The spatial and temporal distribution characteristics and potential mechanisms of local accumulation and transported O3 pollution in Nanjing were investigated using observation data and the WRF-Chem air quality model.The results show that,for the horizontal distribution characteristics the distribution of high values of exogenous O3 pollution is larger and more dispersed than that of local accumulation O3 pollution during the two O3 pollution episodes.However,during the most severe daytime hours of both O3 pollution processes,the distribution of high value areas of local accumulation O3 pollution was large and concentrated,with most of the O3 concentrations exceeding 160 μg/m3,while the distribution of high value areas of external transport O3 pollution was smaller and more dispersed,with concentrations in the range of 120-160 μg/m3 being significantly larger than that of local accumulation O3 pollution.For the vertical distribution characteristics,in the early stage of the pollution episode,local accumulation O3 pollution has high O3 concentrations below 2 km height,with concentrations up to 250 µg/m3,while the high value areas of external transport O3 pollution are mainly concentrated at 2-2.5 km height,and the highest concentration does not exceed 200 μg/m3;in the middle of the pollution process,the high value area of local accumulation O3 pollution concentration is concentrated under 2 km height,and the maximum O3 concentration occurs at about 1 km height;while the high value area of external transport O3 pollution concentration can reach up to about 2.6 km height,and the maximum concentration is concentrated under 1.5 km height,and all exceed 250 μg/m3;in the late stage of the pollution process,the high value area of local accumulation O3 pollution concentration gradually dissipates,but the external transport of O3 pollution still appears in a wide areas within 0.5-3 km altitude.In terms of generation mechanism,high near-surface O3 concentrations are mainly depended on vertical mixing in local accumulation O3 pollution,with a total contribution of 451.7 μg/m3 over 96 h,accounting for 66.2％of the total process volume,while in external transport O3 pollution it is mainly depended on horizontal advection and vertical mixing,with a total contribution of 1 050.2 μg/m3 and 799.1 μg/m3 over 72 h,respectively,with a total contribution of 61.5％to the total process volume.At an altitude of 500 m,the high concentration of O3 in the early stage of local accumulation O3 pollution mainly depends on vertical advection and chemical processes,while it only depends on chemical processes in the middle stage,and mainly depends on horizontal advection and chemical processes in the later stage.However,the medium-high O3 concentration in the early,middle,and late stages of external transport O3 pollution mainly depends on horizontal advection and chemical processes.At an altitude of 1 000 m,the positive contribution of O3 concentration in both episodes is mainly controlled by the chemical process.
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