# Dust Emission Thresholds in Loess Soil Under Different Saltation Fluxes
## 1 Introduction  
The majority of aerosol mass in the atmosphere is related to soil-derived dust particles produced by aeolian (wind) processes [1]. Annual global dust emissions from soils into the atmosphere are estimated to be as high as 3000 million tons, including particulate matter (PM) that is less than 10 micrometers in diameter (PM10). The emission of PM10 from soils is directly associated with loss of soil nutrients (clays and organic matter) [2], air pollution, and substantial health risks [3, 4]. Other environmental impacts of atmospheric dust refer to the Earth's global energy balance and climate sensitivity through radiative effects and climate feedbacks [1]. Considering these various important impacts of soil dust on the Earth system, it is critical to estimate accurately the dust emission. Although models that estimate the dust emission from soils have been improved [5, 6], still there is large disagreement between the empirical results and their estimated values by the models [1]. These discrepancies are partly a result of our gap in understanding the threshold wind friction velocity needed to mobilize soil particles.  
Dust emission depends on the soil properties and on the wind shear stress exerted on the soil surface. The shear stress is characterized by the friction velocity (u,), which express the velocity gradient in boundary layer flow. The threshold friction velocity (u,t) is then the minimum value of the shear velocity (u,) for which the soil experiences dust emission [7, 8]. This threshold friction velocity has been the subject of many experimental studies, starting with the pioneering work of [PERSON] [9]. His work revealed that, for a bed of monodisperse (single particle size) beds, the threshold friction velocity has a minimum around 100 \(\upmu\)m. Larger particles are more difficult to mobilize because of the increased gravitational force, and smaller particles are more difficult to mobilize because of theincreased cohesive forces [10]. The cohesive forces of the clay and fine silt particles limit the dust emission form soil by direct aerodynamic lifting of loose particle [8]. Consequently, dust is normally emitted by the impacts of more easily mobilized sand-sized particles. These \"saltators\" undergo ballistic trajectories that impact the soil surface. The resulted saltation process enables the entrainment of cohesive dust particles by the disintegration of the soil aggregates [7; 8; 11]. However, the threshold of motion of monodisperse beds of sand particles is unrealistic because dust-emitting soils contain a wide range of particle sizes (<1 \(\upmu\)m to ~1000 \(\upmu\)m).  
Many soils throughout the world are subjected to increase human activities, and dust emission, in particular, in semi-arid climates [12]. The soil disturbance resulted in disintegration of the soil aggregates, which produce small aggregates and loose sand particles that are available for saltation [11]. It is generally assumed that soils with a higher amount of large aggregates have stronger resistance against erosive forces [13]. Yet, the effect of the soil particle size distribution on the saltation and dust emission threshold is poorly understood, and hence poorly parameterized in dust emission models. Consequently, predictions of future environmental changes and the resulting changes in soil properties and wind erosion are limited by this missing information of how dust emission threshold are affected by the particle size distribution.  
The main goal of this study was to examine the dependence of dust emission threshold on soils with different distribution of dust and sand-sized particles. To this end, we analyzed soil samples that are characteristics of a semi-arid loess soil and conducted a targeted experiment on dust emission with a boundary-layer wind tunnel to provide empirical information on the link between soil particle size distribution and saltation and dust emission thresholds.