According to observation data from the Ministry of Ecology and Environment,the Beijing-Tianjin-Hebei region (BTH) has been one of the main areas of ozone (O₃) pollution in China since the implementation of the “Action Plan for Air Pollution Prevention and Control” in 2013.This study analyzed the large-scale circulation characteristics corresponding to O₃ heavy pollution (O₃ maximum daily 8-h average (MDA8)>160 μg·m^-3) in the Beijing area from April to September during 2014—2020 based on the T-PCA (Time-dependent Principal Component Analysis) method.Combined with the HYSPLIT backward trajectory model,it interpreted the changes in the regional transport characteristics and potential source areas of O₃ in the Beijing area during two periods (2014—2017 and 2018—2020) under different large-scale circulation characteristics.The results show that during the period of heavy O₃ pollution in Beijing,the main large-scale circulation patterns are uniform pressure field type and low-pressure control type.
The main potential source areas for O₃ heavy pollution in Beijing from 2014 to 2017 were some parts of Beijing-Tianjin-Hebei,Shandong,Shanxi,and Nei Mongol.The days with heavy O₃ pollution in Beijing from 2014 to 2017 were greatly affected by short-distance tracks.Under the equal pressure field behind the high pressure (types 1 and 4),the potential source areas of heavy O₃ pollution in Beijing are mainly located in the southern part of Beijing-Tianjin-Hebei and parts of Shanxi,Shandong,and Nei Mongol;when the Beijing-Tianjin-Hebei region is controlled by low pressure (types 2 and 3),the potential source areas are mainly located in the southern part of Beijing-Tianjin-Hebei and parts of Shandong.Compared with types 2 and 3,which is more strongly influenced by the summer monsoon,shows that parts of Nei Mongol can contribute over 160 μg·m^-3 to Beijing's ozone levels.When Beijing is in an equal pressure field controlled by low pressure (type 5),the potential source area is smaller,mainly in the southern part of Beijing-Tianjin-Hebei.
From 2018 to 2020,the main potential source areas for O₃ heavy pollution in Beijing were some parts of Beijing-Tianjin-Hebei,Shandong,and Jiangsu.The contribution of short-distance trajectories to O₃ pollution in the BTH region became more significant,with the proportion of medium-to-long-distance trajectories from the northwest direction decreasing significantly and that of short-distance trajectories from the south increasing.This shift coincides with the change in potential source areas and emission trends,indicating a transformation in the dominant pollution transport pathways.Compared to 2014—2017,the monthly average emissions of anthropogenic NO,CO,and NMVOCs in 2018—2020 showed a downward trend in the Beijing-Tianjin-Hebei and Shandong regions,as well as in the central and western parts of Nei Mongol.However,anthropogenic emissions increased in the northern regions of Jiangsu and Anhui,and the monthly average emissions of anthropogenic NO and NMVOCs in the central and western parts of Nei Mongol decreased more than in the Beijing-Tianjin-Hebei and Shandong regions.Therefore,while controlling local emissions,it is also necessary to carry out different regional coordinated control in the surrounding areas of BTH,especially the southern part of the Beijing-Tianjin-Hebei region,Shandong and Jiangsu regions.This study can provide a certain theoretical basis for the regional collaborative control of O₃ in the Beijing-Tianjin-Hebei region.