Seasonal Variation Of Aerosol Optical Properties On Near-source Regions And Their Statistical Description

A ground-based sky radiometer was used to measure direct and scattering solar irradiances, as well as aureole radiances, from October 2003 to August 2004 over Yinchuan, one near dust region site, China. The aerosol particles optical depth (AOD), volume size distributions, refraction index and single scattering albedo of aerosols were simultaneously retrieved using the 'SKYRAD' model. The seasonal variation and statistic characteristics of aerosol optical properties and the aerosol direct radiative forcing efficiency were given. The results reveal that during the study period, the AOD varied seasonally, with a vernal maximum and a winter minimum, and the variation in ALPHA was opposite, with a vernal minimum and a winter maximum. The range of seasonal diurnal variability of AOD was 31%, 33%, 24% and 38% in spring,summer,autumn and winter, and showed a similar variation pattern in spring, summer and autumn and had another variation pattern in winter. The range of seasonal diurnal variability of ALPHA in spring was about 50%, and the range seasonal diurnal variability of ALPHA in summer,autumn and winter were smaller than that in spring, about 20%. And there were difference in spring between in other seasons.The frequency distributions of AOD and ALPHA approximately followed a log-normal probability distribution and a normal probability distribution, respectively. The relationship between ALPHA and AOD showed a simple dependence of ALPHA on AOD in spring and can be characterized by one integrated exponential function. And no clear relationship between AOD and ALPHA was evident in other seasons.The aerosol volume size distributions can be characterized by the sum of two log-normals distributions, and represented an accumulation mode with a radius of about 0.15 um, and a coarse mode with a radius of about 7 urn, which was the main aerosol mode in this area. The aerosol single scattering albedo in spring was much higher than in other seasons, and increased dramatically with increase in wavelength, from 0.92 at 400 nm to 0.97 at 1020 nm. In other seasons, it showed a slightly decreased with wavelength. The real parts of the refraction index rose in spring, and exhibited a low sensitivity to wavelengths; the imaginary parts of the refractive index decreased noticeably in spring and also showed a low sensitivity to wavelength. Both their low sensitivities to wavelengths slightly varied with time, and the imaginary parts were more complicated. The aerosol direct radiative forcing efficiency at the short wavelength is -55.31, -59.71, -60.13 and -60.38 at surface and 13.89, 10.39, 2.30 and 2.64 at TOA in spring, summer, autumn and winter, respectively. The value at surface/TOA in spring is lower/higher than that in other seasons, indicating the influence of the dust aerosol.