Number concentration and distribution of airborne particles in the size range 5.6 to 560 nm diameter were measured in Beijing for a 15-d period in winter 2005. Dally average number concentrations of nucleation mode （5.6-20 um）, Aitken mode （20-100 um）, and accumulation mode （100-560 um） particles, and total particles were 17500, 32000, 4000, and 53500 cm^-3, respectively. Average particle size distribution was monomodal with a mode diameter of about 40 um at night and bimodal with mode diameters of about 10 and about 40 um during the daytime. New particle formation events, which were connected to diurnal variation of nucleation mode particles, were observed in more than half of the observation days. The events often started around 10：00-11：00 Chinese Standard Time （CST） and ended up after 3-4 h. Concentrations of Aitken and accumulation mode particles increased from midnight and reached their maxima at about 10：00 CST, and then decreased and became the lowest in the afternoon. Analysis of diurnal cycles in traffic volume and meteorological parameters revealed that the accumulation of the particles in Aitken and accumulation modes in the morning was influenced by formation of an inversion and increase in vehicle emission, and dispersion of such particles in the afternoon was associated with more effective vertical mixing and higher wind speed.
SHI Zong-boHE Ke-binYU Xue-chunYAO Zhi-liangYANG Fu-moMA Yong-liangMA RuiJIA Ying-taoZHANG Jie
