This manuscript, entitled “The impacts of marine-emitted halogens on OH radical in East Asia during summer”, by Shidong Fan and Ying Li, presents a modeling study using WRF-CMAQ, a community model implemented with tropospheric halogen chemistry with a focus on marine emissions of several key reactive halogen compounds. The iodine chemical mechanism appears to be fairly comprehensive. By using process analysis tools (integrated reaction rate and integrated process rate) and several sensitivity tests, the authors show that the reactive iodine chemistry have major impacts on the production of OH radicals in the marine boundary layer in East Asia. The sensitivity tests are nicely designed such that the impacts of various processes can be separated in a logical manner. The conclusions are of interest to the broad community and several research gaps fur future investigation are identified. The paper is generally well organized and structured, although the clarity and logic could use some improvement. However, a few key science questions are not clearly elucidated, which are elaborated in the Major Comments section. I also have several Minor and Technical comments. All these comments and suggestions are intended to strengthen the manuscript and to improve the clarity. I recommend this manuscript for publication after Major Revision.

Major comments/suggestions:

The main results (Figure 2 and Figure 3) are very interesting. Yet, some of the discussion presented in the current version of the manuscript is vague and fails to address two key questions: 

(1) How would anthropogenic pollutants interact with marine halogens? The authors have cited a wide range of previous studies (e.g., Sherwen et al., Wang et al.,) but most of these studies focus on the global scale where halogen chemistry is largely driven by natural processes. In this work, the studied area is subject to major anthropogenic influence. This is a question of great interest that many global models are difficult to address because of the coarser resolution. I would suggest that the authors expand on this, which places this work in the context of previous studies and greatly advances our current understanding of reactive halogen chemistry.

(2) Figure 2 shows some striking discrepancies, which are, again, very intriguing. Section 3.3 and 3.4 are able to identify several processes that are probably less important. However, these sections fail to offer a clear explanation on what exactly drives the spatial variability of iodine-induced OH production. I have a few detailed suggestions in the next section to improve the clarity. In addition, I would also suggest that the authors provide a few more maps showing the surface seawater iodide field, the modeled marine emissions of I2 and HOI, the modeled ozone dry deposition velocities, as well as the modeled HOI and sea salt aerosols in the surface air. Perhaps these can shed insights into the spatial variability of iodine-induced OH production in this region.

Specific comments:

Line 15: Please clarify how monthly OH production (P_OH) is calculated. Does this involve nighttime signals? 

Line 33: This is a vague description. HO2 -> OH is not always a net source of OH, since a major fraction of HO2 is actually produced from OH. Please rewrite this sentence: either tease out the primary fraction of HO2 (e.g., from formaldehyde) or discuss the sum of OH and HO2. After all the interconversion between OH and HO2 is fast.

Line 37: … under low-NOx condition. Also please clarify what exactly is “low-NOx condition”. Is this referring to the condition when RO2 fate is dictated by non-NOx pathways? If this is the case, often the NOx threshold can be very low.

Line 40-43: The end of this paragraph is confusing and unclear how this is relevant for this manuscript.

Line 46: … under high NOx condition.

Line 50: What is “long-term species”? I assume what the authors meant to say is “long-lived” since ozone lifetime is longer than, say, HOx radicals? If this is the case, this is clearly not true since ozone is not always constrained in box models. For instance, there are numerous studies focusing on halogen-induced ozone destruction in the Arctic and the combined effects on HOx and they use box model with ozone unconstrained.

Line 45: “One relevant reaction is that XO (X=Cl, Br, and I) transform HO2 to OH…” This is very confusing as written since this is not a one-step process: HOX is produced first, which may undergo photolysis and produce OH but HOX can also undergo heterogeneous uptake on aerosols (a major driver of halogen cycling).

Line 54: “… but it is not very clear which process will dominate” Respectfully, I disagree. Perhaps this is not explicitly spelled out in some studies, but the final model outcome speaks for itself. For instance, Wang et al., (2021) showed that the net effect of halogen chemistry on global tropospheric HOx is that both OH and HO2 are reduced by 3-4%. This is buffered by many other processes but the primary driver is a global ~10% decrease in HOx production from ozone. This is qualitatively consistent with previous studies. 

