In order to identify the pollutant transport mechanisms for these events, similar to Cheng et al. [2006], available hourly BC data were scanned to screen out the typically strong BC events, which were picked up according to hourly BC exceeding 7 mgm−3 within a continuous 4 h period. As in other cities in Northeast Asia, the reference BC level of 7 mgm at Tongyu was comparable to the mean BC concentration of Seoul in winter [Park et al., 2002], the annual average of Chongju [Lee and Kang, 2001], and less than the mean BC value of Changchun in winter [Cao et al., 2007]. Wu