Seasonal Characteristics of Aerosol Optical Properties at the SKYNET Hefei Site (31.90°N, 117.17°E) from 2007 to 2013

Seasonal characteristics of aerosol optical properties in Sky Radiometer Network (SKYNET) Hefei site are studied using a sky radiometer from March 2007 to May 2013. The aerosol optical depth (AOD), Angstrom exponent (AE), volume size distributions, single-scattering albedo (SSA), refractive index, and asymmetry factor (ASY) of aerosols are simultaneously retrieved using the SKYRAD.pack version 4.2 software. During the study period, the AOD varied seasonally, with themaximum value of 1.02±0.42 at 500nm occurring in the summer, and the highest AOD (1.13±0.42) occurred in June due to stagnant climate conditions and accumulation of polluted aerosols before the East Asian summer monsoon. The variation in AE showed a different pattern, with the minimum (0.97±0.28) and maximum values (1.30±0.22) occurring during the spring and fall seasons, respectively. The relatively low value of AE in spring is related to the emission of Asian dust events. The aerosol volume size distributions can be expressed by the trimodal patterns for each season, consisting of a fine mode with R< 0.6 μm, a coarse mode with R> 2.5 μm, and a middle mode located between them. The real part of the refractive index increased with wavelength (380–870nm) while the imaginary part of the refractive index decreased for all seasons except for the summer. The seasonal mean values of SSA were 0.97±0.02 (summer), 0.95±0.03 (spring), 0.93±0.04 (autumn), and 0.91±0.04 (winter) at 380nm indicating more absorbing aerosol in the autumn and winter months. Furthermore, aerosol properties were greatly modified by condensation growth as evidenced by the positive dependencies of AOD, SSA, and ASY on relative humidity.