Line 56-57: “For example, the conversion of HO2 to OH enhanced by XO would consume HO2, which in turn should decrease the conversion through HO2+NO. Previous CTM studies generally did not consider such an impact” I am confused. These two are competing processes with the same end goal, which is to convert HO2 to OH. As long as these relevant mechanisms are included in the model, the impact will be considered.

Line 58-59: I failed to follow how exactly halogen chemistry can affect the photolysis of HONO and H2O2, ozonolysis of some alkene, … Please clarify.

Line 63: “However, previous studies did not analyze the pathways…” Again I disagree with this statement. Globally, iodine chemistry alone may have largely compensating effect on OH (e.g., Sherwen et al., 2016) but the relative abundance of reactive iodine species and the impacts of iodine chemistry on the global tropospheric OH levels in the context of chlorine and bromine chemistry is shown in Wang et al. (2021): globally, the effect of halogen chemistry (including iodine) on OH, is a net reduction. The relative importance of iodine chemistry on the global scale thus can be inferred.

Line 65-66: This is very vague as written. Do the authors refer to the reactive halogens (e.g., I2/HOI), debromination from sea salt, or the very short-lived substances (VSLS)? Please clarify. But generally this is a valid point, and more recent estimates certainly benefit from more comprehensive observations, thus yielding narrower ranges compared to earlier studies. Either way, a few representative citations are warranted. I’ll list a few more recent studies for each broad topic for the author’s consideration: Iodide-driven I2/HOI emission and ozone deposition (Carpenter et al., 2021; Chance et al., 2019; Inamdar et al., 2020; Karagodin-Doyennel et al., 2021; Pound et al., 2020; Sherwen et al., 2019; Wang et al., 2021); sea salt debromination (Zhu et al., 2019); VSLS (Lennartz et al., 2015; Ordóñez et al., 2012; Wang et al., 2019; Ziska et al., 2013). 

Line 135-136: please show the original and scaled halocarbon emissions. Please note that although this is a wildly use approach (i.e., scale to chlorophyll-a), there is no robust relationship between many VSLS and chlorophyll-a (Carpenter et al., 2009; Chance et al., 2014; Liu et al., 2013)

Line 139: the iodide-driven ozone deposition is coupled with the reactive iodine emission in several recent studies (Karagodin-Doyennel et al., 2021; Pound et al., 2020).

Line 142: Sherwen et al., (2019) showed that the McDonald et al. iodide parameterization underestimates the surface seawater iodide by roughly a factor of 2 on the global scale. Chance et al. is improved but shows wider variability compared to observations.

Line 150-: The amount of ozone measurements used to demonstrate the performance of this model (Figure S1) is remarkable. But this is less relevant for this study since the majority of the stations are located inland and hence are probably not heavily impacted by the marine halogens. What is directly relevant for this study is, the modeled ozone levels in those sensitivity studies, especially in the All_High case, and how would these compare to the coastal ozone measurements when the air masses are primarily originated from the ocean.

Line 158: Almost all…? Also, what is considered as benchmark here?

Table 3: Note that the daytime average IO reported in Großmann et al. does not exceed 1.5 ppt in the Northwest Pacific (10-40N), although the measurement uncertainty is close to ~1 ppt. Koenig et al., (2020) and Karagodin-Doyennel et al., (2021) also reports IO vertical profiles in the western Pacific in the upper troposphere/lower stratosphere. What does the modeled IO in the upper troposphere look like over the Western Pacific? Could also list a few modeling studies for surface IO (the authors listed a few values from GEOS-Chem for BrO, so might as well). Also please mark on Figure 1 the approximate locations of the studies listed in Table 3 for evaluation.

Figure 1: Are those in the surface air or averaged in the marine boundary layer? Please clarify. Also the IO color scale used in Figure 1(b) and (d) are inappropriate since very large areas are saturated at 2 ppt already. 

Lines 197-198: What is this “middle area of the ocean”?

Line 236, 240: photolysis of O3: note that the photolysis of O3 is not really producing OH directly, it is the produced O1D reaction with water vapor that generates OH. Please clarify how this P_OH_O1D term is calculated.

