气溶胶OCEC切割点确定方法改进及应用

Improvement and Application of the Method for Determination of OCEC Split

用MOUDI采样器分级(0.18～18μm,8级)采集北京(BD)及美国亚特兰大(GT)2011年7、8月大气气溶胶样品,测定有机碳(OC)及元素碳(EC).结果表明,由于石英膜结构在灼烧过程中变化,空白膜激光值在OCEC分析过程会随温度改变而变化,且整体呈下降趋势,扣除空白膜激光值变化后手动确定切割点,即激光校正切割可以提高切割点准确性.采用激光校正切割法得到的OC、EC浓度及粒径分布与有氧无氧切割法不同.一些样品切割点出现在通氧前,原因可能是气溶胶中金属等可在无氧环境中催化氧化EC或受热后分解、变色的吸光物质含量较高等.GT气溶胶碳质组分浓度低于BD,且除GT采样点EC外,均呈双峰分布.BD及GT两个采样点OC浓度在0.56～1.0μm、3.2～5.6μm两个粒径段出现峰值,采样期间BD峰值浓度分别为(2.82±1.59)μg.m-3、(1.95±0.76)μg.m-3,GT峰值浓度分别为(1.28±0.41)μg.m-3、(0.64±0.19)μg.m-3.BD采样点EC浓度峰值出现在1.0～1.8μm、3.2～5.6μm,分别为(0.32±0.24)μg.m-3、(0.26±0.19)μg.m-3.GT采样点EC呈三峰分布,集中于粒径更小的气溶胶中,粒径为0.18～0.56μm气溶胶中EC含量占总采样粒径段44.6%.GT采样点OC、EC浓度均较BD更集中于积聚模态,原因可能为夏季GT主要污染源是机动车尾气排放,而BD存在较多工业活动等排放.

Aerosol samples were collected in Beijing（BD） and Atlanta（GT） from July to August in 2011 using a Micro-Orifice Uniform Deposit Impactor（MOUDI）（0.18-18 μm,eight-stage） for organic carbon（OC） and elemental carbon（EC） measurement（Sunset Laboratory Inc,USA）.The laser intensity of blank filters decreased with temperature in the process of OC ＆ EC analysis because the structure of quartz filters was changed when burned which largely affected the determination of low concentration samples’ splits.It would increase the accuracy of OC ＆ EC split to determine it manually after the change of blank filter’s laser intensity was recouped.The concentrations and size distributions of OC ＆ EC using the improved method were different from taking the moment when oxygen was introduced as the split.The split may appear before oxygen addition,when the sample was rich in metal or substances that can be decomposed after heated.The concentrations of carbonaceous components were higher at BD than those at GT.The size distributions of OC showed a bimodal pattern with peaks appeared in the particles with size of（0.56-1.0）μm and（3.2-5.6）μm.The peak concentrations of OC were（2.82±1.59） μg·m^-3nd（1.95±0.76） μg·m^-3t BD,and（1.28±0.41） μg·m^-3nd（0.64±0.19） μg·m^-3t GT.EC showed a bimodal pattern at BD with peaks in particles with size of（0.56-1.0） μm and（3.2-5.6） μm,while showed a trimodal pattern at GT.The peak concentrations at BD were（0.32±0.24） μg·m^-3nd（0.26±0.19） μg·m^-3C at GT was preferably enriched in particles with size of（0.18-0.56） μm,the mass concentrations of EC in this size accounted for 44.6%.The OC and EC were more concentrated in accumulation mode at GT than those at BD,the reason may be that the main pollution source of GT is motor vehicle emission,while there are more industrial gas emissions at BD.