## 4 Discussion  
Loess soils that are associated with dust emission are typically composed of aggregate size distribution at the range of less than 2 \(\upmu\)m to over 2000 \(\upmu\)m. In this study we examined soil samples with aggregate size fraction within this range. Previous studies on dust emission from loess soil reported on aggregate size distribution at similar range of the current study [38; 39; 40]. Yet information on the properties of each aggregate size is important for the study on dust emission from soil aggregates. The aggregate properties in the present study show that the common cement materials are not the major factor in the formation of large aggregates. The amount of organic matter, soil moisture, and carbonate in aggregates was relatively low and similar among the samples (Table 1). [PERSON] et al. [3] show that aggregates at size of 105-1000 \(\upmu\)m contain a large amount of fine sand compared to smaller aggregates containing more clay and organic matter. It can be assumed that the major factor that allows  
Figure 4: The particulate matter that is <10 \(\upmu\)m in dimeter (PM10) concentration calculated from the wind tunnel measurements of the reference samples under various shear velocities. The average concentration is presented by the solid line inside the minimum/maximum box, and standard deviations are in dashed lines. Note the differences in the value rang of Y axis. Figure 5: Nonlinear correlation between PM10 flux and saltation flux for the different soil samples. Note the different value range of saltation and PM fluxes between the soil samples. the large aggregates to exist in our soil is the amount of coarse-silt and fine sand (\(<\)200 \(\upmu\)m) in relation to the content of the clay in the aggregates. The coarse-silt and fine sand may accumulate with the clay to form larger aggregates. The wind tunnel was operated under a range of air velocities that start from low velocity that barely start aeolian processes to velocities that are directly identified with dust emission in various soils and specifically in loess soil [18; 26; 30; 31; 41; 42]. The wind profiles received in this study (Figure 2) have similar trends to profiles of previous works [26; 41]. The wide range of the wind shear velocities (0.24-0.52 m/s) enabled us to define the saltation and dust emission thresholds [25; 33]. According to the results of this study, the threshold value of dust emission in aggregates 63-500 \(\upmu\)m is a shear velocity of 0.31 m/s and in macroaggregates 500-1000 \(\upmu\)m is 0.39 m/s (Figure 5). The aggregate size is critical to determine the emission rate at any given wind velocity. For example, the highest rate of PM10 at velocity of 0.43 m/s is received from aggregates 250-500 \(\upmu\)m. In this size group all the aggregates are at saltator sizes, which move in saltation at the shear velocities of 0.31-0.52 m/s to provide high saltation mass. In the group of 63-250, only the fraction of 125-250 \(\upmu\)m is moving in saltation, while the smaller ones (63-125 \(\upmu\)m) may enter into suspension, and thus lower saltation and dust fluxes are received compared with the aggregate group 250-500 \(\upmu\)m. Saltation of larger aggregates (500-1000 \(\upmu\)m) requires more energy to be transported and release dust by aggregate breakage. Assuming that loose dust-sized particles (\(>\)63 \(\upmu\)m) in most soils are barely available for direct lifting due to cohesive forces, the main mechanism that allows the dust emission is the breaking of the aggregates by the saltation [25; 43]. In all samples, the emission of dust occurs only when the saltation begins, according to the threshold velocity of the different samples (Figures 3 and 4). The reduction in the aggregate size as a result of saltation and aggregate breakage was demonstrated by [PERSON] and [PERSON] [26].