兰州市夏季大气中碳类气溶胶含量变化特征及其来源分析

Characteristics of Carbonaceous Aerosols and Their Source in the Atmosphere during Summer in Lanzhou

利用有机碳/元素碳分析仪(DRI 2001A型)和黑碳积分光谱仪(ISSW)测定了2015年兰州市夏季大气中有机碳(OC)、元素碳(EC)以及黑碳(BC)的含量,并对夏季日夜有机碳、元素碳和黑碳质量浓度变化特征进行了深入的分析。研究结果表明:黑碳和元素碳质量浓度测量结果受滤膜采样效率和测量仪器影响,差异较大,黑碳气溶胶夜间的含量高于白天且变化幅度大,呈现明显的波动上升趋势。有机碳平均质量浓度白天为(3.90±1.23)μg·m^(-3),高于夜间,其值为(3.35±1.24)μg·m^(-3);元素碳平均质量浓度白天为(1.07±0.46)μg·m^(-3),低于夜间,其值为(1.59±0.68)μg·m^(-3)。兰州市夏季尤其是白天二次有机碳(SOC)含量较高,二次源为白天有机碳主要来源,一次源为夜间有机碳主要来源。将元素碳分为低温燃烧时生成的焦碳char(char=EC1-OPC)和高温燃烧时生成的烟炱soot(soot=EC2+EC3),通过分析char和soot日夜变化趋势,发现夏季日夜主要污染源均是机动车尾气,但夜间生物质燃烧和煤炭燃烧污染较白天有所增加,且明显呈现上升趋势。

Organic Carbon （OC）, Blakc Carbon （BC）, and Elemental Carbon （EC） are major kinds of carbo- naceous particles depended on their measurement techniques. We collected 40 samples during summer in Lanzhou by two kinds of filters （nuclepore filters with 0. 4 μm pore sizes and quartz fiber filters with 1 μm pore sizes）, respectively. The concentrations of OC, EC were measured by the thermal/optical method using DRI-2001A carbon analyzer with the IMPROVE-A temperature protocol, while the BC concentrations were measured by the Integrating Sphere/Integrating Sandwich Spectrophotometer （ISSW）. The results indicated that the concentrations of BC and EC were affected by the sampling efficiency and different measurement techniques. Actually, for each kind of filters, there were 20 samples in the daytime and nighttime, respectively. There was a large variation of the BC concentrations at night. The average concentration of OC in the daytime was （ 3. 90± 1. 23） Ixg ~ m-3, higher than that of （3.35±1.24） p^g ~ m-3 at nighttime, but for EC, （1.07±0.46） Ixg ~ m-3 at daytime, lower than that of （1. 59±0. 68） p^g ~ m-3 at nighttime. Through the backward trajectories analysis, we found that the air with short transport distance, slower move speed and sinking motion would accumulate the pollutant. On the contrast, long transport distance and faster move speed were conducive to the spread of pollutants. The high value （greater than 2） of OC/EC indicated that the concentration of secondary organic carbon （SOC） was high, espe- cially during the daytime in summer in Lanzhou. SOC made a major contribution to the mass of OC in the atmosphere with the concentration of （2. 43±0. 90）μg·m^-3 during the daytime, while in the nighttime the concentra- tion of SOC was （ 1. 16±0. 73）μg·m^-3, they were about 62% and 35% of OC in the daytime and nighttime, re- spectively, which manifested that OC was dominated by primary sources at nighttime. In order to further determine the pollution sources, we separated EC into two categories （char and soot） based on different combustion tempera- tures. In fact, in the IMPROVE-A Thermal-Optical Reflectance （TOR） method, we had got OC1, OC2, OC3, OC4, OPC and EC1, EC2, EC3. From the previous studies, we could calculate the concentrations of char and soot （char=EC1-OPC; soot--EC2＋EC3）. By comparing the changes of char and soot concentrations, we found that the main pollution source was motor vehicle exhaust both at daytime and nighttime in summer, but the contribution of the biomass and coal combustion pollution increased and presented an increasing trend at nighttime.