北京典型城区冬季PM2.5水溶性离子特征

Characteristics of water soluble inorganic ions in PM2.5 in winter of Beijing in typical urban areas

为了全面了解北京市煤改气后冬季大气细颗粒PM2.5中无机水溶性离子及其来源,于2017年冬季供暖期间在北京市海淀区连续采集了41个PM2.5样品,采用离子色谱分析了其中10种水溶性离子(Na+、NH4+、K+、Ca2+、Mg2+、F-、Cl-、NO2-、NO3-、SO42-)的浓度。结果表明:采样期间北京市PM2.5的日均质量浓度范围为(94.28±52.49)μg/m3,相较于2016年,2017年冬季PM2.5日均质量浓度降低了29μg/m3,减少幅度达28.2%,污染天数明显降低;污染期颗粒物中NO3-、SO42-、NH4+显著高于优良期;质量浓度比ρ(NO3-)/ρ(SO42-)大于1,移动源贡献相对较大,反映出"煤改气"政策实施引起燃煤等固定源排放硫酸盐减少。采用飞行时间二次离子质谱(TOF-SIMS)结合质量高分辨模式与正负离子成像模式表征了PM2.5。结果显示NH4+在污染期倾向以分子硫酸铵或硫酸氢铵的形态存在。

In order to fully understand the inorganic water-soluble ions and their sources in the atmospheric fine particles PM2.5 in Beijing after coal-to-gas conversion,41 PM2.5 samples were continuously collected in Haidian District of Beijing during the winter heating period in 2017.The concentrations of 10 kinds of water-soluble ions(Na+,NH4+,K+,Ca2+,Mg2+,F-,Cl-,NO2-,NO3-,SO42-)were analyzed by ion chromatography.The results show that the daily average mass concentration of PM2.5 in Beijing during the sampling period is(94.28±52.49) μg/m3.Compared with the winter of 2016,the average daily PM2.5 concentration in Beijing in 2017 decreased by 29 μg/m3,with a decrease of 28.2%,and the number of pollution days decreased significantly.During the pollution period,NO3-,SO42-,and NH4+in the particulate matter were significantly higher than the clean period.The ratio of the concentration of ρ(NO3-)/ρ(SO42-) is greater than 1,and the contribution of the mobile source is relatively large,reflecting that the implementation of the "coal to gas" policy has contributed to the reduction of sulfate emissions from fixed sources such as coal.Time-of-flight secondary ion mass spectrometry(TOF-SIMS)was combined with mass high resolution mode and positive and negative ion imaging mode to characterize PM2.5.The results show that NH4+tends to exist in the form of molecular ammonium sulfate or ammonium hydrogen sulfate during the pollution period.