## 2 Materials and Methods
### Dust Emission Experiment  
The dust experiment was performed by a boundary layer wind tunnel that was used in previous studies [26; 30; 31]. Boundary-layer wind tunnels enable the production conditions for the natural aeolian process that occur in the field and make it possible to provide quantitative information on the emission of dust from various soils [32]. The wind tunnel has a cross sectional area of 0.5 \(\times\) 0.5 m open-floored working sections with a length of 7 m. Air push or air suction flow in the tunnel is generated by an axial fan up to maximum velocity of 18 m s\({}^{-1}\). Instruments installed in the test section of the tunnel enable quantification of: (1) wind profile for the calculation of frictional velocity, (2) saltation flux, and (3) dust concentrations of PM10. The soil samples were tested in the wind tunnel under a range of fan frequencies (11-41 Hz). These frequencies represent a range of common wind velocities that are associated with dust emissions in the field. The wind velocity profile was measured for each fan frequency at several 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 microwave anemometer with vane diameter of 14 mm that measures wind velocities at the range of 0-30 m/s with reading resolution of 0.1 m/s (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 [25]:  
\[\frac{u}{u_{*}}\ =\ \frac{1}{K}ln\left(\frac{z}{z_{0}}\right) \tag{2}\]  
where \(u\) is the wind velocity (m/s) at height \(z\) (m), \(z_{0}\) is the aerodynamic roughness length (m) of the surface \(u^{*}\) is the shear velocity (m/s), and \(K\) is [PERSON]'s constant (\(\approx\)0.4). The soil was placed on the tunnel bed at a soil layer of \(\sim\)2 cm. Each soil sample was tested for all fan frequencies in three replicates--overall 90 runs were performed by the wind tunnel (3 samples, 3 replicates, 10 wind velocities). Each run lasted for 30 s per velocity continuously in order to detect trends of dust emission and saltation. A saltation trap was placed in the wind tunnel to collect saltators in horizontal transport. The saltation mass in the trap was analyzed for the relative contents of sand and dust (\(<\)63 \(\upmu\)m) fractions. The mass of the transported particles was calculated as saltation flux (Q, kg m\({}^{-1}\) s\({}^{-1}\)) that crosses a width of 1 m, e.g., [33]. Dust concentration of particulate matter that is \(<\)10 \(\upmu\)m in diameter (PM10, \(\upmu\)m g m\({}^{-3}\)) was recorded by a light-scattering device (DustTrak DRX 8534, TSI, Shoreview, MN, USA), which was placed at 15 cm above the tunnel bed, in the range of 0.001-150 \(\upmu\)m\({}^{-3}\) (\(\pm\)0.1% of reading) at 1-s interval. The recorded PM10 concentrations were converted into mass flux emitted from the soil surface (kg m\({}^{-2}\) s\({}^{-1}\)) based on the wind tunnel dimensions and area of the sand bed [34]:  
\[F_{PM}\ =\ (C_{PM}-C_{bg})\times V_{air}\times A_{cs}/A_{p} \tag{3}\]  
where \(C_{PM}\) is the recorded PM concentrations (kg m\({}^{-3}\)) from the soil, \(C_{bg}\) is the average PM background concentration (kg m\({}^{-3}\)), \(V_{air}\) is the mean horizontal wind velocity i over height (m s\({}^{-1}\)), \(A_{cs}\) is the cross-section (height \(\times\) width) of the wind tunnel (m\({}^{2}\)), and \(A_{p}\) is the area (m\({}^{2}\)) of the experimental plot (length \(\times\) width).