Contributions of inter-city and regional transport to PM2.5 concentrations in the Beijing-Tianjin-Hebei region and its implications on regional joint air pollution control.

Regional transport plays an important role in the serious PM2.5 pollutions in the Beijing-Tianjin-Hebei (BTH) region, China. Practical regional joint emission control strategies require quantitative assessments of the transport contribution among cities and regions. The Community Multiscale Air Quality model equipped with the Integrated Source Apportionment Model is used to simulate the contributions from 5 major emission sectors in 13 cities of the BTH region, and 4 surrounding provinces outside BTH for the year 2014. Annual averaged local contribution ranges from 32% to 63% for the 13 cities in the BTH region, where secondary components contributing more than primary components. Regional contribution ratio becomes larger and the transport distance longer in July and October than in January and March. For Beijing, local contributions are 62% and 69% in January and March respectively, and the regional transports are mainly from nearby cities such as Zhangjiakou, Baoding and Langfang. In July and October, local contributions in Beijing are only 33% and 38% respectively, and a large range of regions in the south have substantial contributions, where Shandong Province and Henan Province contribute 3.6-5.3 μg/m3. Analysis of daily contributions suggests that regional transport is stronger under higher PM2.5 concentrations. During heavy pollution, local emissions in Beijing contribute 61%, 49%, 23% and 25% in January, march, July and October respectively, while during the clean days, the ratios are 88%, 88%, 76% and 57% respectively. Southerly regional transport during the rising phase of "saw tooth" pattern might be enhanced by weak cold high pressure and its easterly, northerly moving path. Among the major emission sectors, in winter, local domestic combustion is the most important source for Beijing, Tianjin and Shijiazhuang. In summer, transportation and domestic combustion are two important local sources for Beijing, while joint control in other cities should focus on industry.