group in University of Leeds doing that and some of the references are given in the paper.

>>We appreciate this comment, and we have revised the abstract. The LP-LIF was not used for the first time to determine HO2 uptake, however, our method is different from previous approaches. We have revised the expression as follows and more details to indicate the difference is added in the main context.
"…Here, we established a novel approach for detecting the loss rate of HO2 uptake by inorganic aerosols derived from NaCl, (NH4)2SO4, Na2SO4, or diluted seawater using laser-pump and laser-induced fluorescence techniques and…"

- What is the difference between the experimental approach in this paper and those in papers like the ones included in the references: George et al 2013 and Lakey et al 2015? In both cases an aerosol flow tube is used and HO2 is detected after being converted in OH by laser induced fluorescence. What are the advantages of this new set up in relation what is have been already published?

>> Thank you so much for addressing this point of view. We apologize for the unclear expression. The AFT-reactor approach controls the reaction time of radicals with aerosols to determine the loss rate of radicals from varied residence time depending on the injector place. In our apparatus, the residence time was fixed as 1 s, and we detect the loss rate of radicals with varied volume of aerosols. We have added more details in the context (please refer to introduction and section 2.3, which was also required by another reviewer). Also, Table 2 is added to compare the characteristics of our method and previous AFT-reactor-FAGE approach.

In the paper they claim the following advantages:
"The CC/LP-LIF offered the possibility to detect the loss rate of HO2 radicals by controlling the sampling of air, either with or without aerosols, to separate heterogeneous HO2 loss from gas-phase loss. Furthermore, no additional calibration was required. Thus, we established a direct measurement strategy for determining HO2 uptake by applying the CC/LP-LIF method with an atomizer for the first time."
In the set up described in the references is possible as well to introduce aerosol or not as this is necessary to determine kloss homogeneous that is necessary to determine the uptake.

>>We appreciate the valuable comments on this issue. It is true that same technique has been used to detect the HO2 loss onto aerosols, however, the principle is different on how radicals were produced and how to determine the uptake coefficient. We apologize for the misunderstanding expression and have revised the context as following.
"The CC/LP-LIF offered the possibility to detect the loss rate of HO2 radicals by controlling the sampling of air, either with or without aerosols, to separate heterogeneous HO2 loss from gas-phase loss. Also, it is flexible to detect the HO2 loss rate onto varied total aerosol surface area concentrations by changing the flow rate of zero gas passing the atomizer. Note that the residence time of HO2 mixing with aerosols in the reaction cell is short as 1 s, allowing high time resolutions and there is no need to change the interaction time using our apparatus. AFT-reactor approach requires a movable injector to change the reaction time of radicals with aerosols, and detect different radical concentrations at varied time to determine its loss rate. However, we directly measure the loss rate of radicals onto increased/decreased volume of aerosols to obtain its uptake coefficient. Furthermore, no additional calibration was required because we measured HO2 reactivity rather than its concentration. Thus, we established a novel measurement strategy for determining HO2 uptake by applying the CC/LP-LIF method with an atomizer, this is different to AFT-reactor approach which requires controlling of the injection of radicals. Table 2 compares the details between AFT-reactor and our apparatus…"

Another advantage highlighted is that does not require a calibration. In the references they do not claim that they need calibration, they just determine it to know the HO2 initial concentration but for determining the uptake is not a data required.
In the references by Leeds group they are using an atomizer in the same way they are using in this paper.

>>Thanks for pointing this out. In the references, researchers used an AFT-reactor connected with FAGE (this is similar to our LIF system) or PERCA/CIMS (this measure the concentration of radicals). Radicals were injected at varied places of the AFT-reactor for different HO2 concentrations to be measured. We used the technique differently via mixing the radicals and aerosols together in the reaction cell at a fixed time (1 s), and detect the HO2 loss rate with different volumes of aerosols. The advantage has been specified as the following:
"Furthermore, no additional calibration was required because we measured HO2 reactivity rather than its concentration. It is also possible to detect RO2 uptake using our method."

It is not clear how they change the contact time to determine the HO2 heterogeneous rate loss.... Or do they do the experiments at fixed time? This has been done as well by Leeds group for example in the thesis: https://core.ac.uk/download/pdf/30267581.pdf (papers already published, too). It is equivalent to fixed injector experiments.

