# Dust Emission Thresholds in Loess Soil Under Different Saltation Fluxes
## 4 Discussion  
The soil samples tested in this study (Figure 1) represent a range of soil texture in dust sources of sandy and non-sandy soils. The most productive areas for dust are located mainly in arid regions, such as playsa (dry lakes) in the Mojave Desert with about 35% sand content [24]. In the Middle-East and North Africa [25; 26; 27], the source areas are characterized by a silt-loam (loess) soil with a clay fraction up to 30% [28], including loess soils in the northern Negev that are subjected to intensive agricultural activities [12; 29]. Major dust sources in the Chinese and the Mongolian deserts are identified with a range of 0.7-11.9% clay, 1.8-34.1% silt, and 2.3-53.0% sand [30]. Unlike common dust sources, sand dunes have a low content of silt and clay (~2%). However, recently they were considered to be dust sources [15; 23] and as such a sample of sand dune was tested in this study. The results of the dust PM show an increase concentration as the amount of the clay-silt fraction in the sample is higher (Figure 3). [PERSON] et al. [16] have presented a similar trend with highest PM10 flux for a sample with clay-silt content of 85%. Basically, the increase of PM10 concentration under such conditions is related to the higher availability of dust content in the sample. Since the amount of dust particles in the soil samples \"sand 90%\" and \"sand 75%\" is relatively low, it can be assumed that the dust emission at velocity \(>\) 0.4 m s\({}^{-1}\) could have been an underestimation for such soils with unlimited dust-particle supply [10; 16]. In the sample of \"sand 50%;\" however, the highest PM10 concentration was recorded in the strongest shear velocity (0.52 m s\({}^{-1}\)), which indicate on sufficient dust content in the sample for continues emission. the distinct dust threshold for all samples is at 0.24 m s\({}^{-1}\), while in sample \"sand 75%\" a PM concentration was recorder already at 0.19 m s\({}^{-1}\). As for the \"sand dune\" sample, no dust concentration was recorded at all shear velocities. Though, recent studies have shown the potential of dust generation from sand. [PERSON] et al. [23] showed that under wind shear velocity of 0.3 m s\({}^{-1}\) the PM10 fluxes in the Oceano sand is smaller than those fluxes from most non-sandy soils (~1 and 150 ug m\({}^{-2}\) s\({}^{-1}\), respectively). [PERSON] et al. [15] recorded PM10 concentration of 0.01 ug m\({}^{-3}\) from sand dune with \(<\)2% clay content under wind shear velocity of 0.3 m s\({}^{-1}\) in the wind tunnel experiment. They concluded that the dominant dust emission mechanism from sand dunes is clay coatings removal, with a relatively small contribution from re-emission of loose-settled dust. In sands containing higher amounts of dust-sized particles, the relative contribution of the re-emission mechanism increases drastically. It is reasonable to assume that in our study there was a dust emission from the \"sand\" sample by the clay-coating removal. However, our measurement resolution in this study for samples with \(<\)10% dust content were probably not sensitive enough to distinct PM10 concentration values that are significantly above the background value of the experiment (30 ug m\({}^{-3}\)). Dust emission is significantly associated with the saltation process. In all the samples of this study, a distinct saltation starts at shear velocity of 0.31 m s\({}^{-1}\) (Figure 4). A saltation threshold in such velocity was obtained in many studies on sand dunes [8] as well as the trend of increasing sand flux with the shear velocity (Figure 4). The positive linear-correlation between the saltation flux and the PM10 flux was relatively high (R\({}^{2}\) = 0.91-0.99) in all the non-sandy samples (Figure 5). Strong correlations between saltation and dust flux were found also in previews studies on dust emission from non-sandy soils by wind tunnels [17; 31; 32].