太原市冬季大气PM_(2.5)中硫酸盐污染特征及其形成机制研究

Pollution characteristics and formation mechanism of sulfate in PM2.5 during winter in Taiyuan

为更好地研究太原市冬季大气细颗粒物(PM_(2.5))中硫酸盐(SO_(4)^(2-))的污染特征及形成机制,于2020年1月采用在线设备对小时分辨率的PM_(2.5)及其化学组分(水溶性离子、金属元素)、气态污染物进行观测,分析了不同污染水平下SO_(4)^(2-)变化特征,评估了不同形成机制对硫酸盐形成的贡献.结果表明,观测期间太原市ρ(PM_(2.5))平均值为(108.7±72.1)µg·m-3,SO_(4)^(2-)、NO_(3)-和NH_(4)^(+)在PM_(2.5)中的占比分别为18.0%±7.9%、16.8%±4.9%和13.1%±3.5%.清洁天、污染天和严重污染天ρ(SO_(4)^(2-))分别为(9.2±5.3)、(17.9±9.2)和(46.3±17.7)µg·m-3,污染天(0.33)和严重污染天(0.51)硫氧化率(SOR,n(SO_(4)^(2-))/(n(SO_(4)^(2-))+n(SO_(2))))明显高于清洁天(0.26).SOR与RH呈显著正相关(r=0.65,p<0.05),与O_(x)(O_(x)=O_(3)+NO_(2))相关性较弱(r=0.10,p<0.05),表明SO_(4)^(2-)主要来源于液相和非均相反应.观测期间pH范围为4.0~4.7,pH影响的敏感性分析显示Mn表面催化和H2O_(2)途径主导硫酸盐的形成.污染案例分析表明硫酸盐产生速率范围为1.5~7.1μg·m^(-3)·h^(-1),数值计算结果显示Mn表面催化和H_(2)O_(2)途径对SO_(4)^(2-)产生的平均贡献分别为37.7%和36.0%.

In order to better study the pollution characteristics and formation mechanism of sulfate(SO_(4)^(2-))in atmospheric fine particulate matter(PM_(2.5))in winter of Taiyuan,hourly PM_(2.5) and its chemical components(water-soluble ions,metal elements)and gaseous pollutants were monitored by online equipment during January 2020.The characteristics of SO_(4)^(2-)under different pollution levels were analyzed and the contribution of different formation mechanisms to sulfate formation was evaluated.The results showed that the average concentration of PM_(2.5) was(108.7±72.1)µg·m-3 during the observation period in Taiyuan,and the proportions of SO_(4)^(2-),-NO3 and NH_(4)^(+)in PM_(2.5) were 18.0%±7.9%,16.8%±4.9%and 13.1%±3.5%,respectively.The mean mass concentrations of SO_(4)^(2-)were(9.2±5.3),(17.9±9.2)and(46.3±17.7)µg·m-3 during clean,polluted and heavily polluted days,respectively.SOR(n(SO_(4)^(2-))/(n(SO_(4)^(2-))+n(SO_(2))))were significantly higher during polluted days(0.33)and heavily polluted days(0.51)than clean days(0.26).SOR was significantly positively correlated with relative humidity(RH)(r=0.65,p<0.05),weakly correlated with O_(x)(O_(x)=O3+NO_(2))(r=0.10,p<0.05),indicating that the formation of SO_(4)^(2-)was mainly originated from aqueous-phase and heterogeneous reaction.The pH range was 4.0~4.7 during the observation period,and the sensitivity analysis of pH effect showed that Mn surface catalysis and H2O_(2) pathway dominated the formation of sulfate.The results of the pollution case analysis show that the sulfate production rate ranges from 1.5μg·m^(-3)·h-1 to 7.1μg·m^(-3)·h^(-1).The numerical calculations show that the average contributions of Mn-surface catalysis and H_(2)O_(2) pathway to SO_(4)^(2-)production were 37.7%and 36.0%,respectively.