>>Yes, we agree that it is equivalent to a fixed injector experiment since we do not change the contact time of HO2 interacting with aerosols. Many thanks for providing the thesis, we have carefully compared the detail of Leeds group approach with ours and the differences were summed in Table 2.

Further discussion about the atmospheric implications of their results needs to be included.

>>We appreciate this comment. Our experimental results could be informative to model studies to reach a better understanding of radical budget under complex conditions. Also, further study that considers RO2 uptake onto a series of aerosols would be in urgent which is current still unknown. For the atmospheric implications, we have modified the Conclusion section as “These results clarify poorly understood multiphase reactions and can aid in the optimization of model-based simulations of oxidant generation in ambient environments, especially when considering complex TMI-containing aerosols such as sea spray in the marine regions. Our results also suggest that model studies need to consider different HO2 uptake coefficients onto diverse ambient aerosols to bridge the gap with observation results. In future, it would be necessary to investigate the impact of aerosol pH, effective ion concentrations in the aerosol phase, Cu(II) or Fe(II) concentration dependence, and the water content of aerosols (phase-change effects). It would also be necessary to investigate ambient complex aerosols and detect RO2 uptake."

Moreover, the discussion comparing their results with previous results need to be more in depth.

>>Thanks for giving this suggestion. We compared our result with previous data to examine the validity of our method. The main discussion is available in section 3.1 and section 3.2, which focuses on the instrument performance and analysis on the uncertainties to determine uptake coefficient. The results of HO2 uptake onto (NH4)2SO4 were mostly conducted by different groups. Therefore, we collected all published dataset to our knowledge and also compared the result with theoretic simulation result. Figure 4 provided the result, with good agreement from the comparison, although we agree that there are small differences according to the figure. The uptake of HO2 onto (NH4)2SO4 mainly depends on the self-reactions of HO2 (depending on the HO2 concentration) considering there is very slow bulk-phase reactions. Therefore, when we simulate the self-reactions in the aerosol-phase, it was found that experimental results could agree well with the theoretic line, the uncertainty only remains in the case of high concentration of HO2 (above 109 molecule cm–3), and aerosol pH will impact the self-reaction in the bulk greatly. This was proved by the different simulation results from pH=4 and pH=5.2, although further experimental might be necessary to understand this procedure. The HO2 uptake onto pure (NH4)2SO4 should be quite limited at ambient HO2 level. The result in the current study is important for model studies to understand the behavior and impacts of HO2 radicals in the multiphase reaction process.
In section 3.1, line 273–277, "Although there should be discrepancies among our results and other reported owing to different experimental conditions such as aerosol water content, radical concentration, interaction time of radicals and aerosols, and different hypothesized parameters used to quantify the . More discussion in depth will be described in section 3.2 to indicate the difference of our result compared with previous results."

17-Aug-2023

Dear Dr Li:

Manuscript ID: EA-ART-06-2023-000093.R1
TITLE: Investigation of HO₂ uptake onto Cu(II)- and Fe(II)-doped aqueous inorganic aerosols and seawater aerosols using laser spectroscopic techniques

Thank you for your submission to Environmental Science: Atmospheres, published by the Royal Society of Chemistry. I sent your manuscript to reviewers and I have now received their reports which are copied below.

After careful evaluation of your manuscript and the reviewers’ reports, I will be pleased to accept your manuscript for publication after revisions. One reviewer pointed out a number of minor issues, which I hope you may be able to address in your revisions. Once I receive your revision and decide that these issues are addressed in a satisfactory manner, I will not send the manuscript out for another round of review.

Please revise your manuscript to fully address the reviewers’ comments. When you submit your revised manuscript please include a point by point response to the reviewers’ comments and highlight the changes you have made. Full details of the files you need to submit are listed at the end of this email.

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The Royal Society of Chemistry requires all submitting authors to provide their ORCID iD when they submit a revised manuscript. This is quick and easy to do as part of the revised manuscript submission process. We will publish this information with the article, and you may choose to have your ORCID record updated automatically with details of the publication.

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Environmental Science: Atmospheres strongly encourages authors of research articles to include an ‘Author contributions’ section in their manuscript, for publication in the final article. This should appear immediately above the ‘Conflict of interest’ and ‘Acknowledgement’ sections. I strongly recommend you use CRediT (the Contributor Roles Taxonomy, https://credit.niso.org/) for standardised contribution descriptions. All authors should have agreed to their individual contributions ahead of submission and these should accurately reflect contributions to the work. Please refer to our general author guidelines https://www.rsc.org/journals-books-databases/author-and-reviewer-hub/authors-information/responsibilities/ for more information.

