乌兰布和沙漠流动沙丘下风向降尘粒度特征

选择乌兰布和沙漠典型流动沙丘进行定点野外观测,分析了沙丘下风向不同距离及不同高度降尘的粒度特征及其来源。结果表明:乌兰布和沙漠降尘空间变化大,降尘以细沙为主,随着距离的增加,细沙物质逐渐减少,粉沙和极细沙含量显著增加。降尘平均粒径2.5~3.1 Φ,且随距离、高度的增加而变大。分选系数随着距离、高度的增加而变差; 2 m高度偏度近乎对称,若设沙丘高度为H,则只有在距离为8 H时正偏,高0.5 m处降尘在距沙丘0.5~1 H近对称,2~8 H皆是正偏;峰度随距离的增加逐渐变窄,且2 m高度的变化晚于0.5 m高度。乌兰布和沙漠降尘以局地物质为主,远源物质及区域物质比例很低。<70 μm的远源物质及区域物质的降尘与下风向距离呈正相关,主要来源于沙漠边缘的荒地、河湖沉积物、农田草地,而>70 μm的局地物质的降尘与下风向距离呈负相关,主要来源于沙丘下垫面沙粒的气流短距离输送。

Grain Size Characteristics of Dustfall in the Ulan Buh Desert

The dustfall at different height and distance along leeward direction of the typical shifting sand dunes in the hinterland of the Ulan Buh Desert was studied by means of continuous spot observation and particle size analysis. The results show that the spatial variability of the dustfall is significant in the Ulan Buh Desert. The dustfall is mainly dominated by fine sand. Fine sand is gradually reduced with the increase of the distance along leeward direction of the typical shifting sand dunes. Silt and very fine sand increased rapidly with the increase of the distance along leeward direction of the typical shifting sand dunes. The average grain size of the dustfall is in the range from 2.5 Φ to 3.1 Φ, and it grows with the increase of the distance and the height along leeward direction of the typical shifting sand dunes. The sorting coefficient becomes worse with the increase of distance and height along leeward direction of the typical shifting sand dunes. The skewness coefficients is close to symmetry at 2 m height, and positive at 8 H distance when the height of sand dune is H. But at 0.5 m height, it is symmetrical at 0.5 H-1 H distance and positive skewed at 2 H-8 H distance along leeward direction of the typical shifting sand dunes. Kurtosis changes to narrow gradually with distance increase, and the change at 2 m height is later than at 0.5 m height. It can be assumed that the dustfall of the Ulan Buh Desert is mainly from the local material, and the proportion of material from the far sources and regional material is small. The proportion of material from the far sources and regional material (<70 μm) is positively correlated to leeward distance and mainly from the waste land in desert fringe area, the sediment of rivers and lakes, farmland region. But the proportion of local material (>70 μm) is negatively correlated to downwind distance and mainly from the short distance transport of airflow in underlying surface of dune.