Seasonal statistical characteristics of aerosol optical properties at a site near a dust region in China

[1] A ground-based sky radiometer was used to measure direct and diffuse solar irradiances at Dunhuang, China from January 1999 to March 2001. The aerosol optical thickness (AOT), Angstrom exponent (a), volume size distributions, single scattering albedo and refractive index of aerosols were simultaneously retrieved using the ‘‘SKYRAD’’ inversion code and their seasonal variations and statistical characteristics were studied. The results reveal that during the study period, the AOT at Dunhuang varied seasonally, with the maximum AOT occurring in the spring and the minimum AOT occurring in the fall. The variation in a showed an opposite pattern, with a minimum in the spring and a maximum in the fall. A simple exponential function can express the relationship between AOT and a. The frequency distributions of AOT and a approximately follow a lognormal probability distribution and a normal probability distribution, respectively. The aerosol volume size distributions can be characterized by the sum of two lognormals distributions, and represent an accumulation mode with a radius of about 0.25 um, and a coarse mode with a radius of about 7.7 um. A pseudomode with a radius of about 1.69 um located between the accumulation mode and coarse mode is present in the springtime. The single scattering albedo showed a slight increasing trend with wavelength in spring, summer and autumn and a decreasing trend with wavelength in winter. The maximum value of the real part of the refractive index occurs at 400 nm, and the minimum value occurs at 875 nm for all seasons. Two Gaussian models were developed to describe the frequency distribution of the real part of the aerosol refractive index; the results indicate that the differences between the fits are greatest in the spring. The statistical characteristics of the frequency distributions of aerosol properties might provide a way to identify and estimate aerosol optical properties in areas located near dust regions.