新疆独山子石化区域PM_(2.5)中水溶性无机离子的形成机制

Formation mechanism of water-soluble inorganic ions in PM_(2.5) in Dushanzi Petrochemical District,Xinjiang

2015年9月至2016年7月在新疆独山子区采集大气PM_(2.5)样品,对所含的水溶性无机离子和大气气态污染物的季节性变化进行了分析.其结果表明,PM_(2.5)、SO_2、NO_2和O_3的年均浓度分别为70.04、19.36、4.50、83.06μg·m^(-3); PM_(2.5)、SO_2、NO_2的浓度均出现冬季最高,夏季最低的趋势,而O_3浓度在春、夏季节偏高,冬季偏低;总水溶性无机离子的季节变化特征为冬季(68.99μg·m^(-3))>秋季(14.23μg·m^(-3))>春季(10.31μg·m^(-3))>夏季(5.06μg·m^(-3)),其中SO_2^(-4)、NO_3^-、NH_4^+为水溶性无机离子的主要组成部分,占到水溶性总离子质量浓度的70%以上.对硫氧化率(SOR)和氮氧化率(NOR)的估算表明,全年SOR的值均大于0.1,表明SO_2^(-4)主要来自大气二次转化.夏季NOR值远低于其它季节. SO_2^(-4)浓度和SOR在冬季出现较高值,可能是由于冬季取暖导致SO_2排放量增加,同时较高的相对湿度又促进了SO_2的非均相转化.受相对湿度的影响,NO_3^-在冬季主要以非均相反应的方式生成,在春、夏、秋的3个季节主要以均相反应的方式生成;当PM_(2.5)的质量浓度大于75μg·m^(-3)时,NO_3^-/SO_2^(-4)、NOR/SOR和NOR值均显著增加,表明独山子区的硝酸盐污染较为严重.

PM（2.5） samples were collected in Dushanzi area from September 2015 to July 2016. The seasonal variation of water-soluble inorganic ions and gaseous pollutants（ NO2,SO2 and O3） were analyzed. The average concentrations of PM（2.5）,NO2,SO2 and O3 were 70. 04,19. 36,4. 50 and 83.06 μg·m（-3）,respectively. The concentrations of PM（2.5）,NO2 and SO2 were the highest in thewinter and the lowest in the summer,while O3 levels were the highest during spring-summer and the lowest in the winter. The total concentration of water-soluble inorganic ions decrease in the order：winter 〉 autumn〉 spring 〉summer. Sulfate,nitrate and ammonium were the major components in PM（2.5） with the sum accounting for more than 70% of PM（2.5） mass concentration. The sulfur oxidation ratio（ SOR） and nitrogen oxidation ratio（ NOR） were calculated respectively. SOR was higher than0.1 in the four seasons,suggesting that the observed sulfate was mainly from secondary formation.NOR was much lower in the summer than in the other seasons. High sulfate levels and SOR values in the winter were largely due to enhanced emission of SO2 from coal burning for heating and enhanced heterogeneous reactions of SO2 under high relative humidity conditions. Under the influence of relative humidity,nitrate was generated in the homogeneous and heterogeneous reactions in the winter and was generated in of homogeneous reactions in the spring,summer,and autumn. The ratios of nitrate to sulfate and NOR to SOR as well as NOR values were all significantly increased when PM（2.5） exceeded 75 μg·m（-3）,indicating an important role of nitrate during the polluted period in Dushanzi area.