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

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、56.19 g/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。

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

Atmospheric PM2.5 and the corresponding gaseous precursors (SO2、NO2) were simultaneously collectedat two typical sampling sites in Xiangtan, central south China from December 3, 2013 to January 14, 2014, andthen the concentrations of secondary water-soluble inorganic ions (SO2− 4 、NO−3 and NH+4 ) in PM2.5 were measuredand analyzed by ion chromatography (IC) method. The sources, formation mechanism and influencing factors ofsulfate 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 PM2.5and secondary water-soluble ions in Xiangtan City were 148.34 and 56.19 g/m3, respectively. The concentrationsof SO2− 4 、NO−3 and NH+4 accounted for 15.26%, 14.06% and 8.57% of PM2.5 concentration, respectively, and thetotal concentrations of SO2− 4 、NO−3 and NH+4 accounted for 37.88% of PM2.5 concentration. With increasing ofPM2.5 concentration, the concentrations of secondary water-soluble inorganic ions and gaseous precursors (i.e. SO2and NO2) increased correspondingly. The concentration of SO2− 4 、NO−3 and NH+4 in PM2.5 at severe pollution levelwere 1.93, 2.41 and 2.03 times higher than that at good pollution level. SO2− 4 and NO−3 in PM2.5 at different airquality levels mostly existed in the form of NH4NO3 and (NH4)2SO4, while other sulfates and nitrates may alsoexist at both slightly and moderately polluted periods. The average values of SOR and NOR during sampling periodat different pollution levels were above 0.1, indicating that sulfate and nitrate were mainly produced by secondarytransformation of SO2 and NO2 in the atmosphere. The ratio of NO−3 to SO2− 4 in PM2.5 was 0.89, and all the valuesfor PM2.5 at different air quality levels were less than 1 (i.e. 0.78, 0.99, 0.82 and 0.97, respectively), indicating thatthe main source of the water-soluble ions in winter PM2.5 in Xiangtan city was coal combustion.