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

The mass-particle size distributions (MSDs) of 9 elements in ground-based aerosol samples from dust storm (DS) and Iron-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 mu g m(-3), and the MSDs for the elemi:nts were approximately log-normal. On the basis of Al data, the < 10 mu m particles account for similar to 84% of the total dust mass over the deserts. Model-calculated ("100-step" method) dry deposition velocities (V-d) 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 D-99% (the dust particle diameter corresponding to the uppermost 1% of the cumulative mass distribution) and V-d, one can also predict dry velocities, especially when D-99% ranges from 30 to 70 mu m. This provides a simple way to reconstruct V-d 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.