湘潭市PM2.5中二次水溶性无机离子及其气态前体物的污染特征

Pollution Characteristics of Secondary Water-Soluble Inorganic Ions in PM2.5 and Their Gaseous Precursors in Xiangtan, China

2013年12月3日至2014年1月14日,在湘潭市2个功能区(交通、商业、居民区和工业区)采样点对大气PM2.5进行了采集,并同步采集了SO2、NO2;进而利用离子色谱法对PM2.5中二次水溶性无机离子(SO42-、NO3-和NH4+)的浓度进行测试分析。通过分析不同空气质量级别下硫、氮氧化速率(SOR和NOR),探讨了PM2.5中硫酸盐和硝酸盐的来源、形成机制和影响因素等。结果表明,采样期间湘潭市PM2.5及其二次水溶性无机离子(SO42-、NO3-和NH4+)的质量浓度分别为148.34、56.19μg/m3,其中SO42-、NO3-和NH4+分别占PM2.5浓度的15.26%、14.06%和8.57%,三者累计值占PM2.5质量浓度的37.88%。随着PM2.5浓度增加,二次水溶性无机离子及其气态前体物SO2、NO2的浓度也逐渐增加,且"重度"污染时SO42-、NO3-和NH4+浓度较"良"时分别上升了1.93、2.41、2.03倍。不同空气质量级别下PM2.5中的SO42-、NO3-主要以NH4NO3和(NH4)2SO4的形式存在,但在"轻度"和"中度"污染时可能存在其它的硫酸盐和硝酸盐。采样期间SOR和NOR的平均值分别为0.18和0.17,不同污染级别下二者均在0.15以上(大于0.1),表明湘潭市PM2.5中的硫酸盐和硝酸盐主要是经转化形成的二次污染物。大气PM2.5中NO3-/SO42-为0.89,不同空气质量级别下二者比值分别为0.78、0.99、0.82、0.97(均小于1),表明湘潭市冬季PM2.5污染以燃煤源排放为主。

Atmospheric PM2.5 and the corresponding gaseous precursors(SO2、NO2)were simultaneously collected at two typical sampling sites in Xiangtan,central south China from December 3,2013 to January 14,2014,and then the concentrations of secondary water-soluble inorganic ions(SO42-、NO3-and NH4+)in PM2.5 were measured and analyzed by ion chromatography(IC)method.The sources,formation mechanism and influencing factors of sulfate and nitrate in PM2.5 were discussed by analyzing the oxidation rates of sulfur(SOR)and nitrogen(NOR)at different air quality levels.The results show that,during the sampling time,the mass concentrations of PM2.5 and secondary water-soluble ions in Xiangtan City were 148.34 and 56.19μg/m3,respectively.The concentrations of SO42-、NO3-and NH4+accounted for 15.26%,14.06%and 8.57%of PM2.5 concentration,respectively,and the total concentrations of SO42-、NO3-and NH4+accounted for 37.88%of PM2.5 concentration.With increasing of PM2.5 concentration,the concentrations of secondary water-soluble inorganic ions and gaseous precursors(i.e.SO2 and NO2)increased correspondingly.The concentration of SO42-、NO3-and NH4+in PM2.5 at severe pollution level were 1.93,2.41 and 2.03 times higher than that at good pollution level.SO42-and NO3-in PM2.5 at different air quality levels mostly existed in the form of NH4 NO3 and(NH4)2SO4,while other sulfates and nitrates may also exist at both slightly and moderately polluted periods.The average values of SOR and NOR during sampling period at different pollution levels were above 0.1,indicating that sulfate and nitrate were mainly produced by secondary transformation of SO2 and NO2 in the atmosphere.The ratio of NO3-to SO42-in PM2.5 was 0.89,and all the values for PM2.5 at different air quality levels were less than 1(i.e.0.78,0.99,0.82 and 0.97,respectively),indicating that the main source of the water-soluble ions in winter PM2.5 in Xiangtan city was coal combustion.