The regional transport of particulate matter with diameter less than 2.5?μm (PMsub2.5/sub) plays an important role in the air pollution of the Beijing–Tianjin–Hebei (BTH) region in China. However, previous studies on regional transport of PMsub2.5/sub mainly aim at province level, which is insufficient for the development of an optimal joint PMsub2.5/sub control strategy. In this study, we calculate PMsub2.5/sub inflows and outflows through the administrative boundaries of three major cities in the BTH region, i.e., Beijing, Tianjin and Shijiazhuang, using the WRF (Weather Research and Forecasting model)-CMAQ (Community Multiscale Air Quality) modeling system. The monthly average inflow fluxes indicate the major directions of PMsub2.5/sub transport. For Beijing, the PMsub2.5/sub inflow fluxes from Zhangjiakou (in the northwest) and Baoding (in the southwest) constitute 57?% of the total in winter, and Langfang (in the southeast) and Baoding constitute 73?% in summer. Based on the net PMsub2.5/sub fluxes and their vertical distributions, we find there are three major transport pathways in the BTH region: the northwest–southeast pathway in winter (at all levels below 1000?m), the northwest–southeast pathway in summer (at all levels below 1000?m), and the southwest–northeast pathway in both winter and in summer (mainly at 300–1000?m). In winter, even if surface wind speeds are low, the transport at above 300?m can still be strong. Among the three pathways, the southwest–northeast happens along with PMsub2.5/sub concentrations 30 and 55?% higher than the monthly average in winter and summer, respectively. Analysis of two heavy pollution episodes in January and July in Beijing show a much (8–16 times) stronger transport than the monthly average, emphasizing the joint air pollution control of the cities located on the transport pathways, especially during heavy pollution episodes.
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