Streets et al. [2001, 2003a] reported that Black Carbon emissions in China were about 74% from the residential sector and were scattered in a swath curving across the agricultural heartland west from Sichuan Province and east to Hebei Province. As Figure 6 shows, most southerly air masses arriving at Tongyu traveled over those major documented BC emission source areas in China such as Hebei, Henan, Jiangsu, and Shangdong provinces. Although source V and VI were identified as the potential source contribution areas (Figure 5), few air mass trajectories directly passed through these regions.