Compared with these measurements, herein the reference hourly Black Carbon of 7 mgm−3 for Tongyu strong events was close to the average Black Carbon level in the typical Chinese polluted urban regions, comparative to a visibility below 10 km estimated by the method of Deng et al. [2008]. Table 3 shows the Black Carbon events and corresponding mixing heights calculated from HYSPLIT4 model at maximal hourly Black Carbon concentrations during these events. Although 50% of Black Carbon events occurred in spring, the Black Carbon events with a longest duration only occurred in early autumn. Mixing heights of atmospheric boundary layer at maximal hourly Black Carbon concentrations generally showed low values, with an average height of 518.7 m, indicating poor dispersion conditions during all Black Carbon events.