Grain-size distribution of fall-outs of an exceedingly large dust storm in Harbin City and its implications

Abstract：Grain-size analysis was performed on sand and dust fall-outs of an exceedingly large dust storm that occurred in Harbin, Heilongjiang Province, on March 20, 2002. The results indicate that sand and dust fall-outs are silty sandy loam. The silt-sized (4–63 μm) fraction predominates, making up 71.18%; the sand-sized fraction (>63μm) accounts for 21.7% and the clay-sized (<4μm) fraction is scarce, accounting for only 7.13%. The grains are of trimodal distribution: the coarse silt-sized (29.2–34.7 μm) fraction forms the first main peak, making up ~7.4%; the medium sand-sized (420.5–500 μm) fraction forms the second main peak, accounting for ~1.29%; and the clay-sized (0.69–0.82 μm) forms the third main peak, accounting for ~0.52%. The mean grain diameter Mz is 28.4 μm, with a sorting coefficient of 1.81, a skewness of 0.044, a kurtosis of 1.494 and a silt/clay ratio of 7.3. The grain-size distribution shows that sand-dust fall-outs in Harbin are the coarsest ones reported ever since in China. The fall-outs were a mixture of sands and dusts derived from different sources and transported over long and short distances, and coarse particles were near-sources materials that were transported by low-altitude airflow. Harbin and its peripheral areas should be the dust fall-out center of this exceeding large dust storm and belong to the center or near-central zone with relatively strong atmospheric dynamic conditions. Nude loose soils, river beds and floodplain fine alluvium, sand-dust of city construction sites and so on in the urban expansion areas of Harbin may all be main material sources of sand and dust fall-outs of Harbin. The focus of the work for preventing sand-dust storms in Harbin should be on controlling sand-dust source areas at the peripheries of Harbin.