Anomalous Ozone Pollution in Xiamen During Spring 2025

Ozone (O3) pollution is highly sensitive to meteorological variability and regional transport, particularly in coastal southeastern China. During April–May 2025, Xiamen experienced an atypical, persistent springtime O3 episode substantially exceeding the 2014–2024 baseline. Using surface observations and ERA5 reanalysis data, this study investigates the meteorological drivers and formation mechanisms. At Hongwen station, the MDA8 O3 > 160 μg m−3 exceedance frequency reached 11.5% (historical average: 0.1%). This anomaly was closely linked to an anomalous Western Pacific Subtropical High (WPSH) configuration, characterized by northward displacement and accompanying westward extension. Compared to historical high-pollution conditions, surface temperature and downward solar radiation increased by 2.32 °C and 51 W m−2, while wind speed and planetary boundary layer height decreased by 15.3% and 24.2%, favoring O3 production and precursor accumulation. Two distinct pollution periods were identified. Period 1 (29 April–1 May) featured local photochemical enhancement under stagnant conditions; regional mean NO2 increased by 31 μg m−3 before the peak, indicating substantial precursor accumulation. Simultaneously, the mean nighttime O3 concentration at the Huli site during Period 1 was 50.5 μg m−3 (43% lower than that at Hongwen) due to enhanced NO titration from port emissions. Period 2 (12–14 May) involved regional transport, where persistent 850-hPa southwesterly flow facilitated pollutant transport along the coastal corridor, increasing O3 and PM2.5 by 40 μg m−3 and 38 μg m−3. Thus, extreme springtime O3 over southeastern coastal China resulted from anomalous large-scale circulation, regional transport, and local photochemical processes.