机动车排放细微/超细颗粒物消光特性的Mie理论研究

摘要

机动车等燃烧源排放颗粒物离散系统的消光特性主要取决于颗粒群的物理化学组分和粒径分布特征.利用内部混合模型,确定典型机动车排放颗粒物的密度和光学复折射系数(m),通过Mie理论模型研究不同粒径下单体颗粒物的光学 特性,从而研究机动车排放的nm/μm级粒径下细微/超细颗粒物的无量纲光散射率(Qscat)、无量纲光吸收率(Qabs)以及无量纲消光率(Qext).结果表明,不同国家的轻型柴油车(LDV)排放超细颗粒物的化学组分差异较大,而重型柴油车(HDV)则没有显著差别.相关性分析表明,机动车排放颗粒物的EC组分所占比例与复折射系数的实部(n)和虚部(k)均有极强相关性,相关系数(R2)分别为0.995 6和0.993 8.通过Mie理论计算4种复折射系数下典型机动车排放颗粒物Qscat,Qabs和Qext随粒径的变化特征发现,粒径为400～600 nm范围的单体颗粒物消光作用最大.当颗粒物粒径＞1 μm时,Qscat,Qabs和Qext逐渐稳定为常数1.2,1.0和2.2,此时消光率与复折射系数无关.%The characteristics of light extinction produced by fine and ultrafine particles in the exhaust of vehicles and other combustion sources mainly depend on their physical and chemical composition as well as the particle size distribution of the discrete system. This study first presents an internal mixed model to quantify the density and the complex refractive index (m) of typical vehicle exhaust particles. The dimensionless scattering efficiency ( Qacat ), the dimensionless absorbing efficiency ( Qabs ) and the dimensionless extinction efficiency ( Qext ) in all size ranges of vehicle exhaust particles were investigated by the Mie theory methodology. The results showed that the chemical composition of ultrafine particles from light-duty vehicle emissions varied significantly in different countries, but no significant differences were found from heavy-duty vehicles. The results of correlation analysis indicated that the percentage of the EC component of particles emitted from motor vehicles correlated very well with the real part (n) and the imaginary part (k) of the complex refractive index. The correlation coefficients R2 were 0. 9956 and 0. 9938,respectively. The extinction effect was found to be strongest when the particle sizes ranged from 400 to 600 nm. This was proved by using the Mie theory for calculating the changing characteristics of the Qacat, Qabs and Qaxt of typical motor vehicle exhaust particles,varying with the particle sizes under four types of complex refractive indexes. When the particle size Dp was larger than 1 μm, the Qacat Qabs and Qext gradually became constant, being 1.2,1.0, and 2.2, respectively. Under this situation, the extinction rate had no correlation with the refractive index m.