## 2 Materials and Methods
### Model Setup and Description  
For this study, the WRF-Chem model version 3.6.1 has been utilized in the numerical domain depicted in Figure 1. It is important to remark that all the code alterations reported by [PERSON] et al. [38] and [PERSON] et al. [23], have been manually incorporated in this version. The numerical domain covers the northern regions of Sahara Desert and part of Europe, with 300 \(\times\) 300 points with a horizontal grid spacing of 10 km. The simulation started on 10 April 2018 (0000 UTC) and ended on 17 April 2018 (0000 UTC). Boundary and initial conditions are at 1-degree resolution and provided from NCAR/NCEP Final Analysis from Global Forecast System (FNL from GFS). Based on the WRF setup described in the work by [PERSON] et al. [39] for the Northern Sahara and the Mediterranean basin, the following physics parameterizations are utilized: the Mellor-Yamada-Nakanishi and Niino (MYNN) 2.5 level turbulent kinetic energy (TKE) parameterization is used to describe the planetary boundary layer and surface layer scheme [40]. The Rapid Update Cycle (RUC) Land Surface Model is chosen to represent the land surface processes [41]. The radiation physics is parameterized using the Rapid Radiative Transfer Model [42] for both short-wave (ra_sw_physics = 4) and long-wave (ra_lw_physics = 4). The Improved Bulk Microphysics Scheme of [PERSON] et al. [43] is used for the treatment of the microphysics processes (Table 1). The WRF-Chem setting related to the aerosol model concerns the hybrid bulk/sectional Georgia Tech/Goddard Global Ozone Chemistry Aerosol Radiation and Transport (GO-CART, [44]) model. This is implemented in WRF-Chem model selecting the namelist variable chem_opt = 300 (Table 2). As shown in Tables 3 and 4 it considers five and four dust bins with diameters in the range (0-20) \(\upmu\)m for the two dust emission schemes described below. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Case** & **Chem\_Opt** & **DUST\_Opt** & **Dust\_Schme** & **Scheme** \\ \hline run\_2 & 300 & 4 & 1 & UoC-S01 \\ run\_3 & 300 & 3 & - & AFWA \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Weather Research and Forecasting model, coupled with chemistry (WRF-Chem) chemistry setup. Figure 1: Numerical domain and position of the XMed-Dry Stations. Tables 3 and 4 show the effective diameter that is used in the specialized routine that calculates the gravitational settling deposition. In the WRF-Chem releases above 4.0 (June 2018) the number and diameter of dust bins are consistent between the two emission schemes. The physics of the emission of dust is described with great details in a recent review by [PERSON] et al. (2018), in which the authors have analyzed the principal mechanisms responsible for the process of soil particles mobilization. In particular, three basic processes are responsible for the entrainment of dust particles in the atmospheric flows: (1) aerodynamic lift, (2) saltation bombardment, and (3) particle disaggregation. These three processes generate the lifting of a wide size spectrum of soil particles, from sand sized grains at around 70 \(\upmu\)m by aerodynamic wind shear, down to smaller dust-sized particles at 0.1 \(\upmu\)m (saltation bombardment and disaggregation). There are actually three different dust emission schemes implemented into the WRF-Chem 3.6.1 model using the GOCART aerosol chemistry core (chem_opt = 300): the GOCART-WRF scheme (dust_opt = 1), the AFWA scheme (dust_opt = 3), and the University of Cologne (UoC-S01) scheme (dust_opt = 4). Both AFWA and UoC-S01 emission schemes were introduced in WRF-Chem, because the GOCART-WRF original model revealed some issues with the dust emission scheme ([PERSON], 2018), making this parameterization ([PERSON], 2018) not appropriate under certain environmental conditions ([PERSON], 2018). Specific details of the AFWA dust emission scheme, which may be considered a modification of the [PERSON] and [PERSON] (2018) emission model, is already described in a work by [PERSON] et al. (2018) so it will not be replicated here.