Impact of sea-land breezes on a severe ozone pollution episode over the western Pearl River Estuary

In this paper, the role of sea-land breezes in a severe ozone (O₃) pollution episode was studied. The analysis revealed that under the control of the periphery of Typhoon Nanmadol, Zhongshan City on the western coast of the Pearl River Estuary experienced severe O₃ pollution on 16 September 2022, with the daily maximum 8-h average O₃ concentration reaching 274 μg m⁻³. From morning to noon, rapid O₃ accumulation under intense solar radiation and weak northerly winds produced the first peak. During early evening hours, sea breezes occurred and initiated convergence with northerly synoptic winds over central Zhongshan. The recirculation factor of the V-component of ground winds decreased from 1.0 to 0.52, accompanied by an 86 % increase in net O₃ influx compared to the midday level. Concurrently, ventilation index below 1000 m altitude dropped by 58 %, driving secondary O₃ peaks at both ground level and boundary layer heights. After the occurrence of sea breezes, dominant updrafts generated negative vertical O₃ flux at ground. However, the vertical flux removal accounted for merely 4–13 % of horizontal O₃ influx, leading to the accumulation of ground O₃. Concurrent upward transport caused significant O₃ increases at 300 m and 400 m altitude by 49 μg m⁻³ and 45 μg m⁻³ respectively, markedly exceeding the concentration variations at 500–800 m layers.