聊城冬季一重污染过程PM_(2.5)污染特征及成因分析

PM_(2.5) Pollution Characterization and Cause Analysis of a Winter Heavy Pollution Event,Liaocheng City

北方秋冬季为重污染过程频发季节,为了解聊城市冬季重污染过程中PM_(2.5)及化学组分污染特征,于2016年1月7~11日在聊城市区开展PM_(2.5)样品采集并分析了其中水溶性离子、碳成分及无机金属元素这3种化学组分,并对污染特征及成因进行了分析.结果表明,此次污染过程PM_(2.5)浓度呈现明显的倒V字型,平均浓度为238.3μg·m^(-3),超过国家环境空气质量标准(GB 3095-2012)二级浓度限值2.2倍;NH_4^+、NO_3^-和SO_4^(2-)为PM_(2.5)的主要水溶性离子成分;随污染加重或减轻,NH_4^+、SO_4^(2-)、NO_3^-、Cl-和Mg^(2+)浓度呈现增加或降低趋势,而Ca^(2+)变化趋势与之相反.污染鼎盛时,NH_4^+、NO_3^-和SO_4^(2-)浓度分别为48.96、68.45和80.55μg·m^(-3),达到起始阶段的6.29、7.31和7.84倍;过程期间OC和EC的浓度为20.8~60.2μg·m^(-3)和3.0~7.5μg·m^(-3),OC浓度高于EC且变化幅度明显偏大;过程期间各日无机金属元素浓度和分别为10.2、22.4、16.0、19.6和8.2μg·m^(-3),富集因子(EF)结果显示,各元素EF均小于10,未被富集,表明污染过程中其主要来源于地壳等自然源;PM_(2.5)质量浓度重构结果表明,有机物(OM)、SO_4^(2-)和NO_3^-为PM_(2.5)的主要组分,其次为NH_4^+、地壳物质和其他离子,EC和微量元素含量相对较低.随着PM_(2.5)污染加重,二次无机盐(SO_4^(2-)、NO_3^-及NH_4^+)浓度及所占比例均随之增加,OM浓度随之增加但比例有所下降,而地壳物质浓度及比例均下降,表明二次无机转化是此次污染过程的主要原因,主要受燃煤和机动车排放影响.

Heavy pollution events frequently occur during fall and winter seasons in northern areas. In order to understand the characteristics and chemical composition of PM2.5 during heavy pollution in winter in Liaocheng City,ambient PM2.5 samples were collected between January 7-11,2016. Mass concentration,water-soluble ions,carbonaceous species,and elements were analyzed,as well as the causes of pollution. Results showed that PM2.5 mass concentration was 238. 3 μg·m^-3 with the trend clearly that of an inverted V; this concentration represents exceedance of the National Ambient Air Quality Standard（GB 3095-2012） by more than 2. 2 times. NO3^-,SO4^2-,and NH4^＋（SNA） were the main water-soluble ions. As pollution increased or decreased,NH4^＋,SO4^2-,NO3^-,and Cl-exhibited the same trend,which contrasted with that of Ca^2＋. During the peak of pollution,NH4^＋,NO3^-,and SO4^2- concentrations were 48. 96,68. 45,and 80. 55 μg·m^-3,with these representing levels 6. 29,7. 31,and 7. 84 times those of the initial stage,respectively. During the pollution event,OC and EC concentration variation ranges were 20. 8-60. 2 μg·m-（-3）,and 3. 0-7. 5 μg·m^-3,respectively. The concentration of OC was significantly higher than that of EC and the variation amplitude was significantly larger.During the event,the mass concentrations of 27 inorganic elements on each day were 10. 2,22. 4,16. 0,19. 6,and 8. 2 μg·m^-3,respectively. Enrichment factors（EF） of all elements were less than 10,indicating lack of enrichment and showing that sources were mainly natural. PM2.5 mass concentration reconstruction results showed that organic matter（OM）,SO4^2-,and NO3^- were major components,followed by NH4^＋,crustal material,and other ions. EC and trace element content was relatively low. As PM2.5 pollution worsened,secondary inorganic salt（NH4^＋,SO4^2-,NO3^-） concentrations and proportions increased,OM concentration increased but its proportion decreased,while crustal material concentration and proportion both decreased,showing that secondary inorganic conversion was the main cause of this pollution event,mainly driven by coal and motor vehicle emissions.