气溶胶光学厚度谱特征判断粒子大小方法初探

The Application of AOD's Spectral Curve Parameter to Judgment of Aerosol Particle Size

当气溶胶谱满足Junge分布时,Angstrom指数(α)可以准确地描述粒子大小,但真实大气气溶胶很少完全满足这一条件,仅用α判断粒子大小会有较大出入。基于北京、香河、兴隆、太湖4个Aeronet观测站21世纪以来各站历时都超过1年的气溶胶光学厚度资料,获得5511组lnτ与lnλ的二次拟合参数a_3,a_1,尝试找到一种结合α,a_2,a_1判断粒子大小的方法。结果表明:当气溶胶为粗粒子时(V_(fine)/V_(total)<0,2),α均小于0.75,仅用α就可以较好地判断粒子大小,但当气溶胶以细粒子为主时(V_(fine)/_(total)>0.7),该方法会有较大出入,此时a_2,a_1可以有效地辅助α判断粒子大小,α>0.75,a_2<-0.5或a_2<-0.5,a_1<-1.0是较好的判据。此外,分析发现国外研究提出的用a_2-a_1判断粒子大小的方法效果并不理想,尤其在1<a_2-a_1<2的情况下,粒子的组成有多种可能。

Aerosols play important roles in earth＇s climate system by scattering and absorbing radiance and changing surface radiation budge, so they are recognized as one of the most important factors inducing global dimming during 1960 to 1990. On the other side, aerosols can also act as cloud condensation nuclei, changing the optical and microphysical property, altering hydrological cycle. The aerosol particle size is an important factor which could determine the aerosol physical and optical characteristic, so the knowledge of the particle size information is an urgent and necessary job for aerosol research. Angstrom exponent （a） is a convenient parameter for describing particle size, in general, as a〈l.0, it means coarse particle is dominant; as a〉l.0, it means fine particle is dominant. However, this application is based on Angstrom power law which is just valid for the Junge size distribution, not suitable for all environments and locations. Improper application would introduce considerable uncertainty and lead to misunderstanding. Fortunately, previous measurements note that when the aerosol doesn＇t meet Junge size distribution, the Angstrom exponent would vary with wavelength, and the spectral curvature of the Angstrom exponent （a2, al） is useful for describing particle size. Analysis is conducted to find the relationship between these parameter and particle size based on Aeronet Level 2.0 data from Beijing, Taihu, Xinglong, Xianghe sites. The data contain 440 nm, 675 nm, 870 nm, 1020 nm AODs, and particle volume concentration, the data temporal series of each site is more than one year, a, a2, al are derived by least square method. The results show that although theaerosol is mixed by fine and coarse particle sometimes, a is still larger than 1.5 which indicate that aerosol mainly consists of fine particle in typical Angstrom law. Correspondingly, median a （0.8〈a〈1.2） also appears for fine particle condition at which a should be larger than 1.2 in typical Angstrom law. On the other side, when a is less than 0.75, the aerosol is dominated by coarse particle certainly which is in accord with Angstrom law. It means that a is a good particle size indicator for coarse particle but not for all conditions. Although a2, al contain particle size information, they are not sufficient for describing particle size just by themselves, they could be nice comple ment to a. There is significant relationship between particle size and a, a2, a~ , when a^0.75, a2〈-0.5 or a2〈 -0.5, al〈-1.0, it means that aerosol is dominated by fine particle（Vfine/Vto〉O.7） ; when a〈O. 75 or a2〉 -0.25, a 1）-- 1.0, it means that aerosol is dominated by coarse particle（V,~,,~/V,ot^IGO. 2）. It has been proposed that a2-al is a first approximation of a, which could also be a good particle size indicator. But the examination shows that a2-a1 does not perform as well as expected, especially when 1〈a2-a〈2, the particle size is very complicated, thus a2-al provides no assistance to judgment.