北京城区夏季PM2.5中碳组分和二次水溶性无机离子浓度特征

Characteristics of carbon components and secondary water-soluble inorganic ions in PM_(2.5) in summer in urban Beijing

为探索北京城区大气细颗粒物浓度水平及其碳组分和二次水溶性无机离子的浓度特征,于2014年6月1日至7月15日在车公庄地区使用微量振荡天平(TEOM+FDMS)、EC/OC在线分析仪以及水溶性离子在线分析仪对PM_(2.5)质量浓度及其主要化学组分(OC、EC、SO_4^(2-)、NO_3^-和NH_4^+)进行了实时监测.研究结果表明,北京市城区夏季PM_(2.5)质量浓度平均值为69.0±47.9μg·m-3,PM_(2.5)中OC、EC、SO_4^(2-)、NO_3^-和NH_4^+所占的比例分别为15.8%、2.4%、23.0%、15.7%和19.2%,SNA(SO_4^(2-)、NO_3^-和NH_4^+)合计达到了PM_(2.5)质量浓度的57.9%.研究各组分的日变化特征发现,OC和SO_4^(2-)白天浓度变化较小,夜晚浓度稍高;NO_3^-和NH_4^+则随着光照和温度的增加而逐渐降低;EC呈现出夜晚浓度高白天浓度低的特点.研究各组分的相关性及比值发现,OC和EC的相关系数为0.62,OC/EC大于2.0,说明北京城区夏季存在着较为严重的二次污染;此外,NO_3^-/SO_4^(2-)平均比值为0.68,SOR和NOR的变化趋势基本一致,两者的平均值分别为0.55和0.14.通过分析北京市城区夏季不同浓度级别各组分的变化发现,随着PM_(2.5)质量浓度的增加,OC和EC所占的比例不断降低,而SNA比例则不断升高,其中NO_3^-浓度水平的增加最为显著.

To investigate the characteristics of fine particalate matters in urban Beijing,on-line measurements of PM2.5mass and the main chemical composition in PM2.5（ OC,EC,SO4^2-,NO3^-and NH4^＋） were conducted in the area of Chegongzhuang from June 1stto July 15 th,2014 by using TEOM＋ FDMS,the Sunset OC / EC field analyzer and the URG ambient ion monitor. The results showed that the average mass concentration of PM2.5was 69. 0 ± 47. 9 μg·m^-3 and the mass contribution of OC,EC,SO4^2-,NO3^-and NH4^＋in PM2.5was 15. 8%,2. 4%,23. 0%,15. 7% and 19. 2%,respectively. The sum of these three secondary water-soluble inorganic ions（ SNA） accounted formore than one half of the PM2.5mass concentration（ 57. 9%）,which had become the major source of PM2.5in urban Beijing. In addition,the daily of OC and SO4^2-was small in the daytime and higher at night. NO3^-and NH4^＋decreased gradually with the elevation of sunshine and temperature;EC was higher at the night and lower in the daytime. The correlation coefficient between OC and EC was 0. 62,and the ratios of OC / EC were all larger than 2. 0,suggesting the importance of secondary organic aerosols in the formation of particle pollution. Furthermore,the ratio of NO3^-/ SO4^2-was 0.68,and similar variation pattern of SOR and NOR were observed,the daily values of which were 0.55 and 0. 14,respectively. Finally,elevated contributions of secondary inorganic species and a corresponding decrease of organics were observed as the increasing of PM2.5. Among the inorganic species,nitrate presented the largest increase from 12. 8% to 17. 8%,indicating the enhanced roles of secondary species in the particle pollution.