## 2 Materials and Methods
### WRF-Chem Model
#### 2.3.1 Dust Schemes in WRF-Chem  
From version 3.7 onwards, the WRF-Chem model provides a choice of dust schemes. The most important dust schemes in the WRF-Chem model are GOCART, AFWA and University of Cologne (UoC) [58, 59, 22]. In this study, according to the volume of the study and also according to the previous research [60], GOCART and AFWA schemas were considered suitable for this region. As follows:  
GOCART Dust Scheme  
In the GOCART scheme, the particle source (S) function is defined as the ratio of erodible soil subject to the wind erosion process:  
\[\text{S}=\left(\frac{\text{z}_{\text{max}}-\text{z}_{\text{i}}}{\text{z}_{ \text{max}}-\text{z}_{\text{min}}}\right)^{5} \tag{2}\]  
where S can be interpreted as the probability of sediment accumulation at point i of height z; zmax and zmin are the maximum and minimum elevations in a range of \(10^{\circ}\times 10^{\circ}\) around the grid point, respectively. Dust particle emission fluxes in the GOCART scheme are defined by the following empirical relationship [61]:  
\[\left\{ \begin{array}{ll}\text{F}_{\text{p}}=\text{C}_{\text{G}}\text{S}_{ \text{p}}\text{U}_{10}^{2}(\text{U}_{10}-\text{U}_{\text{t}}),\text{ U}_{10}>\text{U}_{\text{t}}\\ 0&\text{otherwise}\end{array} \right. \tag{3}\]  
where Fp is (ug/m\({}^{2}\)s) represents the emission flux for size bin p, CG is a dimensionality coefficient (ug s\({}^{2}\) m\({}^{-5}\)), (CG = 1.2 ug s\({}^{2}\) m\({}^{-5}\)), sp is the fraction of each particle size class, S is the source function representing the fraction of alluvium available for wind erosion [41] and Ut is a threshold velocity at 10 m, U10 is horizontal wind speed at 10 m, and Ut is the threshold velocity (m s\({}^{-1}\)) below which dust emission does not occur and is a function of particle size, air density and surface moisture [41]. [PERSON] et al. [19] suggested that Ut \(\approx\) ut\({}_{\text{t}}^{\prime}\), where ut\({}_{\text{t}}^{\prime}\) is a threshold friction velocity, using the formula of [62] and considering the effect of soil moisture. The parameterization method proposed by [PERSON] and [PERSON] [62] for the dust emission was implemented as a scheme or dust module (AFWA) by the United States (US) Air Force Meteorological Center in the WRF-Chem model. In the AFWA scheme, the correction factor is applied to ut as follows:  
\[\text{u}_{\text{t}}=\text{u}_{\text{st}}\Big{(}\text{D}_{\text{p}}\Big{)} \frac{\text{f}(\text{moisture})}{\text{f}(\text{roughness})} \tag{4}\]  
where f (roughness) is a drag partition correction--if f (roughness) is 1, it means the surface is smooth; Value decreases with increasing amounts of large rocks, cobbles, vegetation, etc. f (moisture) is the soil moisture content, and Dp is the particle diameter in bin p. Thus, ut increases with increasing rockiness,vegetation, and other surface types with high roughness. The f (roughness) value for Southwest Asia is currently set to 1, because the great deserts of Saudi Arabia, Iraq, Syria, Iran, etc. are in this region. Vegetation in these areas is very limited, the land surface is bare and wind erosion systems are active in these areas. For these reasons, the f (roughness)t for these areas is estimated to be 1 (smooth surface). Dust emission is restricted to areas with roughness length \(Z_{0}\leq 5\) m (typically barren lands and sparsely vegetated surfaces) [63].