I look forward to receiving your revised manuscript.

Yours sincerely,
Dr Tzung-May Fu
Associate Editor
Environmental Science: Atmospheres
Royal Society of Chemistry

************

Reviewer 1

The authors have well addressed my comments, and I would like to recommend the manuscript for publication.

Reviewer 3

General comments
The role of Fe on atmospheric HO2 uptake in ambient aerosols is not known well quantitatively. The authors presented and analyzed the results from laboratory measurements of HO2 uptake onto Fe-doped aqueous inorganic aerosols and others. If the paper were published in its current form, the point made by the previous reviewer would be critical. I can recommend for publication in Environmental Science: Atmospheres after the authors address the issues raised below.

Specific comments
p. 7, l.261: Please rephrase the change rate. Please show the input and output variables for E-AIM and ISORROPIA in Appendix. How did you treat L-ascorbate? Please show the input parameters for the organic compound properties in Appendix.

p.9, l.316: Please indicate the salinity and ionic strength of seawater for Izu and Hachijojima in Appendix.

p.9, l.319: Please specify the mechanism that further suppresses HO2 uptake.

p.12, l.424: I agree that both Cu and Fe should be considered in ambient aerosols given Cu-Fe redox coupling. However, the results for Cu-Fe redox coupling are not shown in this paper. Please clarify this point.

p.23, l.425: I agree that Fe(II) in the presence of O2 is quickly oxidized to Fe(III) with no Fe-binding organic ligand. However, you should justify the laboratory conditions to sustain Fe(II) as the major redox state of Fe in Fe-doped aqueous inorganic aerosols. If not, the estimated Fe(II) in liquid water would be overestimated, because the dissolved Fe(III) can be precipitated as Fe oxyhydroxide nanoparticles at the high pH conditions. Thus, ferrihydrite in addition to the reactions in Table 4 must be considered in the thermodynamic equilibrium model. In this case, please consider the redox state and ferrihydrite in aerosols throughout the manuscript. For instance, please consider the redox state and Fe solubility of ferrihydrite at pH 4 in Fig. 5, and so on.

Dear Dr. Editor:

Thank you so much for your emails dated 17 August, 2023 enclosing the editor’s decision and reviewers’ comments. We also thank reviewers for taking time and giving us valuable remarks. We have carefully reviewed the comments and have revised the manuscript accordingly.

We hope the revised version is now suitable for publication and look forward to hearing from you in due course.
***************
Referee 3:
General comments
The role of Fe on atmospheric HO2 uptake in ambient aerosols is not known well quantitatively. The authors presented and analyzed the results from laboratory measurements of HO2 uptake onto Fe-doped aqueous inorganic aerosols and others. If the paper were published in its current form, the point made by the previous reviewer would be critical. I can recommend for publication in Environmental Science: Atmospheres after the authors address the issues raised below.

>>Thanks for spending time on our manuscript and emphasizing the impacts of Fe irons on HO2 uptake. We have added the explanation and revised accordingly as suggested.

Specific comments
p. 7, l.261: Please rephrase the change rate. Please show the input and output variables for E-AIM and ISORROPIA in Appendix. How did you treat L-ascorbate? Please show the input parameters for the organic compound properties in Appendix.

>>We appreciate this comment. The change rate has been modified as change of ratio in the paragraph.
We assessed the TMI concentrations indirectly by assuming the same change of ratio as water content in aerosol phase compared to the liquid phase.
The input and output variables for model simulation were added in the appendix Table A2. We calculated the concentration of TMIs in the aerosol phase using the same change of ratio as water content from the liquid to the aerosols. L-ascorbate was not treated in both models. Because L-ascorbate was not used for further analysis in the bulk phase, thus we did not consider the organic compound properties.
Table A2. Input and output variables for E-AIM and ISORROPIA.
E-AIM ISORROPIA
Input Temperature, Relative humidity, [H+], [NH4+], [Na+], [SO42-], [Cl-] Temperature, Relative humidity, [Na+], [SO42-], [NH4+], [Cl-]
Output moles of species in the aqueous phase, moles of gases, and aerosol water content Equilibrium concentrations and aerosol water content

p.9, l.316: Please indicate the salinity and ionic strength of seawater for Izu and Hachijojima in Appendix.

