# Dust Emission Thresholds in Loess Soil Under Different Saltation Fluxes
## 2 Materials and Methods
### Wind Tunnel Experiment  
The experiment on dust emission threshold was conducted with a boundary layer wind tunnel [16; 17; 18]. The wind tunnels enable aeolian simulations under standardized quasi-natural wind conditions and provide quantitative information on aeolian particle transport in the field and dust emission rates from soils [19; 20; 21; 22]. The wind tunnel has a cross sectional area \(0.5\times 0.5\) m with open-floored working sections of up to 10 m length. The tunnel fan was operated at nine fan frequencies (8, 11, 14, 17, 23, 26, 29, 32, 35, and 38 Hz) to represent a wide range of wind velocities. The wind velocity profile was measured for each fan frequency at different heights above the tunnel bed: 0.02, 0.035, 0.05, 0.075, 0.10, 0.15, 0.20, 0.25, 0.30, 0.35, 0.40, and 0.45 m. The wind measurement was conducted with a micro-vane anemometer with vane diameter of 14 mm that measure wind velocities at the range of \(\sim\)0-30 m s\({}^{-1}\) with reading resolution of 0.1 m s\({}^{-1}\) (KIMO vt 200, Ontario, Canada). The data are logged at time interval of 5 s for each test. The data were analyzed to determine the height of the boundary layer, the average wind velocity in the boundary layer, and the wind shear velocity (u\({}_{*}\)). The wind shear velocity is expressed by the Prandtl-von Karman equation [8]:  
\[\frac{\mathrm{u}}{\mathrm{u}_{*}}=\ \frac{1}{\mathrm{K}}\ln\left(\frac{\mathrm{ z}}{\mathrm{z}_{0}}\right) \tag{1}\]  
where u is the wind velocity (m s\({}^{-1}\)) at height z (m), z\({}_{0}\) is the aerodynamic roughness length of the surface (m), u\({}_{*}\) is the shear velocity (m s\({}^{-1}\)), and K is [PERSON]'s constant (\(\approx\)0.4). Each soil sample was tested under all the fan frequencies/wind velocities. Overall 108 tests were conducted in this study (4 soil samples, 9 wind velocities, 3 replicas). Each run was last 30 s to record the trend of sand transport and dust emission. A series of traps were placed in the tunnel along the wind direction. The transported particles were collected at height of 0-0.10 m above the tunnel bed. After the experiment, the sediments in the traps were analyzed for the relative contents of sand and dust (\(<\)63 um) fractions. In addition, the mass of the transported particles calculated as saltation flux (Q, kg m\({}^{-1}\) s\({}^{-1}\)) that cross a width of 1 m [15]. Dust concentrations (\(\mu\)g m\({}^{-3}\)) PM10 were recorded by a light-scattering device, DustTrak DRX 8534 (TSI, Shoreview, MN, USA), in the range of 0.001-150 \(\mu\)g m\({}^{-3}\) (\(\pm\)0.1% of reading) at sec-1 intervals, was placed at 15 cm above the tunnel bed. The recorded PM10 concentrations were converted into mass flux (Fpm) emitted from the soil surface (kg m\({}^{-2}\) s\({}^{-1}\)) based on the wind tunnel dimensions and the area of the sand bed [15]:  
\[\mathrm{F}_{\mathrm{PM}}=(\mathrm{C}_{\mathrm{PM}}-\mathrm{C}_{\mathrm{bg}}) \times\mathrm{V}_{\mathrm{air}}\times\mathrm{A}_{\mathrm{cs}}/\mathrm{A}_{ \mathrm{p}} \tag{2}\]  
Figure 1: **(a)** Location of the loess soil in the Negev (Israel), Eastern Mediterranean. **(b)** Particle size distribution of the soil samples used for the experiments: sand dune and various mixes of sand and dust fractions that were used for the wind tunnel experiment. The dust fraction, which used for the preparation the mixed samples, was extracted from a loess (bulk) soil. where C\({}_{\rm PM}\) is the recorded PM concentrations (kg m\({}^{-3}\)) from the soil, C\({}_{\rm bg}\) is the average PM background concentration (kg m\({}^{-3}\)), V\({}_{\rm air}\) is the mean horizontal wind velocity in m s\({}^{-1}\) (average over height), A\({}_{\rm cs}\) is the cross-section (height \(\times\) width) of the wind tunnel (m\({}^{2}\)), and A\({}_{\rm P}\) is the area (m\({}^{2}\)) of the experimental plot (length \(\times\) width).