Line 252: “Since HOX is a primary source of OH…” I do not think HOX should be considered as a primary source of OH. Essentially what it does is converting HO2 to OH:

X + O3 = XO + O2

XO + HO2 = HOX + O2

HOX + hv = X + OH

Net: O3 + HO2 = OH + 2*O2

Lines 259-261: I do not understand what the authors mean by “delta_P_OH_HO2 was generally ignored in previous CTM studies”. This term denotes the OH production from HO2 + NO/O3/…, which are some of the most essential reactions in the atmosphere. I doubt these are actually ignored in any major modern CTMs. Please list the models that actually ignores these extremely important reactions. And again, not all box models are observationally constrained.

Line 265-: this paragraph attempts to explain the striking spatial variability shown in Figure 2 but it does not really deliver a clear answer. Figure 3, as currently shown, does not help with this purpose at al: the sum of panel (a), (e), and (f), should yield panel (g). But the color scales of Panel (f) is poorly chosen (mostly saturated at ~1e+6 cm^-3 s^-1), which, I think, is key to explain the spatial discrepancy shown in Panel (g). I would expect that the largely positive delta_P_OH_HOX term is partially compensated in Bohai Sea and the Sea of Japan but completely overturned in the Yellow Sea and East China Sea, by the sum of detla_P_OH_O1D and delta_O_OH_HO2. In light of this, I would suggest the authors tweak the color scales in Panels (a), (e), and mostly importantly (f). Could remove Panels (b), (c), (d), and (h) and describe in the text that these pathways are virtually neglectable.

Line 285 & Line 307: It remains unclear in the current version of the manuscript that how debromination from sea salt is implemented in this model, which, is the dominant reactive bromine source in the troposphere (Wang et al., 2021; Zhu et al., 2019). Please compare the sea salt debromination rates derived from this work to the literature values.

Line 287: … the impact of inorganic iodine is stronger/larger/more pronounced than…

Line 314-: Sure Cl itself is a strong oxidant but why would Cl oxidation affect OH radical production? Is it thru RO2  HO2  OH channel? Please clarify.

Line 395: the point of having debromination is not to solve “excessive BrO”. Debromination is, however, the largest net source of reactive bromine in the global troposphere (Wang et al., 2021; Zhu et al., 2019). Please compare the sea salt debromination rate derived from this work to the literature values.

References

Carpenter, L. J., Jones, C. E., Dunk, R. M., Hornsby, K. E., & Woeltjen, J. (2009). Air-sea fluxes of biogenic bromine from the tropical and North Atlantic Ocean. Atmospheric Chemistry and Physics, 9(5), 1805–1816. https://doi.org/10.5194/acp-9-1805-2009

Carpenter, Lucy J., Chance, R. J., Sherwen, T., Adams, T. J., Ball, S. M., Evans, M. J., et al. (2021). Marine iodine emissions in a changing world. Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences, 477(2247), 20200824. https://doi.org/10.1098/rspa.2020.0824

Chance, R., Baker, A. R., Carpenter, L., & Jickells, T. D. (2014). The distribution of iodide at the sea surface. Environmental Science: Processes & Impacts, 16(8), 1841–1859. https://doi.org/10.1039/C4EM00139G

Chance, R. J., Tinel, L., Sherwen, T., Baker, A. R., Bell, T., Brindle, J., et al. (2019). Global sea-surface iodide observations, 1967–2018. Scientific Data, 6(1), 286. https://doi.org/10.1038/s41597-019-0288-y

Inamdar, S., Tinel, L., Chance, R., Carpenter, L. J., Sabu, P., Chacko, R., et al. (2020). Estimation of reactive inorganic iodine fluxes in the Indian and Southern Ocean marine boundary layer. Atmospheric Chemistry and Physics, 20(20), 12093–12114. https://doi.org/10.5194/acp-20-12093-2020

Karagodin-Doyennel, A., Rozanov, E., Sukhodolov, T., Egorova, T., Saiz-Lopez, A., Cuevas, C. A., et al. (2021). Iodine chemistry in the chemistry–climate model SOCOL-AERv2-I. Geoscientific Model Development, 14(10), 6623–6645. https://doi.org/10.5194/gmd-14-6623-2021

