干旱区生态环境知识资源中心

Whether grazed semi-arid grassland can be regarded as a net-sink or net-source of aeolian dust is difficult to detect since deposition and emission processes are in gradual transition. In grassland, dust arrives from sources far away or is directly emitted and immediately part of the suspension load. The processes of dust emission or deposition can be mainly identified by vertical concentration measurements close to the ground and close to the sources. Often the concentration differences at the used measuring heights are too small to derive the direction of vertical fluxes precisely, especially at dust storms of light-to-medium intensities. For this reason, particle mass (PM10, PM1) and particle number concentrations were measured simultaneously at ground level and at heights of up to 80 m using a dust monitoring system lifted by a kite. The measured data of dust concentration gradients are used to calculate the vertical fluxes with the necessary certainty to derive the direction of the dust fluxes. Additionally, dust arriving from different directions and crossing different land use/land cover patterns was used to interpret the interaction of source/sink relationships along the transport direction upstream of the measuring point. Results show that dust measurements above grassland have to span greater height differences to derive vertical fluxes certainly, i.e., concentration gradients (Delta c/Delta z) of PM10 and PM1 provide an indication of the direction of the dust flux. Furthermore, the results indicate a close relationship between the particle size compositions of the dust and land use/cover patterns. Arable land in the surroundings causes an emission flux (vertical upward), especially of the coarser fractions >8 mu m, whereas grassland causes deposition fluxes of size classes >2 mu m during a dust storm of low intensity. Sources from far away are reflected by higher concentrations of finer particles, measured during a super-regional dust storm. Both, dust concentration gradients and particle size composition reflect the potential source areas and deposition processes adequately with regard to different land use/cover patterns up to 50 km away.