## 5 Results of Aerosol Model and the Optical Properties Inter-Comparison
### Evaluation of the VIIRS_ML Aerosol Model Type  
Using the same way of evaluating the M_VIIRS_EDR, the aerosol model type assumed in the VIIRS_ML (M_VIIRS_ML) in the NCP sites is evaluated and shown in Table 5 and Figure 3. In Table 5, it can be found that the biases of SSA670 nm between the VIIRS_ML and CE318 are \(\leq\)0.04 in almost all sites and all seasons except for Xinglong during the winter. The average bias for all seasons is \(-\)0.015, 0.0075 and \(-\)0.0075 at Beijing, XiangHe and Xinglong, respectively. These biases are less than the differences between the VIIRS_EDR and CE318 at the corresponding site. That is likely because the M_VIIRS_ML is defined according to the AERONET sunphotometer inversion in local regions while M_VIIRS_EDR is from the predefined aerosol model types at five sites located in other places. Notably, the obvious undervaluation of the VIIRS_ML SSA over Beijing during almost all seasons may cause the overestimation of the VIIRS_ML AOD over Beijing (MB \(\geq\) 0.26 shown in Table 2) versus the VIIRS_EDR.  
However, there are some large differences of SSA670 nm between the VIIRS_ML and CE318 in some seasons. The moderately absorbing model in spring over Beijing used in the VIIRS_ML may be inappropriate because the CE318 inversion shows weakly absorbing type. The largest difference is found at Xinglong (regional background site). The absolute biases in all seasons over this site are \(\geq\)0.3 and the highest bias 0.6 is occurred in the winter. The CE318 shows weakly absorbing aerosols in the winter that is in contrast to the moderately absorbing aerosols used by the VIIRS_ML. This may indicate that the M_VIIRS_ML over the NCP may be unsuitable for regional background sites.  
In Figure 3, it can be found that the accuracy rates of the M_VIIRS_ML over Beijing, XiangHe, and Xinglong are 0.47, 0.51, and 0.46, respectively (Figure 3b). The average accuracy rate of the M_VIIRS_ML over the NCP region (0.48) is higher than that of M_VIIRS_EDR (0.27). As for each aerosol model type (Figure 3d), the accuracy rate of the weakly absorbing fine model is higher than the moderately absorbing fine model in Beijing and XiangHe. However, in Xinglong, the accuracy rate of the weakly absorbing fine model is lower than the moderately absorbing fine model. The large difference of the accuracy rate of the two model types in Beijing reflects that the moderately absorbing fine model assumed in the spring and winter may require careful consideration because of the dust in the spring and more strongly absorbing aerosols in the winter cannot be neglected [39].  
For different sites, the accuracy rates of the M_VIIRS_EDR over Beijing and XiangHe are higher than their corresponding values of the M_VIIRS_EDR. From Beijing to Xinglong, the accuracy rate of the M_VIIRS_EDR decreases, while M_VIIRS_ML varies little with lowest value over Xinglong site. This reflects that the M_VIIRS_EDR is unsuitable over the NCP urban and suburban sites, while the M_VIIRS_ML is suitable over urban and suburban sites. The highest accuracy rate of the M_VIIRS_EDR over the NCP sites is only 0.37. Thus, the aerosol model type selection in the VIIRS_EDR algorithm is inappropriate in the NCP region, which may cause an important error in the AOD inversion.