Koenig, T. K., Baidar, S., Campuzano-Jost, P., Cuevas, C. A., Dix, B., Fernandez, R. P., et al. (2020). Quantitative detection of iodine in the stratosphere. Proceedings of the National Academy of Sciences, 117(4), 1860–1866. https://doi.org/10.1073/pnas.1916828117

Lennartz, S. T., Krysztofiak, G., Marandino, C. A., Sinnhuber, B.-M., Tegtmeier, S., Ziska, F., et al. (2015). Modelling marine emissions and atmospheric distributions of halocarbons and dimethyl sulfide: the influence of prescribed water concentration vs. prescribed emissions. Atmospheric Chemistry and Physics, 15(20), 11753–11772. https://doi.org/10.5194/acp-15-11753-2015

Liu, Y., Yvon‐Lewis, S. A., Thornton, D. C. O., Butler, J. H., Bianchi, T. S., Campbell, L., et al. (2013). Spatial and temporal distributions of bromoform and dibromomethane in the Atlantic Ocean and their relationship with photosynthetic biomass. Journal of Geophysical Research: Oceans, 118(8), 3950–3965. https://doi.org/10.1002/jgrc.20299

Ordóñez, C., Lamarque, J.-F., Tilmes, S., Kinnison, D. E., Atlas, E. L., Blake, D. R., et al. (2012). Bromine and iodine chemistry in a global chemistry-climate model: description and evaluation of very short-lived oceanic sources. Atmospheric Chemistry and Physics, 12(3), 1423–1447.

Pound, R. J., Sherwen, T., Helmig, D., Carpenter, L. J., & Evans, M. J. (2020). Influences of oceanic ozone deposition on tropospheric photochemistry. Atmospheric Chemistry and Physics, 20(7), 4227–4239. https://doi.org/10.5194/acp-20-4227-2020

Sherwen, T., Schmidt, J. A., Evans, M. J., Carpenter, L. J., Großmann, K., Eastham, S. D., et al. (2016). Global impacts of tropospheric halogens (Cl, Br, I) on oxidants and composition in GEOS-Chem. Atmos. Chem. Phys., 16(18), 12239–12271. https://doi.org/10.5194/acp-16-12239-2016

Sherwen, Tomás, Chance, R. J., Tinel, L., Ellis, D., Evans, M. J., & Carpenter, L. J. (2019). A machine learning based global sea-surface iodide distribution. Earth System Science Data Discussions, 1–40. https://doi.org/10.5194/essd-2019-40

Wang, S., Kinnison, D, Montzka, S, Apel, E., Hornbrook, R., Hills, A., et al. (2019). Ocean biogeochemistry control on the marine emissions of brominated very short-lived ozone-depleting substances: a machine-learning approach. Journal of Geophysical Research. https://doi.org/DOI:10.1029/2019JD031288

Wang, X., Jacob, D. J., Downs, W., Zhai, S., Zhu, L., Shah, V., et al. (2021). Global tropospheric halogen (Cl, Br, I) chemistry and its impact on oxidants. Atmospheric Chemistry and Physics, 21(18), 13973–13996. https://doi.org/10.5194/acp-21-13973-2021

Zhu, L., Jacob, D. J., Eastham, S. D., Sulprizio, M. P., Wang, X., Sherwen, T., et al. (2019). Effect of sea salt aerosol on tropospheric bromine chemistry. Atmospheric Chemistry and Physics, 19(9), 6497–6507. https://doi.org/10.5194/acp-19-6497-2019

Ziska, F., Quack, B., Abrahamsson, K., Archer, S. D., Atlas, E., Bell, T., et al. (2013). Global sea-to-air flux climatology for bromoform, dibromomethane and methyl iodide. Atmospheric Chemistry and Physics, 13(17), 8915–8934. https://doi.org/10.5194/acp-13-8915-2013

The authors present a study of the effects of halogen chemistry on the concentrations and production rates of OH radicals in the East Asia region in July 2019 using the CMAQ model with WRF meteorology. The methodology applied is appropriate to the study, and the results will be of interest to the atmospheric science community.

The emission schemes and chemical mechanisms for halogens (Cl, Br, and I) employed in the model are generally well described, and there is some comparison of model results with observations of halogen species where these are available. However, the manuscript would benefit from some additional discussion of the uncertainties in the mechanisms with a view to highlighting which processes in the model should be targeted for improvements that will reduce model uncertainty.

