Taking Handan, Xingtai, Hengshui, and Cangzhou, four cities in southwest Hebei Province along the Beijing-Tianjin-Hebei typical transport route, as examples, we analyzed the variation characteristics of 3a meteorological conditions and PM2.5 concentration in winter from 2019 to 2021 and used potential source contribution analysis (PSCF) and concentration weight analysis (CWT) to identify the transport characteristics of PM2.5 in the four cities during the study period. Based on the meteorological air quality model (WRF-CMAQ) transmission matrix method and transport flux method, the contribution of PM2.5 transport between Handan, Xingtai, Hengshui, and Cangzhou and the surrounding areas was quantitatively assessed; the vertical distribution characteristics of PM2.5 net transport flux were revealed; and the two main transport routes of PM2.5 pollution were further identified. The results showed that during the study period, the PM2.5 concentration decreased by 45.85%, 49.45%, 42.40%, and 31.65%, respectively. The potential source contribution of Handan and Xingtai was mainly distributed in south-central Shanxi (Linfen, Changzhi, and Jinzhong), northern Henan (Xinxiang, Kaifeng, and Zhengzhou), and a small part of Inner Mongolia (PSCF > 0.9). The potential contribution areas of Hengshui and Cangzhou were mainly concentrated in southern Hebei (Handan and Shijiazhuang), central Shanxi (Taiyuan and Yangquan), and some Shandong regions (PSCF > 0.7), and the CWT results were similar to those of PSCF. During the study period, the local contribution (51.11%-62.99%) was slightly higher than the regional contribution (37.01%-48.89%) during winter in the four cities. Affected by horizontal turbulence and vertical diffusion, the impact of regional transmission in 2020 was slightly higher than that in other years (0.50%-9.52%). In 2021, the influence of regional transmission was slightly lower than that of other years (-2.15%--9.52%) due to low PM2.5 concentration and meteorological factors. For Handan, Xingtai, Hengshui, and Cangzhou, the total inflow (outflow) flux intensity of the four cities during winter and the surrounding areas was in 2020 > 2021 > 2019. For the total net flux, the total inflow (outflow) flux intensity of the four cities in winter was 0.094, -0.070, and 0.087 kt·d-1 (Xingtai:0.212, 0.395, and 0.544 kt·d-1; Hengshui:-0.040, -0.228, and 0.185 kt·d-1; Cangzhou:0.062, 0.126, and 0.128 kt·d-1). During the study, Handan, Xingtai, and Cangzhou were mostly used as transport receptors, whereas Hengshui was mostly used as a transport source. In the range of 0-1 260 m, the net transport flux intensity of PM2.5 increased basically with the increase in height, and the maximum net flux of the various cities in different periods was different. The maximum net flux of Handan, Xingtai, and Hengshui was 252-1 261 m, 817 m, and 252-817 m, respectively. The maximum net flux in Cangzhou was 252-359 m. By analyzing the transmission characteristics of the four cities, it was found that there were two main transport directions of PM2.5, that is, the northwest-southeast direction (Shanxi → Handan → Henan and Shandong; Shijiazhuang → Xingtai → Handan and Shandong; Baoding → Cangzhou → Shandong) and the southwest-northeast direction (Shanxi → Xingtai → Hengshui → Cangzhou → Bohai Bay).
Keywords: PM2.5; concentration-weighted trajctory（CWT）; meteorological air quality model （WRF-CMAQ）; potential source contribution function （PSCF）; transmission contribution; transmission flux.