## 3 Results
### Comparison with AOD/MAIAC and AERONET  
The comparison of satellite data with model results is performed by computing the columnar AOD from the extinction coefficient that is calculated by the model optical routines [56] at 0.55 \(\upmu\)m ([23], eq.36). Figure 4 shows the binned average of the AOD at 0.55 \(\upmu\)m elaborated for 14 April (a-c), 15 April (d-f) and 16 April (g-i). Left column (a,d,g) represents the Modis/MAIAC AOD, central column (b,e,h), the AOD from WRF-Chem/run_2 and finally the right column the AOD from WRF-Chem/run_3 configuration. Both Modis/MAIAC retrievals and model outputs show the generation/emission of mineral dust during 14 April (Figure 4a-c) to the dust source area corresponding to the Chott region in Northern Tunisia [4,9]. The spatial pattern is typical of a low-pressure system with the associated high wind speed at the surface (10 m) as described above. The following days (15 and 16 April) are characterized by the long-range transport toward Italy (Figure 4d-f) and toward Greece on 16 April (Figure 4g-i). More in detail, it may put in evidence: (i) the quite good spatial reproduction; and (ii) the overestimation of model AOD for both configurations, even if the run_2 produces a better comparison for all the investigated days. It should be also notified the consistent presence of clouds that prevented the aerosol retrievals from the MAIAC/ AOD algorithm.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline  & **Athens** & **Run_2** & **Run_3** \\ \hline April & \(\upmu\)g m\({}^{-3}\) & \(\upmu\)g m\({}^{-3}\) & \(\upmu\)g m\({}^{-3}\) \\
12 & 27.2 & 18.6 & 86.5 \\
13 & 36.2 & 29.4 & 86.2 \\
14 & 45.2 & 51.2 & 129.0 \\
15 & 35.1 & 39.7 & 106.3 \\
16 & 35.2 & 181.0 & 641.8 \\ \hline \hline \end{tabular}
\end{table}
Table 9: Daily mean PM\({}_{10}\) at Athens and predicted with the WRF-Chem configurations run_2 and run_3.  
To complement the comparison with the satellite retrievals and to better follow the temporal evolution of the AOD field over the central Mediterranean, we used the AOD Level 2.0 (Version 3) AERONET measurements at Athens, Rome (Tor Vergata) and Malta. These data are automatically cloud cleared and quality assured with pre-field and post-field calibration applied.  
Figure 5a-c depicts the hourly resolved AOD at 550 nm from the run_2 simulation (red line), run_3 (blue line) and the corresponding AERONET measurements (black crosses).  
The measurements show AOD peaks (<1.0) on 16 April at Athens, on 15 and 16 April at Rome Tor Vergata (~1.0) and the highest peak (~1.5) on 15 April at Malta. These measurements agree with the synoptic transport of the dust plume that is described in Section 3.1. The comparison between AERONET measurements with the two WRF-Chem runs reveals clearly the optimal reproduction of this columnar optical property by the run_2 configuration, with a good performance in terms of peak value and timing. On the other side, the AFWA setup (run_3) produces a relevant overestimation of the AOD, with several unrealistic peak values registered during the simulation period.  
Figure 4: Daily averaged maps for AOD at 0.55 \(\upmu\)m for 14 April (**a–c**), 15 April (**d–f**), and 16 April (**g–i**). Left column (**a**,**d**,**g**) represents the Modis/MAIAC AOD, central column (**b**,**e**,**h**) the run_2 AOD and right column the run_3 AOD.