Similarly, how do uncertainties in measurements (or in the availability of measurements) impact tests of model performance and the model results? Are there key species or locations which should be targeted for future observations that would help to validate models?

Can the authors comment on any expected seasonal effects? How representative are the conditions in July 2019 of other summer months and of other years?

Are there any impacts of halogens on OH loss processes as well as processes involved in the production of OH.

Is it possible to comment on the wider significance of the results? What do the results imply about our understanding of methane lifetimes for example?

More specific comments are given below:

Line 33: HO2 should not really be considered as a source of OH owing to its production via OH reactions.

Line 37: Define NOx and the concentration range used to define the low NOx regime. Note that the studies mentioned in which modelled OH concentrations are underestimated typically have both low NOx and high biogenic VOCs. It’s not clear that these studies are relevant to marine regions.

Lines 56-69: The chemistry HO2 + XO and HO2 + NO was included in previous studies and the impacts were thus considered as part of the overall effects.

Line 100: Is a 10 day period appropriate to spin-up any long-lived species in the model?

Line 113: It would be preferable to reference a peer-reviewed result rather than a figure in a preprint version.

Lines 237, 252, 265: Although HOX photolysis is potentially a primary source of OH, it may also be considered a secondary source if significant HOX is produced via HO2 + X since HO2 is primarily produced via OH chemistry. It would be helpful if the authors could comment on the relative importance of chemical production of HOX and direct emission of HOX. What fraction of OH is produced via photolysis of HOX?

Line 242: Please clarify the meaning of FORM + O. Is there any impact of halogen chemistry on formaldehyde?

Line 259: Changes in OH and HO2 have been considered as part of the net result in previous CTM studies, it isn’t really correct to say that these changes were ignored.

Line 260: The box model studies do provide accurate model results, but they are typically directed at different processes and different timescales. It is not really correct to say that a model is not accurate.

Line 368: Are the changes to the photolysis of HOX and HO2 + Y linked via HO2 + X?

Line 302 onwards: Please show the charges on ionic species.

Line 411: Is it possible to perform any sensitivity analysis to HO2 uptake?

There are also a number of language improvements that should be corrected. Some of these are listed below:

Title: This would read better as ‘on OH radicals’ or ‘on the OH radical’.

Line 9: ‘Hydroxyl (OH) radical... OH’ to ‘hydroxyl (OH) radicals... The OH’.

Line 11: ‘OH level’ to ‘OH levels’.

Line 16: ‘response of OH’ to ‘response of the OH’.

Line 23: ‘while increase’ to ‘while increasing’.

Line 25: ‘the southern’ to ‘southern’.

Line 26: ‘the coastal’ to ‘coastal’.

Line 31: ‘Hydroxyl’ to ‘The hydroxyl’.

Line 32: Define VOCs, ‘produce’ to ‘producing’.

Line 34: ‘O1D’ to ‘O(1D)’, ‘At urban’ to ‘In urban’.

Line 44: ‘impact’ to ‘impacts’, ‘The marine-emitted halogen’ to ‘Marine-emitted halogens’.

Line 50: ‘long-term’ to ‘long-lived’.

Line 53: Define HOx.

Line 54: ‘CTMs studies’ to ‘CTM studies’.

Line 81: It would help to define CMAQ and WRF in full.

Line 89: ‘mechanisms’ to ‘mechanism’.

Line 90: ‘Rosenbrock’ to ‘The Rosenbrock’.

Line 105: ‘inline’ to ‘online’?

Line 134: ‘global annual’ to ‘by global annual’.

Line 161: ‘though’ to ‘although’, ‘in-situ’ to ‘in situ’, ‘area is’ to ‘area are’.

Line 164: ‘BrO and IO very’ to ‘BrO and IO are very’.

Line 165: ‘IO concentration’ to ‘the IO concentration’.

Line 166: ‘Western Pacific’ to ‘the Western Pacific’.

Line 171: ‘largest’ to ‘the largest’, ‘while smallest’ to ‘and the smallest’.

Line 184: ‘average’ to ‘the average’.

