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

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 micro-physical 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 (α) is a convenient parameter for describing particle size, in general, as α < 1.0, it means coarse particle is dominant; as α > 1.0, 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, a₁) 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, α, a₂, a₁ are derived by least square method. The results show that although the aerosol is mixed by fine and coarse particle sometimes, α is still larger than 1.5 which indicate that aerosol mainly consists of fine particle in typical Angstrom law. Correspondingly, median α (0.8 < α < 1.2) also appears for fine particle condition at which α should be larger than 1.2 in typical Angstrom law. On the other side, when α is less than 0.75, the aerosol is dominated by coarse particle certainly which is in accord with Angstrom law. It means that α is a good particle size indicator for coarse particle but not for all conditions. Although a₂, a₁ contain particle size information, they are not sufficient for describing particle size just by themselves, they could be nice complement to α. There is significant relationship between particle size and α, a₂, a₁, when α > 0.75, a₂ < -0.5 or a₂ < -0.5, a₁ < -1.0, it means that aerosol is dominated by fine particle (V_fine/V_total > 0.7); when α < 0.75 or a₂ > -0.25, a₁ > -1.0, it means that aerosol is dominated by coarse particle (V_fine/V_total < 0.2). It has been proposed that a₂-a₁ is a first approximation of α, 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-a₁ < 2, the particle size is very complicated, thus a₂-a₁ provides no assistance to judgment.