Review of "Sea Breeze-Driven Daytime vertical Distributions of Air Pollutants and Photochemical Implications in an Island Environment" submitted by Bohai Li et al.

This manuscript presents an analysis of air pollutant daytime measurements of NO₂, HCHO and CHOCHO obtained using the MAX-DOAS technique in a rural coastal site in Hainan Island, China from June to August 2024. The analysis focuses on the dependence of air pollutant levels to the dominant airflow patterns at the measurement site, and distinguishes between days with and without sea-breeze, as well as days with tropical cyclones. The main results are: (i) days without sea breeze show higher pollutant concentrations compared to sea breeze days; (ii) typhoon days show vertical transport of pollutants to higher altitude; (iii) high values of glyoxal-to-NO₂ and HCHO-to-NO₂ ratios are indicative of a VOC-limited regime, and the glyoxal-to-NO₂ ratio is reliable for studying the ozone formation sensitivity. The article is generally well-written and the performed analysis seems to be robust, although I could not check the details because the MAX-DOAS dataset is not openly accessible. The findings of the paper concern pollutant patterns in coastal environments, but I am not convinced that they can be generalized to other environments. The article could be considered for publication once the following points are adequately addressed in a revised version.



Specific comments



- l.158-167: Please mention that the ERA5-L fields are provided at 0.1 degree. Despite the finer resolution of ERA5-L compared to the parent ERA5 dataset, it is still relatively coarse. An evaluation of the ERA5 or ERA5-L reanalysis against local meteorological measurements at the site should be included. It would be useful to evaluate ERA5-L under cyclonic conditions.



- l.208-210: The existence of additional sources is suggested here to explain the delayed HCHO enhancement (at 10am). Can you give more details? Note that the delayed HCHO peak formation could be due to VOCs that due to their longer lifetimes produce HCHO with delay. Please clarify.



- l.215-218: The residence time of CHOCHO and HCHO are actually quite similar, globally 2.9 h for CHOCHO and 5 h for HCHO. The profile shapes of Fig.3 for both species are also very similar, which contradicts the view expressed in the paper that CHOCHO remains confined within 500 m of the surface because of its shorter lifetime. In addition, it should be noted that methane oxidation contributes to HCHO at the higher levels (especially above 2km), where the CHOCHO levels are close to zero. Can you further elaborate why the lower photolysis rates would contribute to the late afternoon rebound for both species? I wonder why the reduced mixing, as illustrated in Fig.3, is not enough to explain the rebound. After all, reduced solar radiation simultaneously increases the lifetime and depletes the photochemical production of secondary species such as HCHO and CHOCHO. I recommend to include a figure similar to Fig.3 but showing the diurnal variation of the columns.  Vertical mixing variations is expected to affect less the columns than the surface concentrations.



- The surface VMRs for HCHO and CHOCHO are different in Section 4 and in Section 3.1. Which one is correct?



- The manuscript attributes the high glyoxal-to-NO₂ and HCHO-to-NO₂ ratios to BVOC sources and suggests that they are dominant compared to anthropogenic sources. Could you include bottom-up emission maps of BVOC, anthropogenic NOx and VOCs over the island?



- The manuscript should include a comparison with air pollutant levels from other coastal locations or islands based on literature studies. Besides the typhoon days, what is special about Hainan Island? Can we generalize the findings to other locations?  





Technical comments



- l.40: read 'due to the fact that it originates...'

 

- l.47: remove 'regimes' (repetition)



- l.52: 'Coastal atmospheric environments are significantly influenced by sea-land breeze circulation and typhoons': Typhoons only affect a specific region of the globe. Rephrase.



- l.63: 'in island', something is missing here



- l.64: 'located far from mainland China', be more specific



- l.64-72: Add information about the island (area, population, large cities, powerplants, fraction of vegetation)



- l.66: 'Given the superior air quality', sounds weird. Replace 'superior' by 'good'



- l.72: 'are still unclear'. Replace by 'are not investigated yet'



- l.73: read 'measurements'



- l.83: Beibu Gulf is not shown on Fig.1



- l:83: 'Wuzhi mountain (18.9°N, 109.7°E)'. What are these coordinates?



- l.91: add an 'r' to 'dime'



- l.97: remove 's' from 'Supplements', here and elsewhere in the paper



- Fig.1a: The color of highways is orange, not yellow   



- l.146-156: Fig.S4 is central to the analysis and the filters mentioned are not explained in the main manuscript. Either include a new section on ORA and move Fig.S4 to the main manuscript, or keep it as is and include a brief description on the filters in the main manuscript. Could you also provide the percentage of the days identified as sea breeze days or refer to the relevant section?     



