the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 14 Aug 2025
Sea Breeze-Driven Daytime Vertical Distributions of Air Pollutants and Photochemical Implications in an Island Environment
Abstract. Atmospheric pollutants in island and coastal environments are profoundly modulated by sea breezes (SB) and typhoons. Understanding the characteristics of pollutants and their photochemical indicators under different airflow regimes is crucial for effective pollution control. Utilizing Multi-Axis Differential Optical Absorption Spectroscopy (MAX-DOAS) and a sea-land breeze objective identification algorithm, we reveal distinct spatiotemporal patterns in NO2, HCHO, CHOCHO, and associated photochemistry under varying airflow patterns in a rural coastal area of Hainan. Non-sea breeze days (NSBDs) showed higher pollutant levels with broader vertical distribution range under conducive meteorological conditions. Conversely, on sea breeze days (SBDs), SB limits pollutant dispersion and its cooling effect suppresses ozone formation. Furthermore, SB also enhances transport of NO2 and biogenic volatile organic compounds (BVOC) below 300 m, influencing ozone formation sensitivity (OFS) throughout SB phases. Typhoons effectively scavenge pollutants via strong winds and precipitation but also facilitate mid-to-high altitude BVOC transport and vertical dispersion of surface pollutants. Photochemical indicator analysis (HCHO/NO2 [FNR] and CHOCHO/NO2 [GNR]) indicates that the VOC-limited regimes persist even at high altitudes during typhoons. Elevated FNR and GNR thresholds suggest that existing OFS classifications are inadequate for low-NO2 tropical coastal rural areas, underscoring the need for region-specific assessments. Given the BVOC-dominated environment and additional inputs from SB and typhoons, GNR proves more reliable than FNR for OFS determination. This study emphasizes the necessity of integrating local meteorology and environment conditions in O3 control strategies, providing a scientific basis for pollution mitigation in tropical coastal regions prone to SB and typhoons.
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Status: final response (author comments only)
- CC1: 'Comment on egusphere-2025-2588', Hai GUO, 23 Aug 2025
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RC1: 'Comment on egusphere-2025-2588', Anonymous Referee #1, 17 Oct 2025
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 hereCitation: https://doi.org/10.5194/egusphere-2025-2588-RC1 -
RC2: 'Comment on egusphere-2025-2588', Anonymous Referee #2, 31 Oct 2025
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.
Citation: https://doi.org/10.5194/egusphere-2025-2588-RC2
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General Comments
This study employs Multi-Axis Differential Optical Absorption Spectroscopy (MAX-DOAS) observations to characterize the vertical distributions of aerosols, NO₂, HCHO, and CHOCHO under different air current conditions (NSBDs, SBDs, and TDs) in a rural coastal area of Hainan Island, China. The results show that NSBDs are associated with higher pollutant levels and broader vertical distribution ranges. During SBDs, sea breezes limited pollutant dispersion, while their cooling effect suppressed ozone formation. Under TDs, typhoons scavenge pollutants but facilitate mid-to-high altitude BVOC transport and enhance the vertical dispersion of surface pollutants. These findings provide valuable insights into coastal atmospheric processes and pollutant behaviors. The manuscript is generally well-structured and detailed. However, several issues need to be addressed before it can be considered for publication.
Specific Comments
1. Line 30: References should be consistently ordered, either alphabetically by author or chronologically.
2. Line 85: Please clarify the time resolution of the MAX-DOAS observations.
3. Line 130: The sentences beginning with “Accurate identification of … of SB and LB” are redundant and should be revised.
4. Lines 140–150: I suggest moving Figure S4 from the Supplementary Material to the main text and citing it appropriately.
5. Line 165: Please provide the formula used to calculate relative humidity. In addition, the ERA5 data may be too coarse to capture local weather conditions around the in-situ site. For example, uncertainties in planetary boundary layer height are significant. Verification against nearby weather station data is necessary to ensure the robustness of the meteorological analysis.
6. Line 175: Please provide the actual values instead of vague expressions such as “values recorded in Chinese megacity centers.”
7. Figure S5: This figure is not cited in the text.
8. Line 225: The statement of “Notably, most of the SBDs occur consecutively.” can be explained by meso-scale circulations, which usually occur under stable atmospheric conditions dominated by persistent high pressure.
9. Line 245: The claim that “southwesterly winds persist under the typhoon’s peripheral airflow” needs clarification. Since both the typhoon’s location and the sampling site (n this and other studies) change over time, is the peripheral airflow always associated with southwesterly winds?
10. Line 260: Please add vertical wind components to the correlation figures to better support the conclusions.
11. Section 3.3: It is unclear why ozone concentrations are clustered instead of directly
analyzing their evolution under NSBDs, SBDs, and TDs. Please clarify.
12. Lines 315–320: Is it reasonable to use the minimum early-morning ozone concentration as background ozone? Have the effects of NOx titration been excluded?
13. The dataset includes only seven typhoon days, which may be insufficient to represent general typhoon-related airflows. This appears more like a case study.
14. Please clarify whether the results apply only to Hainan Island or can be generalized to other coastal/island environments. Which findings can be extended to similar regions?
Technical Corrections
1. Ensure consistent use of abbreviations: introduce the full term at first mention, followed by the abbreviation, and use the abbreviation thereafter.
2. Line 60: Add a comma before “Nevertheless.”
3. Line 135: Use “Fig. 1a and b” instead of “Fig. 1a,b.”
4. Correct typos, grammatical errors, and syntactic mistakes throughout the manuscript.
5. The English should be polished further, ideally by a native speaker.