南京北郊冬季霾天PM_(2.5)水溶性离子的污染特征与消光作用研究

Pollution Characteristics and Light Extinction Effects of Water-soluble Ions in PM_(2.5) During Winter Hazy Days at North Suburban Nanjing

为了探讨霾天下大气细颗粒物(PM2.5)中水溶性离子的污染特征及其对大气消光的影响,在2013年1月25日至2月3日于南京北郊进行了PM2.5连续在线监测.利用颗粒物-液体转换采集系统(PILS)连续采集水溶性样品,与离子色谱联用分析了其中SO2-4、NO-3、NH+4、Cl-、Na+、K+、Mg2+和Ca2+的含量;同时采用扫描电迁移率粒径谱仪(SMPS)和空气动力学粒径谱仪(APS)测量细粒子的粒径谱分布;采用三波长光声黑碳光度计(PASS-3)实时在线测量细粒子的散射和吸收消光系数;并实时监测痕量气体浓度.结果表明,观测期间霾与非霾天PM2.5中水溶性离子的总质量浓度分别为70.3μg·m-3和22.9μg·m-3;二次吸湿性组分SO2-4、NO-3和NH+4为主要的污染离子.霾天有利于SO2和NOx向SO2-4和NO-3的转化,尤其是NOx的氧化.运用多元线性回归统计方法,建立了PM2.5干消光系数与气溶胶化学成分之间的经验公式,发现NH4NO3对南京冬季消光的贡献最大,其次为(NH4)2SO4、有机碳(OC)和元素碳(EC).两次重污染事件中,污染前体物的一次排放和二次转化的增加分别是造成离子浓度升高的主要原因.

To investigate the characteristics of water-soluble ions in PM2. 5and their contribution to light extinction in haze days,on-line monitoring of PM2. 5was conducted at North Suburban Nanjing from 25 January through 3 February,2013. Water-soluble components were collected with a particle-into-liquid sampler( PILS),and analyzed by ion chromatography( IC) for the contents of SO2-4,NO-3,NH+4,Cl-,Na+,K+,Mg2 +and Ca2 +.Simultaneously particle size distributions were measured using scanning mobility particle sizer( SMPS) and Aerodynamic Particle Sizer( APS). The absorption and scattering coefficients were measured by three-wavelength photoacoustic soot spectrometer( PASS-3). Trace gases( SO2,NO2 etc.) were also monitored. The results showed that the average concentrations of total water-soluble ions were 70. 3 and 22. 9 μg·m- 3in haze and normal days,respectively. Secondary hygroscopic components including SO2-4,NO-3and NH+4were the major ionic pollutants. Hazy days favored the conversion of SO2 and NOxto SO2-4and NO-3,respectively,and in particular the oxidation of NOx. Using multiple linear regression statistical method,the empirical relationship between the dry aerosol extinction coefficient and the chemical composition was established. NH4NO3 was found to be the largest contributor to aerosol extinction in winter in Nanjing,followed by( NH4)2SO4,OC and EC. In two heavy pollution events,the increase of ion concentrations was influenced by the increase of primary emissions and secondary transformation.