## 3 Results
### Comparison with MERRA2 Reanalysis and Surface PM\({}_{10}\)  
The final set of comparison is realized considering the dust column mass density (DUCM) and in situ particulate mass concentration (PM\({}_{10}\)) data. The variable DUCM represents the columnar content of dust aerosol particles in the atmosphere. This quantity is calculated in the WRF-Chem output system by multiplying for each grid point the dust mass mixing ratio (\(\mu\)g kg\({}^{-1}\)) with the dry-air density (kg m\({}^{-3}\)) and summing over each vertical layer depth (m). The resulting quantity is expressed in \(\mu\)g m\({}^{-2}\) and gives an indication of the amount of dust in the atmosphere. In the panels of Figure 6 we have reported the comparison of the daily averaged DUCM from WRF-Chem run_2 and run_3 with the corresponding M2T1 NXAER-DUCMASS variable of the MERRA2 Time-averaged, Single Level, Assimilation, Aerosol Diagnostics (V5.12.4) (GMAO, 2015). More in detail, in Figure 6 is reported the daily averaged maps for dust column mass density for 14 April (a-c), 15 April (d-f), and 16 April (g-i). Left column (a,d,g) represents run_2 outputs, central column (b,e,h) the run_3 outputs and right column (c,f,i) represents the MERRA2 reanalysis maps. There is in general an agreement for the spatial pattern of the daily averaged field between MERRA2 reanalysis and the WRF-Chem predictions. Model runs and reanalysis show that the dust plume emitted on 14 April is transported toward Southern/Central Italy on 15 April and then dislocated eastward in the following day. This spatial pattern is quite well reproduced by both schemes, we have to notify once more the considerable overprediction in term of intensity of the run_3 configuration and the good matching between run_2 and MERRA2, especially during 15 April. Figure 5: Comparison between WRF-Chem run_2 (red line), run_3 (blue line) and AERONET level 2.0 data (black crosses) at Athens (**a**); Rome Tor Vergata (**b**); and Malta (**c**). If we consider particulate mass concentration (PM\({}_{10}\)) data, in the second column of Tables 7-9, we have reported the daily mean PM\({}_{10}\) concentrations measured at Gharb (Malta; Table 7), Lecce (Table 8), and Athens (Table 9) monitoring stations. In third and fourth columns are reported the corresponding PM\({}_{10}\) daily average from the WRF-Chem run_2 and run_3, respectively. Being surface values, all the quantities are expressed in \(\mu\)g m\({}^{-3}\). If we consider the dust-grain distribution showed in Tables 3 and 4, the dust contribution to PM\({}_{10}\) is calculated, in the WRF-Chem model, using the formulas [38]  
\[\text{PM}_{10}\;=\;\text{dust\_1}\;+\;\text{dust\_2}\;+\;\text{dust\_3}\qquad \text{run\_2-scheme}\]  
\[\text{PM}_{10}\;=\;\text{dust\_1}\;+\;\text{dust\_2}\;+\;\text{dust\_3}\;+\; 0.737\;*\;\text{dust\_4}\qquad\text{run\_3-scheme}\]  
Due to the proximity to North African coasts (Figure 1), Malta is impacted by the dust plume on 15 April with a quite high daily mean value (158 \(\mu\)g m\({}^{-3}\)) reported by the authority's measurement station in Gharb (Malta; Table 7). The XMed-Dry sample SLS_149 (Table 6) revealed that, during the sampling period from 13 (17:30 UTC) to 15 (10:15 UTC) April 2018, the sea salt deposition rate (not showed) is nearly as high as the dust rate for the same size range of 0.625-10 \(\mu\)m. On the other side, on 15 April the run_2 output underestimated the daily mean PM\({}_{10}\) concentration by about 40 \(\mu\)g m\({}^{-3}\). This bias was caused by neglecting the contribution of sea spray to PM\({}_{10}\) concentrations in the present study. In conformity with the above results, run_3 configuration provides a considerable over-prediction for this and the other PM\({}_{10}\) stations described below. Figure 6: Daily averaged maps for dust column mass density for 14 April (**a–c**), 15 April (**d–f**) and 16 April (**g–i**). Left column (**a,d,g**) represents run_2 outputs, central column (**b,e,h**) the run_3 outputs and right column (**c,f,i**) MERRA2 model reanalysis.