Impacts of Sea–Land Breeze on Coastal Ozone Pollution in the Western Pearl River Delta

Ozone (O3) pollution in coastal urban regions is strongly influenced by sea–land breeze (SLB) circulation. However, linking its long-term surface effects with vertically resolved observations remains an important research need in the western Pearl River Delta (PRD). This study combines meteorological and surface O3 observations from 2014–2023 with lidar measurements and ERA5 reanalysis for a case study on 25–26 July 2023. We examine three complementary aspects: SLB frequency and temporal characteristics, surface O3 concentrations and peak timing, and vertical O3 redistribution. SLB days accounted for 13.1–19.4% of days annually, with mean sea-breeze and land-breeze durations of 8.79 and 6.63 h, respectively. Surface O3 concentrations were generally higher on SLB days in all seasons, with larger increases at inland sites, particularly in spring and autumn. Changes in peak timing varied by season and site. The case study suggests a vertical recirculation pathway in which nocturnal return flow transports O3-rich air inland aloft, followed by downward redistribution during daytime boundary-layer development. By integrating a decade of surface observations with vertically resolved evidence from an individual event, this study connects regional statistical patterns with a physical interpretation of coupled horizontal recirculation and vertical redistribution. These findings highlight the importance of considering breeze timing, elevated O3 layers, and boundary-layer mixing in coastal air-quality forecasting, chemical transport modeling, and regional pollution mitigation.