Abstract: Nighttime O₃ pollution was observed in four coastal cities in central and northern Fujian from 20:00 on May 8 to 03:00 on May 9,2024,with the ρ(O_3-8h)(8 h running average O₃ concentration)exceeded the secondary air quality standard.Based on ground observations from environmental monitoring sites and meteorological stations,O₃ lidar profiles,meteorological sounding data and ERA5 reanalysis data,we investigated the dynamic mechanisms and sources of enhanced ozone during the episode through statistical analysis and meteorological diagnostics.The nighttime O₃ pollution mainly occurred in the coastal areas of central and northern Fujian.In Fuzhou City,ρ(O₃)remained above 130 μg·m^-3 from 18:00 on May 8 to 06:00 on May 9(non-photochemical period),and two ozone-rich air masses with different characteristics were observed.The first air mass was characterized by increased concentrations of ozone and other pollutants.In contrast,the second air mass featured elevated ρ(O₃)levels but decreases in the concentrations of other pollutants and relative humidity.The first peak value of ρ(O₃)at 194 μg·m^-3 appeared at 20:00 on the 8th,and the ρ(CO),ρ(PM_2.5)and ρ(PM₁₀)in the air mass showed positive anomalies compared with the average concentrations during May 6-12.High values of ρ(O₃)could be found above 2km according to lidar data.The second peak occurred at 01:00 on the 9th.After a slight decline,ρ(O₃)suddenly rose to 191 μg·m^-3 at the 5th hour.while the ρ(CO),ρ(PM_2.5)and ρ(PM₁₀)showed negative anomalies.The concentrations of pollutants and relative humidity showed a significant decrease,and the height of the high-O₃ layer reduced to less than 1 km.