Identification of long-range transport pathways and potential sources of PMand PMin Beijing from 2014 to 2015

Trajectory clustering, potential source contribution function（PSCF） and concentration-weighted trajectory（CWT） methods were applied to investigate the transport pathways and identify potential sources of PM2.5and PM10 in different seasons from June 2014 to May 2015 in Beijing.The cluster analyses showed that Beijing was affected by trajectories from the south and southeast in summer and autumn. In winter and spring, Beijing was not only affected by the trajectories from the south and southeast, but was also affected by trajectories from the north and northwest. In addition, the analyses of the pressure profile of backward trajectories showed that backward trajectories, which have important influence on Beijing, were mainly distributed above 970 h Pa in summer and autumn and below 950 h Pa in spring and winter. This indicates that PM2.5and PM10 were strongly affected by the near surface air masses in summer and autumn and by high altitude air masses in winter and spring. Results of PSCF and CWT analyses showed that the largest potential source areas were identified in spring, followed by winter and autumn,then summer. In addition, potential source regions of PM10 were similar to those of PM2.5. There were a clear seasonal and spatial variation of the potential source areas of Beijing and the airflow in the horizontal and vertical directions. Therefore, more effective regional emission reduction measures in Beijing's surrounding provinces should be implemented to reduce emissions of regional sources in different seasons.