Transport Patterns, Size Distributions, and Depolarization Characteristics of Dust Particles in East Asia in Spring 2018

The long-range transport of Asian dust and its complex interaction with anthropogenic pollutants have a significant yet poorly quantified effect on the global climate. In this study, dust events were observed based on an optical particle counter with a polarization detection module at an urban site in North China. The temporal variations in the size distribution and morphological changes of dust particles were analyzed. The results demonstrate that the dust events were induced by a deep Siberian low-pressure anomaly and/or Mongolian cyclone in the low troposphere. The morphology of dust particles, as indicated by the depolarization ratio (δ = s/(s + p), the ratio of the parallel signal to the total backscattering signal), varied significantly at different transport heights. A decreasing tendency was found when dust was trapped within the boundary layer over polluted areas, particularly when the atmospheric loading of pollutants and the relative humidity were high. During a typical polluted dust case, the δ value of mineral dust (Dp = 5 μm) decreased by 39%, as the conditions were stagnant in East China, consistent with the Cloud-Aerosol LIDAR with Orthogonal Polarization results. The morphological variation was evaluated according to the aspect ratio, presuming that the dust particles were ellipsoidal. The interaction between dust particles and water vapor could lead to an increase of at least 6% in volume when the relative humidity increased up to 90%. An aerosol classification based on δ values and size information could be performed to estimate the contributions of different aerosol types to pollution days. The results should be helpful for evaluating satellite-borne and ground-based light detection and ranging data analyses.