Particle Size Distribution Measurement And Analysis At Shangdianzi In Beijing

Atmospheric particles have been recognized to play a key role in the balance of global climate, notably through their direct and indirect radiative effects. And the particles also affect human health. The characteristics of particle number size distribution in the atmosphere are essential to the overall understanding of the behavior and effects of ambient particles.From March, 2008 to February, 2009, measurements of particle number size distribution at SDZ have been taken for one year, using TDMPS (Twin Differential Mobility Particle Sizer) and APS(Aerodynamic Particle Sizer) at Shangdianzi station. The effective observed days account 85% for all days. The measured particle diameter ranges from 3nm to 10μm with a 10-minute time resolution. During the measurement period, total particle number concentration, particle surface concentration and volume concentration are (1.1±0.7)×104 cm-3, (7±6.5)×102μm2 cm-3 and (42±46)μm3 cm-3, respectively. The particle number concentration at SDZ is significantly higher than that in Mt. Waliguan, one of the Global Atmosphere Watch station and some rural sites of other countries, especially for accumulation mode particles.The particle number concentration and size distribution have noticeable seasonal variation. The highest particle number concentration is observed in spring, and the lowest in summer. Nucleation mode particles have the largest number concentration in spring, mainly due to the frequent new particle formation events. The growth of nucleation mode particles results in high Atiken mode particles in spring. The condensation and coagulation of Atiken mode particles and long transport with polluted air mass contribute to high accumulation mode particles in spring. High coarse mode particle number concentration appears also in spring due to dust storm events. Particle number size distributions can be expressed by modal parameters for the different seasons, the results can provide valuable parameters for climate models and atmospheric chemistry models.The particle number size distribution is significantly influenced by the meteorological parameters. Both wind direction and wind speed strongly affect the particle number concentration and particle number size distribution. With the increasing of wind speed, the Aitken mode and accumulation mode particle number concentration decrease, and nucleation mode particle number concentrations increase. The dominant surface wind directions in SDZ are normally northeast and southwest. When the wind comes from southwest, the number concentration of Aitken mode and accumulation mode particle increase, and nucleation mode particle concentration decrease. Precipitation is another factor effecting the particle number size distribution, which can remove accumulation mode particles most effectively.Cluster analysis of backward trajectory shows that there are mainly five types of trajectories affecting the particle size distributions at SDZ. High accumulation mode particle number concentrations are observed when air masses come from the surrounding of SDZ or the polluted areas, with short back trajectory length. High number concentrations of nucleation mode particles are associated with air masses from clean regions, with long trajectory length.During the measurement, new particle formation events have been observed at 122 days, 39% of 315 effective measured days, with nucleation mode particle number concentration increasing sharply. New particle formation events occur more frequently in spring than other seasons. The sunny and low relative humidity in spring and winter favors the new particle formation. While in summer the temperature and relative humidity are high and wind speed is low, which favors the accumulation of particles, thus inhibited the new particle formation. For all the events, the observed formation rates are in the range of 0.4～13.5 cm-3s-1, and growth rate 0.4～12nm h-1. The maximum of formation rate appears in spring and minimum in winter, while the maximum of growth rate appears in summer and minimum in winter.