- l.170: 'series daily', add 'of' between the two



- l.179: 'This likely stems from biogenic emissions'. Could you provide biogenic emission maps?



- l.181: 'refined', can you be more specific?



- l.206: Replace 'The early peaks' by 'The high HCHO and CHOCHO morning levels'



- l.207: 'in the previous night', replace by 'during daytime'



- l.209: remove 'beyond this mechanism'



- l.212: replace 'A' by 'a'



- Section 3.3. Move Table S5 to the main manuscript to quick reference



- l. 326: Yang et al. 2024. Replace by a textbook reference here



- l.326: add space '1and'



- l.379: 'to the local area'. Do you mean the measurement site?



- l.456: '...several megacities', Please provide reference here

Li et al. combine MAX-DOAS observations with an objective sea–land breeze identification algorithm to characterize pollutants, e.g. NO2, HCHO, CHOCHO, spatiotemporal distributions and photochemical indicators for an island environment under distinct airflow regimes, including sea breezes and typhoons. The results prove the critical role of local meteorology in modulating pollution levels, vertical profiles, and further photochemical indicators (FNR and GNR), which are influencing the ozone formation sensitivity across different altitudes. These findings provide valuable insights into the interplay between meteorological dynamics and atmospheric chemistry in tropical coastal environments. Overall, the manuscript is logically organized, clearly illustrated, and well-written. I recommend its acceptance after minor revisions addressing the following points.

Specific comments:

P1, L15: Hainan→Hainan, China

P1, L22: “existing OFS classifications” is unclear. Does it mean the classification methodology or the thresholds?

P1, Abstract: it could briefly clarify the observational period or duration (e.g., season or year), which would help readers contextualize the results temporally.

P2, L37-38: As described, the MAX-DOAS method has been successfully applied for atmospheric compositions monitoring. How about the performances especially for NO2, HCHO and CHOCHO?

P2, L44, “elevated RGF at higher altitudes” means even more glyoxal and less formaldehyde, what is the relationship between this ratio variation and anthropogenic VOCs?

P3, L57: distribution? Or formation?

P3, L62: Please add a full stop between “layers” and “Nevertheless”

P4, L83: altitude …. above sea level

P4, L94-95: What is RMS? And why did higher error thresholds set for HCHO and CHOCHO?

P5, L110: Regarding the surface concentration of the a priori profile, is it reasonable to set HCHO even higher than as twice as NO2 and twenty as HCHO? How the surface concentration of the a priori profile influencing the profile retrieval?

P5, L115: The unit for ozone should be corrected.

Fig 1: the label for typhoon track should be added in Fig. 1a.

P6, Line127-132: There are two sentences with a certain degree of repeatability.

P7, Line 140: Please supplement a wind rose plot for the campaign, better with a separate daytime and nighttime one.

P7, Sect. 2.2 and 2.3: Regarding the identification of SBL, there are two questions: 1) how to evaluate the identification performance? Can it be validated? 2) how about the representativeness of the meteorological data used in the identification? Would it cause some uncertainties?  

P8, Line 170, daily averaged

P8, Line 173-175, Even in Beijing and Shanghai, AOD should also be lower in summer, how to get the conclusion of “typically exceeds 0.5”?

P9, Line 186: how to quantitatively evaluate “a clear decoupling between surface and column measurements for NO2, HCHO, and CHOCHO, which was more pronounced in aerosols”?

P10, Line 199-202: Similarly, could the authors provide any quantitatively evidence between humidity and extinction to certificate the aerosol hygroscopic growth?

P14-16: The discussion are too informative. Better to give a brief summary to characterize the difference of NO2 and HCHO/HCHO between NSBDs and SBDs? And how these differences regulated by the ACPs.

Fig. S11: net ozone production (∆O3)→net ozone increment

P21, Line 390-401: Can be draw a conclusion that which indicator, i.e. GNR and FNR, is better for diagnose the ozone formation sensitivity?

Fig. 10: It can be seen that the results of ozone formation sensitivity are quite different obtained by using different indicators. So which one is more reliable? Or any preference for using under different ACPs? 

Also the OFS regimes shifted across different layers, can it be driven by some key meteorological parameters in view of the ACPs?

A general comment that it is too informatively described in the manuscript. I recommend the authors can simplify and shorten some parts to some degree.