Ground-based remote sensing of aerosol optical properties in one city in Northwest China

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, China. The aerosol particles optical depth (AOD), Angstrom exponent (ALPHA), volume size distributions, refraction index and single scattering albedo of aerosols were simultaneously retrieved using the ‘SKYRAD’ code. 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 diurnal variability of AOD was significant, showed a similar variation pattern in spring, summer and autumn and had another variation pattern in winter. The frequency distributions of AOD and ALPHA approximately follow 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-normal distributions, and represented an accumulation mode with a radius of about 0.15 μm, and a coarse mode with a radius of about 7 μm, which was the main aerosol mode in this area. 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, but no clear patterns were found. 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 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.