In order to investigate the sources of Black Carbon captured at Tongyu, the HYSPLIT model was used to trace the outflow trails of air pollution. Figure 4 shows 5 d back trajectories arriving at 150 m above the sampling site for each Black Carbon event listed in Table 3. The main outflow patterns in fall were very similar to those of spring, with outflows popular in northerly, northwesterly, and southerly pathways.