Probability distribution of endpoints of backward trajectories from Chongming, Danyang, Jingjiang, Jiaxing, and Zhangjiagang in different available source areas.

Notes: Area 1 includes northern Jiangsu and northern Anhui; Area 2 includes southeastern Henan and eastern Hubei; Area 3 includes Southern and central Anhui; Area 4 includes the Yangtze River Delta; Area 5 includes northeastern Hunan and northwestern Jiangxi; Area 6 includes northeastern Jiangxi, northwestern Fujian, and western Zhejiang; Area 7 includes southern and central Zhejiang; Area 8 includes southern Jiangxi and southern and central Fujian. These probable source area were showed in Fig. 1.