Atmospheric Aerosol Nonsphericity Over the Loess Plateau in Northwest China

Aerosol nonsphericity, which is not well depicted in model calculations, seriously affects aerosol optical properties and subsequently alters the radiative forcing of the earth-atmosphere system. Based on aerosol backscattering linear depolarization ratio data observed by a polarization lidar at the Semi-Arid Climate and Environment Observatory of Lanzhou University (SACOL) from September 2009 to August 2012 and numerical computations, the spatial and temporal distribution of the aerosol depolarization ratio, parameterization of the derived aspect ratio and influence of water vapor on aerosol nonsphericity were investigated. The aerosol depolarization ratios decreased with increasing height. Aerosol nonsphericity exhibited considerable seasonal variations with a pronounced maximum in spring, when more nonspherical aerosols were transported upward to the free troposphere. The column-averaged lidar depolarizationratios were 0.13, 0.09, 0.08, and 0.10 in the spring, summer, autumn and winter, respectively.Aerosol aspect ratios were derived by combining the lidar observed depolarization ratios and numerical computations. The derived aspect ratios ranged from 1.00 to 1.30, and the frequency distribution was akin to a log-normal distribution that peaked at approximately 1.06. A modified lognormal function was fitted to the frequency distribution of the derived aspect ratios. When the precipitable water was quite small, there was no obvious dependency between the aerosol depolarization ratio and precipitable water. However, in summer, when there was sufficient precipitable water, there was a significant decreasing trend of the depolarization ratio with increasing precipitable water. Moreover, the precipitable water explained 80.8% of the variation in the averaged depolarization ratio in summer.