Vertical flux density and frequency profiles of wind-blown sand as a function of the grain size over gobi and implications for aeolian transport processes

Vertical profiles of wind-blown sand as a function of the grain size are significant to better understand the microscopic process of heterogeneous saltation. Here, vertical flux density and frequency profiles of wind-blown sand as a function of the grain size over three typical gobi surfaces during three transport events were revealed. The results indicate that given the three gobi surfaces examined, the sand flux density of smaller sand particles (69–316 μm) exponentially decayed with the height, while the sand flux density of larger grains (363–976 μm) gradually deviated from the above exponential decay with the height and exhibited nonmonotonic variation with the height. The frequency of coarse grains (209–976 μm) continuously increased with the elevation until an inflection occurred at a certain height above the ground (0.17–1.3 m), and above the inflection point, the frequency of coarse grains exponentially decreased with the height. However, the frequency of fine grains (67–163 μm) initially decreased with the elevation. This trend was reversed at heights ranging from 0.17 to 0.73 m above the ground, after which the frequency exponentially increased. In contrast, the frequency profile of ∼180-μm diameter sand grains revealed an exponential decay curve throughout the entire elevation range examined. These results indicate that grains larger than 180 μm participated in the grain-bed collision process over gobi, and the rebound height was positively related to the grain size, while grains smaller than 180 μm were more notably affected by turbulence.