## 2 Materials and Methods
### Model Setup and Description
It is important to remark that in this scheme the dust emission is controlled by the saltation of larger particles that are triggered by wind shear at the surface leading to the emission of smaller particles by saltation bombardment. The vertical bulk dust flux is calculated as ([PERSON], 2018; [PERSON], 2018)  
\[\left\{ \begin{array}{cc}F_{bulk}=G\times EROD^{\gamma}\times\beta&z_{0}\leq 0.2\;\text{m}\\ F_{bulk}=0&z_{0}>0.2\;\text{m}\end{array} \right. \tag{1}\]  
where \(G\) is the total streamwise horizontal saltation flux (([PERSON], 2018), eq.13), \(\beta=10^{0.134(\% clay-6)}\) is the sandblasting mass efficiency calculated considering only the soil clay fraction; \(\gamma\) is the exponential tuning constant for erodibility. The fraction of erodible grid cell is represented by the \(EROD\) parameter which is provided by the WRF-Chem preprocessor (WRF Preprocessing System). Once the total bulk emission (\(F_{bulk}\)) is established, size-resolved dust emission fluxes (g cm\({}^{-2}\) s\({}^{-1}\)) are obtained according to a size distribution that is hardcoded in the AFWA dust emission subroutine. The University of Cologne (UoC-S01) scheme is activated in WRF-Chem by setting dust_opt = 4 and dust_schme = 1, in the chem-section of the WRF input namelist. It \begin{table}
\begin{tabular}{c c c} \hline
**Dust-Bin** **UoC-S01** & **Lower-Upper Diameter (\(\upmu\)m)** & **Effective Diameter (\(\upmu\)m)** \\ \hline
1 & 0–2.5 & 1.25 \\
2 & 2.5–5 & 3.75 \\
3 & 5–10 & 7.5 \\
4 & 10–20 & 15 \\ \hline \end{tabular}
\end{table} 
Table 4: Effective diameter and size bin range for the University of Cologne UoC-S01 scheme. \begin{table}
\begin{tabular}{c c c} \hline
**Dust-Bin AFWA** & **Lower-Upper Diameter (\(\upmu\)m)** & **Effective Diameter (\(\upmu\)m)** \\ \hline
1 & 0.2–2 & 1.1 \\
2 & 2–3.6 & 2.8 \\
3 & 3.6–6 & 4.8 \\
4 & 6–12 & 9 \\
5 & 12–20 & 16 \\ \hline \end{tabular}
\end{table} 
Table 3: Effective diameter and size bin range for the Air Force Weather Agency AFWA scheme. was firstly introduced by [PERSON] [30] and further documented in [PERSON] et al. [48], [PERSON] and [PERSON], [22], [PERSON] et al. [9; 25], [PERSON] et al. [23], and [PERSON] et al. [49]. The UoC-S01 scheme may be considered a spectral emission scheme because it is based on a size-resolved dust emission equation by supposing that particles are divided into a finite number of particle size intervals. Then the total dust flux (F) is expressed as an integral of the dust emission rate  
\[F=\int_{d_{i}}^{d_{s}}\int_{0}^{d_{i}}f(d_{i},d_{s})p(d_{i})p(d_{s})\delta d_{i} \delta d_{s} \tag{2}\]  
where \(f(d_{i},d_{s})\) is the emission rate for particles of size \(d_{i}\) generated by the saltation of particles of size \(d_{s}\)[9; 30]. Full details of the UoC-S01 formulation may be found in [PERSON] [30], [PERSON] et al., [48] and [PERSON] et al. [23]. In this context, UoC-S01 and AFWA schemes in WRF-Chem follow the same general algorithm. The main difference between the two schemes is that the more elaborated UoC-S01 algorithm uses size-resolved saltation particle bins already at the level of the saltation process. Another difference concerns the calculation of the threshold friction velocity, the UoC-S01 scheme uses an additional correction factor caused by surface roughness elements, and commonly referred to as the drag partition correction [23]. Concerning the calculation of the threshold friction velocity, the differences between the two schemes is showed in Figure 11 of Appendix A. It is evident the largest positive difference of the quantity (\(u_{*}-u_{*t}\)) for the AFWA scheme (panels b, d) compared to UoC (panels a, c). This quantity is of primarily importance because it defines the emission of dust at each appropriate grid point. The removal processes consider both dry and wet deposition. A dry deposition scheme [50] accounting for gravitational settling and surface deposition is used to simulate the dry removal of mineral dust.