Particle number size distribution in Beijing was simulated during the period 15 January to 13 March 2006 using NAQPMS+APM (Nested Air Quality Prediction Modeling System with Advanced Particle Microphysics). The simulated number size distribution of fine particles agreed well with observations. The simulation results showed that new particle formation occurred frequently. The nucleation-mode particle number concentration increased drastically due to the nucleation process during new particle formation. In pollution periods, the number concentration of accumulation-mode particles increased, while the nucleation-mode particle number concentration deceased obviously. Moreover, the particle number size distribution shifted towards larger diameters. In heavy pollution episodes, the particle size increased significantly due to intense mixing through microphysical processes. The sizes of primary particles could be 50%larger than their core size due to secondary species coating on them. Accumulation-mode secondary and primary particles together contributed to pollution formation. For Beijing and surrounding areas, the simulation results showed that primary particles dominated particle numbers in the south of Beijing and Hebei, while secondary particles were dominant in the northern part of Hebei.%利用包含详细微物理动力学机制的NAQPMS+APM (Nested Air Quality Prediction Modeling System with Advanced Particle Microphysics)模式，对北京城市大气2006年1月15日至2月13日期间的粒子数浓度谱分布进行了模拟，模式模拟结果合理，能够很好地再现北京城市大气细粒子的数浓度谱分布演变特征。分析表明，北京冬季大气新粒子形成事件频发，核化作用使核模态粒子数浓度急剧升高；污染累积时，积聚模态粒子数浓度显著增大，而核模态粒子数浓度很小，粒子谱分布向大粒子端移动；重污染期间，粒子微物理混合作用强烈，二次成分在一次粒子上的附着使一次粒子粒径显著增大，二次成分可使一次粒子粒径增大50%以上，积聚模态的二次粒子与一次粒子共同促进了污染的形成。在北京及其近周边区域，北京南部和河北南部一次粒子数量多，占据主导地位，而在河北北部二次粒子则占主导地位。
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