2008年北京市PM_(10)的粒度分布分形维数变化特征

2007年11月-2008年10月间在北京市市中心和西北城区采集了不同季节的PM10样品,并借助扫描电子显微镜和图像分析软件对其进行粒度分布分形维数分析。结果表明,颗粒物中细颗粒物越多,粒度越细,则颗粒物粒度分布分形维数值越大。2008年西北城区的PM10的粒径分布分形维数较大,市中心的较小。市中心的粒度分布分形维数在1.95～2.59之间,各个季节的分形维数呈现冬季春季夏季秋季,西北城区的粒度分布分形维数在2.58～2.72,各个季节的分形维数呈现秋季夏季冬季春季,说明在市中区冬季PM10粒度较细,而在西北城区秋季的颗粒物偏细。与2005年同季相比,2008年的总体颗粒物和烟尘集合体的粒径分布分形维数较大,矿物颗粒的粒度分布分形维数较小,并且总分布的季节变化一致。2008奥运期间的PM10粒度分布分形维数在2.28～3.39之间变化,标志着颗粒物总体变细的趋势。

The Variation of Fractal Dimensions of Size Distribution of Inhalable Particulates in Beijing Air During 2008

In this paper, the PM₁₀ samples were collected at the center and northwestern sites of Beijing during different seasons from Nov 2007 to Oct 2008, and the fractal dimension of size distribution (FDSD) of PM₁₀ was obtained by means of scanning electron microscopy and image analysis software. It is found that the higher percentages of the finer particles in PM₁₀ are associated the higher FDSD values. The FDSD values of PM₁₀ at the northwestern site are generally larger than those at the center site. The FDSD of the PM₁₀ at the center site ranges between 1.95 and 2.59, with the highest value being the winter samples, followed by the spring, summer and autumn samples in descending order. The FDSD of the PM₁₀ at the northwestern ranges between 2.58 and 2.72, with the highest value being the autumn samples, followed by the summer, winter and spring samples in descending order. These results indicate that in the city center area, the winter PM₁₀ samples contain a higher proportion of finer particles; while in the northwestern urban area, the autumn PM₁₀ samples contain a higher proportion of coarser particles. The FDSD values of total particles and soot aggregates of the PM₁₀ in 2008 are larger than those in 2005, and the FDSD of mineral particles in PM₁₀ is smaller than that of 2005. During the 2008 Olympic Games, the FDSD of PM₁₀ ranges from 2.28 to 3.39, indicating an overall fining trend of particles.