>>Thanks for pointing this out. We agree that the salinity and ionic strength of the seawater samples are important which we did not mention. The salinity of seawater from Hachijojima was measured and provided in appendix Table A1, which is 3.5% w/v (35 g/L). The ionic strength of seawater increases with salinity, which is generally around 0.7. However, here we could not do the measurement on the chemical analysis of seawater samples. We have added the reference value of salinity and ionic strength of seawater in the manuscript. More mechanism on HO2 uptake onto seawater will be done in our future study and we plan to collect more seawater samples for both chemical analysis and the HO2 uptake experiment.
The salinity is 35.16504 g kg–1 for standard seawater34 and ionic strength < 1 M.

p.9, l.319: Please specify the mechanism that further suppresses HO2 uptake.

>>The mechanism we supposed to be the organic components contained in seawater samples, which suppresses HO2 uptake. Previous research has found that HO2 uptake decreased with the coating of organic species. However, further experimental proof is needed to support our hypothesis.
Lakey et al found that organics suppress HO2 uptake onto aerosols even containing TMIs,34 and possibly the marine organisms drive discrepancies on HO2 uptake onto seawater.

p.12, l.424: I agree that both Cu and Fe should be considered in ambient aerosols given Cu-Fe redox coupling. However, the results for Cu-Fe redox coupling are not shown in this paper. Please clarify this point.

>>We appreciate this viewpoint, and has revised as suggested. The Cu-Fe redox coupling currently is on available in models (as mentioned in our paper). Further experiments are necessary given its atmospheric significance in the ambient air.
However, the experimental results regarding Cu-Fe redox coupling are not available in the current paper.

p.23, l.425: I agree that Fe(II) in the presence of O2 is quickly oxidized to Fe(III) with no Fe-binding organic ligand. However, you should justify the laboratory conditions to sustain Fe(II) as the major redox state of Fe in Fe-doped aqueous inorganic aerosols. If not, the estimated Fe(II) in liquid water would be overestimated, because the dissolved Fe(III) can be precipitated as Fe oxyhydroxide nanoparticles at the high pH conditions. Thus, ferrihydrite in addition to the reactions in Table 4 must be considered in the thermodynamic equilibrium model. In this case, please consider the redox state and ferrihydrite in aerosols throughout the manuscript. For instance, please consider the redox state and Fe solubility of ferrihydrite at pH 4 in Fig. 5, and so on.

>>Many thanks for addressing this comment. We conducted the experiments with fresh solution kept in an airtight container every time. As the reviewer mentioned, the Fe(II) oxidation by molecular oxygen over the pH range of 4.0–5.5 without the existence of organic ligand also occurs. However, the whole oxidation process last essentially for several hours even with Fe-blinding organic ligand in a higher concentration of Fe(II) condition (the oxidation occurs faster with ligand and in a higher concentration of iron; refer to Adele M. Jones et al., 2015, Geochimica et Cosmochimica Acta, 160, 117-131). In our case we used diluted reagent (0.0015% w/v) in the solution. It takes less than 1 min from the atomization (atomized from the airtight container) to interaction with HO2. Therefore, the major redox state of Fe in the aerosols should be Fe(II). We agree that there might be scarce Fe(III) exist in the system but its effect could be ignorable (within uncertainty). The uptake coefficient did not change with time and therefore the system sustain stable during our measurement. Ferrihydrite is not considered in the model.
Fe(II) in the aqueous or aerosol phase may be oxidized to Fe(III) in the presence of molecular oxygen or HO2, which should be more stable than sole Fe(II). However, the relative low concentration of Fe(II) in the solution, absence of Fe-binding ligand, fast time resolution, and experimental procedure sustain Fe(II) as the major redox state. The effect of Fe(III) can be ignorable (within uncertainty) regarding the value of uptake coefficient in our laboratory conditions. Note that the co-existence of Fe(II) and minor Fe(III) may bring uncertainties to unconsidered side-reactions, which is beyond the scope of the current study.

23-Aug-2023

Dear Dr Li:

Manuscript ID: EA-ART-06-2023-000093.R2
TITLE: Investigation of HO₂ uptake onto Cu(II)- and Fe(II)-doped aqueous inorganic aerosols and seawater aerosols using laser spectroscopic techniques

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