Assessment of inter-city transport of particulate matter in the Beijing-Tianjin-Hebei region - 王书肖教授课题组

The regional transport of PM2.5 plays an important role in the air quality over the Beijing-Tianjin-Hebei (BTH) region in China. However, previous studies on regional transport of PM2.5 are mainly at province level, which is insufficient for the development of an optimal joint PM2.5 control strategy. In this study, we calculate PM2.5 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) &ndash; CMAQ (Community Multiscale Air Quality) modelling system. The monthly average inflow fluxes indicate the major directions of PM2.5 transport. For Beijing, the PM2.5 inflow fluxes from Zhangjiakou (on the northwest) and Baoding (on the southwest) constitute 57&thinsp;% of the total in winter, and Langfang (on the southeast) and Baoding constitute 73&thinsp;% in summer. Based on the net PM2.5 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&thinsp;m), the Southeast-Northwest pathway in summer (at all levels below 1000&thinsp;m), and the Southwest-Northeast pathway both in winter and in summer (mainly at 400&ndash;1000&thinsp;m). In winter, even if surface wind speeds are small, the transport at above 400&thinsp;m could still be strong. Among the three pathways, the Southwest-Northeast happens along with PM2.5 concentrations 30&thinsp;% and 55&thinsp;% 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 stronger (8&ndash;16 times) transport than the monthly average, emphasizing the joint air pollution control of the cities located on the transport pathways, especially during heavy pollution episodes.