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, China during the summer of 2024. 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 classification thresholds 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.