Line 189: ‘related to O3’ to ‘related to the O3’.

Lines 203, 204, & 207: ‘slight’ to ‘a slight’.

Line 205: ‘decrease’ to ‘a decrease’.

Line 206: ‘increase’ to ‘an increase’.

Line 208: ‘while increase’ to ‘while increasing’.

Line 225: ‘literature’ to ‘the literature’.

Line 235: ‘CB6’ to ‘the CB6’.

Line 236: ‘source’ to ‘sources’.

Line 251: ‘whole’ to ‘the whole’.

Line 252: ‘daytime’ to ‘the daytime’.

Line 311: ‘daytime maximum’ to ‘the daytime maximum’.

Review of Fan and Li, the impact of marine-emitted halogens on OH radical in East Asia during summer

General comments:

This manuscript analyzes regional model simulation results regarding the impact of tropospheric halogen chemistry (Cl, Br, and I) on the production rates and concentrations of OH over East Asia and Western Pacific during summer. Three major pathways, i.e., O3 photolysis rate, HOX photolysis, and HO2 +Y were identified and the changes were quantified per emission types (SSA, inorganic iodine, and halocarbons) and their associated processes (chemistry, radiation, and deposition). Although not proposing new processes, the authors describe the overall impact in an integrated manner and then their attribution to individual processes. The Western Pacific low-latitude region is with fairly high sea surface temperature during summer and thus potentially high impact of iodine chemistry (due to high [I-] in the surface seawater) is expected. The findings are mostly reasonable and the logical flow of the manuscript appears sound.

Nonetheless, there are several points needing clarification. First, additional fundamental information of this study is required for justification. For example, a table of chemical reactions relevant to tropospheric halogen chemistry taken into account in the simulations is needed at least in the supplement. Maps of the assumed SSA, HOI and halocarbon fluxes will also help understanding. Typical air transport patterns during July 2019 should be described.

Second, clarification is necessary for some processes. For example, I am afraid that the impact of sea spray aerosols on J(O1D) (Fig. S5, Fig. 6a) is a bit too large. The aerosol optical depth and assumed single scattering albedo in the model need evaluation. Also I do not understand why the InorgI_chem can result in "negative" values over the Philippine Sea, although some explanation is given in lines 379-382.

Third, readability of the manuscript should be improved, as very similar figures (from Figure 2 to 7) appear from one to another. Overall, the manuscript is acceptable after major revisions, responding to the points raised above and to specific comments listed below.

Specific comments:

1. Line 20. Show what are the "three" major pathways in Abstract.
2. Line 22. The increased ozone deposition due to enhanced I- ion levels in the surface seawater would be rather "chemical" than physical?
3. Line 41. What are the "artificial" and “mechanistic" pathways?
4. Line 50. longer-lived?
5. Line 51 resulting in
6. Line 52. It is not box models that are to be accused. Maybe "constraining O3 levels in the model" causes the difficulty that the authors described.
7. Lines 57 and 260. I believe previous CTM studies implicitly take the impact from HO2 into account, unless they constrain HO2 levels to observations.
8. Line 77. What are "its extreme uncertainties"?
9. Line 90. List tropospheric halogen chemistry reactions from the CB6r3m mechanism in a supplementary table. Are they identical to those involved in GEOS-Chem or other recent studies (e.g., Sherwen et al., 2016, Stone et al., 2018, Wang et al., 2021), which are cited in line 202 and compared?
10. Lines 99-100. The typical air mass transportation pattern for this region during July 2019 should be described.
11. Line 140. I believe these two processes are coupled in Sekiya et al. (2020).
12. Table 1. One idea would be to add one column in the right to indicate in which Figure the results of the cases are studied.
13. Line 242. Explain what FORM+O is.
14. Section 3.1. The model performance is only checked with O3 monitoring over the continent. It would be useful to compare the simulation results also with O3 observations at Yonaguni available from http://ebas.nilu.no.
15. Figures 3-7. In their figure captions, some more explanation should be included, to clarify cross linkage of the figures. For example, Figure 3 caption could include that they are breakdown of Figure 2a down to processes. Figure 4 would be the breakdown of Figure 3g, a, e, and f. etc.
16. Lines 305 and 312 and Fig. S5. I am afraid that the impact of sea spray aerosols on J(O1D) and then P(OH) (close to 30% for daytime maximum) is a bit too large. The aerosol optical depth and assumed single scattering albedo in the model need evaluation.
17. Line 370. What is the process that "iodine" diminishes the photolysis of O3?
18. Lines 379-382. Why can the InorgI_chem result in "negative" values over the Philippine Sea? I believe that the photolysis of HOI always tend to "increase" OH and thus should have positive values. Coupling with air mass transport may produce this type of feature? Clarification is needed.
19. Line 392. Rely much on the current halogen chemistry
20. Line 395. The authors should explain how the debromination is included.
21. Line 400. Indeed, the uptake of HOI onto the sea salt particles could significantly alter the fate of HO2 and then OH (i.e. HO2 + Y term) and most of the results in this study. This was previously studied by Kanaya et al. GRL 2002 in the Asian domain.
22. Line 409. There are a series of laboratory studies examining uptake of HO2 onto sea-spray aerosol particles (Taketani et al., 2008, 2009).
23. Line 413. What is the "mass interaction"?
24. Throughout the manuscript: Was all P(OH) studied always at the lowest model layer? How will the results change, when studying the whole atmospheric boundary layer?

