Concentration, size-distribution and deposition of mineral aerosol over Chinese desert regions

Abstract

The mass-particle size distributions (MSDs) of 9 elements in ground-based aerosol samples from dust storm (DS) and non-dust storm (N-DS) periods were determined for 12 sites in 9 major desert regions in northern China. The masses of the 9 elements (Al, Fe, K, Mg, Mn, Sc, Si, Sr and Ti) in the atmosphere were dominated by local mineral dust that averaged 270 mg m−3, and the MSDs for the elements were approximately log-normal. On the basis of Al data, the < 10 mm particles account for ˜84% of the total dust mass over the deserts. Model-calculated (‘‘100-step’’ method) dry deposition velocities (Vd) for the 9 dust-derived elements during N-DS periods ranged from 4.4 to 6.8 cm s−1, with a median value of 5.6 cm s−1. On the basis of a statistical relationship between D99% (the dust particle diameter corresponding to the uppermost 1% of the cumulative mass distribution) and Vd, one can also predict dry velocities, especially when D99% ranges from 30 to 70 mm. This provides a simple way to reconstruct Vd for dust deposits ( like aeolian loess sediments in the Loess Plateau). The estimated daily dry deposition fluxes were higher during DS vs. N-DS periods, but in most cases, the monthly averaged fluxes were mainly attributable to N-DS dust. Two regions with high dust loading and fluxes are identified: the ‘‘Western High-Dust Desert’’ and the ‘‘Northern High-Dust Desert,’’ with Taklimakan Desert and Badain Juran Desert as their respective centers. These are energetic regions in which desert-air is actively exchanged, and these apparently are the major source areas for Asian dust.