中文摘要：

2008年6月至9月,在北京城区清华大学和郊区密云水库开展大气颗粒物观测,采集了PM2.5样品共180个,并获得了PM2.5及12种水溶性离子的质量浓度.观测期间城区和郊区PM2.5浓度接近,分别为68.9μg·m^-3和52.9μg·m^-3;二次无机离子SO4^2-、NO3^-和NH4^＋是PM2.5中含量最高的水溶性离子（三者占水溶性离子的比例达90%以上）并具有较高的区域贡献.奥运前后北京及周边地区的一系列临时减排措施使得污染物（SO2、NOx）排放量呈逐时段降低的趋势,并使得奥运期间（8月8日～8月24日）PM2.5及水溶性离子浓度均降至观测期间最低值（C2O4^2-除外）.然而,观测期间PM2.5水溶性离子浓度的变化特征并不完全一致,NH4^＋占PM2.5比例的变化幅度明显小于SO4^2-和NO3^-,NO3^-浓度受源排放变化的影响程度比SO4^2-更加明显.这些复杂的变化特征表明,除污染物源排放外,气象条件也是影响PM2.5及其水溶性离子浓度的重要因素.气团来源分析显示,南方气流的区域性缓慢传输是导致奥运前（7月20日～8月7日）两个PM2.5重污染过程的关键成因,而北方洁净气流频繁、快速的输入则是导致奥运期间PM2.5浓度显著降低的重要原因之一.

英文摘要：

A total of 180 daily PM2.5 samples were collected simultaneously at an urban site （Tsinghua University） and a suburban site （Miyun Reservoir） during 2008-6-2008-9. Mass concentrations of PM2.5 associated with its 12 water-soluble ions were obtained. The average mass concentrations of PM2.5 at the urban and suburban sites were 68.9 μg·m^-3 and 52.9 μg·m^-3 respectively during the sampling period. Secondary inorganic species SO4^2-,NO3^- and NH4^＋ were the major components of water-soluble ions in PM2.5 （the ratio was over 90%） and displayed high regional contributions. A series of temporary measures for atmospheric pollutant reduction were carried out in Beijing and its surrounding areas before,during,and after the 2008 Olympiad. Reductions of the major atmospheric pollutants （SO2,NOx） were observed on account of these measures. And all of the water-soluble ions decreased to a minimum except for C2O4^2-. However,the variations of water-soluble ions were not always consistent with each other throughout the sampling periods. The amplitude of variation for NH4^＋ is much less than those for SO4^2- and NO3^-,and the influence of the emission source variations on NO3^- is more obvious than that on SO4^2-. These complex variations of PM2.5 associated with its 12 water-soluble ions implied that apart from the influence of emission sources,meteorology could also significantly affect the PM2.5 and its water-soluble ions. Backward trajectory analysis indicated that the slow-moving air masses from the south prior to the Olympiad （7-20~8-7） were the main reason for two PM2.5 heavily polluted processes; in contrast,the quick movement of air masses from the north during the Olympiad was a factor in the improved air quality in Beijing.