In general, the diurnal profile of summer was comparatively flat while those of other seasons showed much more variation. Compared with spring and autumn, the morning peak of BC showed an advance in summer and a delay in winter. The afternoon peak of BC was somewhat opposite to the morning peak, especially in winter. One possible explanation for these small time movements of BC peaks may be the launch of daybreak or dusk, to a certain extent which affects pollutant dispersion, and people’s activity in different seasons.