References

Kanaya, Y., Yokouchi, Y., Matsumoto, J., Nakamura, K., Kato, S., Tanimoto, H., Toyota, K., and Akimoto, H.: Implications of iodine chemistry for daytime HO2 levels at Rishiri Island, Geophys. Res. Lett., 29( 8), doi:10.1029/2001GL014061, 2002.

Sekiya, T., Kanaya, Y., Sudo, K., Taketani, F., Iwamoto, Y., Aita, M. N., Yamamoto, A., and Kawamoto, K.: Global Bromine- and Iodine-Mediated Tropospheric Ozone Loss Estimated Using the CHASER Chemical Transport Model, Sola, 16, 220-227, 10.2151/sola.2020-037, 2020.

Sherwen, T., Schmidt, J. A., Evans, M. J., Carpenter, L. J., Grossmann, K., Eastham, S. D., Jacob, D. J., Dix, B., Koenig, T. K., Sinreich, R., Ortega, I., Volkamer, R., Saiz-Lopez, A., Prados-Roman, C., Mahajan, A. S., and Ordonez, C.: Global impacts of tropospheric halogens (Cl, Br, I) on oxidants and composition in GEOS-Chem, Atmospheric Chemistry and Physics, 16, 12239-12271, 10.5194/acp-16-12239-2016, 2016.

Stone, D., Sherwen, T., Evans, M. J., Vaughan, S., Ingham, T., Whalley, L. K., Edwards, P. M., Read, K. A., Lee, J. D., Moller, S. J., Carpenter, L. J., Lewis, A. C., and Heard, D. E.: Impacts of bromine and iodine chemistry on tropospheric OH and HO2: comparing observations with box and global model perspectives, Atmospheric Chemistry and Physics, 18, 3541-3561, 10.5194/acp-18-3541-2018, 2018.

Taketani, F., Kanaya, Y., and Akimoto, H.: Heterogeneous Loss of HO2 by KCl, Synthetic sea salt and natural seawater aerosol particles”, Atmos. Environ. 43, 1660–1665, 2009.

Taketani, F., Kanaya, Y., and Akimoto, H.: Kinetics of heterogeneous reaction of HO2 radical at ambient concentration levels with (NH4)2SO4 and NaCl aerosol particles, J. Phys. Chem. A, 112 (11), 2370 -2377, doi:10.1021/jp0769936, 2008.

Wang, X., Jacob, D. J., Downs, W., Zhai, S., Zhu, L., Shah, V., Holmes, C. D., Sherwen, T., Alexander, B., Evans, M. J., Eastham, S. D., Neuman, J. A., Veres, P., Koenig, T. K., Volkamer, R., Huey, L. G., Bannan, T. J., Percival, C. J., Lee, B. H., and Thornton, J. A.: Global tropospheric halogen (Cl, Br, I) chemistry and its impact on oxidants, Atmos. Chem. Phys. Discuss., 2021, 1-34, 10.5194/acp-2021-441, 2021.