Diurnal variation of number concentration and size distribution of ultrafine particles in the urban atmosphere of Beijing in winter

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. Daily average number concentrations of nucleation (5.6-20 nm), Aitken (20-100 nm) and accumulation (100-560 nm) mode 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 nm at night and bimodal with mode diameters of about 10 and about 40 nm 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 maximum at about 10:00 CST, and then decreased and became lowest in the afternoon. Analysis of diurnal cycles in traffic volume and meteorological parameters revealed that the accumulation of the particles 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.