北京城区大气PM_(2.5)主要化学组分及污染特征

Characteristics of PM_(2.5) and Its Chemical Composition in the Urban Area of Beijing

为了解北京城区大气PM2.5主要化学组成特征,于2012年8月─2013年7月对城区石景山、东四和通州3个采样点及城区对照点定陵和区域传输点榆垡开展为期1 a的PM2.5组分研究,共获得268组样品.结果表明：城区平均质量浓度大于1.0μg/m3的组分有OC、NO3^-、SO4^2-、NH4^＋、EC、Cl^-、Si、Ca、Al、K^＋,其中ρ（OC）、ρ（NO3^-）、ρ（SO4^2-）、ρ（NH4^＋）分别为（22.2±17.1）、（21.5±25.9）、（19.8±23.7）、（14.3±16.8）μg/m^3,分别占ρ（PM2.5）的17.9%、17.3%、15.9%、11.5%,城区各主要组分的平均质量浓度明显大于对照点;城区各采样点之间主要组分所占比例相差不大,与城区对照点、区域传输点差异明显;春、夏、秋、冬四季城区采样点的主要组分均为OC、NO3^-、SO4^2-、NH4^＋,这4种组分质量浓度之和分别占各季ρ（PM2.5）的62.5%、54.2%、46.0%、62.7%,其中春季ρ（NO3^-）、夏季ρ（SO42-）、秋冬季的ρ（OC）相对较高;北京城区各采样点均受SOC影响较大,OC/EC〔ρ（OC）/ρ（EC）〕的平均值为5.7,城区SNA（二次无机气溶胶）占ρ（PM2.5）的比例（15.0%-53.1%）和NO3^-/SO4^2-〔ρ（NO3^-）/ρ（SO4^2-）〕（0.47-1.36）均随空气质量指数上升而增加,同时观测期间北京城区PM2.5中NO3^-/SO4^2-的平均值为1.14,较往年明显增大,表明目前北京城区的PM2.5排放源逐步由以固定源为主向固定源和移动源并重的方向发展.

In order to understand the characteristics and chemical compositions of PM2. 5in the urban area of Beijing,268 groups of PM2. 5samples were collected from July 2012 to August 2013 at Dingling（ background site）,Yufa（ regional transmission site）,Gucheng,Dongsi and Tongzhou（ urban sites）. The samples were analyzed for water-soluble ions,organic and elemental carbon and inorganic elements. The average concentrations of OC,NO3^-,SO4^2-,NH4^＋,EC,Cl^-,Si,Ca,Al and K＋in the samples were all above 1. 0μg/m^3. The average concentrations of OC,NO3^-,SO4^2-and NH4＋were（ 22. 2 ± 17. 1）,（ 21. 5 ± 25. 9）,（ 19. 8 ± 23. 7） and（ 14. 3 ±16. 8） μg/m3respectively,accounting for 17. 9%,17. 3%,15. 9% and 11. 5% of the PM2. 5. The concentrations of all species in the samples from non-background sites were higher than the samples from the background site. The concentration differences and chemical compositions among the three urban sites were not significant,but were quite different from those of Dingling and Yufa. OC,NO3^-,SO4^2-and NH4^＋were the most abundant species in the urban PM2. 5. The sum of the four species accounted for 62. 5%,54. 2%,46. 0% and62. 7% of PM2. 5in the spring,summer,autumn and winter,respectively. The species with the highest concentration was NO3-in the spring,SO4^2-in the summer and OC in the autumn and winter. The value of OC/EC（ 5. 7） showed a higher level of secondary organic carbon in the urban samples. The fractions of SNA and the ratios of NO3^-/SO4^2-increased with the AQI. The average ratio of NO3^-/SO4^2-in urban area was 1. 14,larger than previous studies in Beijing. There are signs that the sources of PM2. 5are gradually shifting from fixed sources to a combination of fixed and mobile sources in Beijing.