How sea-land breezes affect ozone in coastal cities: spatial and temporal patterns

The influence of sea-land breeze (SLB) circulation on ozone (O₃) pollution has been widely reported in coastal regions worldwide. However, its urban-scale regional and seasonal variabilities, as well as the underlying mechanisms, remain insufficiently understood. Based on 2 years of multi-source observational data and multiple statistical methods, this study systematically investigated the characteristics and mechanisms of SLB-induced O₃ pollution in Tianjin. The results showed clear seasonal dependence, with the SLB effect on O₃ being most pronounced in autumn, followed by summer, and negligible in spring and winter. The most affected period was 12:00-18:00 in summer and extended until 23:00 in autumn. Spatially, SLB exacerbated O₃ pollution in urban areas with relatively high background concentrations, while increasing pollution frequency in coastal and suburban areas with lower O₃ levels. SLB circulation significantly enhanced the influence of meteorological conditions on O₃ formation, particularly in autumn. In summer, O₃ enhancement under SLB conditions resulted from the combined effects of intensified photochemical production and SLB-driven pollutant transport and recirculation, with the region generally exhibiting a NOx-limited regime. In contrast, the direct contribution of transport weakened in autumn, while temperature-driven photochemical production became dominant, and the region generally shifted to a transitional precursor-sensitivity regime. In addition, the longer duration of autumn SLB events favored the persistence of elevated O₃ levels. These results demonstrate that the SLB-O₃ relationship is strongly season-dependent and spatially heterogeneous, highlighting the need for season-specific and region-specific control strategies for coastal O₃ pollution mitigation.