Ground-based observation of aerosol optical properties in Lanzhou, China

Aerosol optical properties from August 2006 to July 2007 were obtained from ground-based and sky radiance measurements in Semi-Arid Climate and Environment Observatory of Lanzhou University (SACOL),China.Higher aerosol optical thickness (AOT) associated with lower (A)ngstr(O)m exponent (α) was mainly observed in spring,which was consistent with the seasonal dust production from Hexi Corridor.The maximum monthly average of AOT 0.56 occurred in March of 2007,which was two times larger than the minimum 0.28 in October of 2006.Approximately 60% of the AOT ranged between 0.3 and 0.5,and nearly 93% of α varied from 0.1 to 0.8 occurred in spring.The significant correlation between aerosol properties and water vapor content was not observed.The aerosol volume size distribution can be characterized by the bimodal logarithm normal structure: fine mode (r < 0.6 μm) and coarse mode (r > 0.6 μm).Aerosols in spring of SACOL were dominated by large particles with the volume concentration ratio of coarse to fine modes being 7.85.The averages of asymmetry factor (ɡ) in the wavelength range 440-1020 nm were found to be 0.71,0.67,0.67 and 0.69 in spring,summer,autumn and winter,respectively.