## 5 Results of Aerosol Model and the Optical Properties Inter-Comparison
### Evaluation of the VIIRS_EDR Aerosol Model Type  
Figure 2 shows the frequencies of each aerosol model type used in the VIIRS_EDR (hereafter called M_VIIRS_EDR) AOD retrieval at the Beijing, XiangHe and Xinglong sites during the evaluation period. For M_VIIRS_EDR, the dust and clean urban aerosol models are the two dominant model types used in NCP region. In Beijing, the M_VIIRS_EDR shows that dust and clean urban models account for more than 80% of the aerosols during the evaluation period. The frequency of the polluted urban model is less than 1%. However, Beijing is a mega city with a population of approximately 21 million and five million vehicles are located in the heavy polluted NCP region. It is undisputed that polluted urban aerosol is the dominant aerosol [38]. Thus, the M_VIIRS_EDR is unsuitable at the Beijing site. Because XiangHe is located 50 km to the east of Beijing, the M_VIIRS_EDR for XiangHe shows similar results to Beijing. However, XiangHe also shows some differences from Beijing: dust and clean urban models decrease and other models increase. As for Xinglong station (regional back ground station), the M_VIIRS_EDR shows more polluted urban and low absorption smoke and less dust models than the Beijing and XiangHe stations. From Beijing to Xinglong, the M_VIIRS_EDR shows that the frequency of polluted urban aerosol models increase, which is inconsistent with the fact that pollution decreases from Beijing to Xinglong according to past study results in the NCP region [12; 28; 39]. To show the aerosol model type differences between the VIIRS retrievals and CE318 sunphotometer observations, the seasonal values of SSA670 nm of the CE318 inversion, VIIRS_EDR and VIIRS_ML in the NCP three sites are shown in Table 5. Comparing the VIIRS_EDR and CE318, the SSA670 nm values from the VIIRS_EDR are higher than those from the CE318 during almost all seasons and over all the three sites. This result reflects more frequent weakly absorbing aerosol model type used in VIIRS_EDR retrievals in the NCP sites, as Figure 2 shows more frequency of dust and clean urban aerosol models. The largest difference between the VIIRS_EDR and CE318 at Beijing and XiangHe sites is shown during winter with 0.07 at Beijing and 0.08 at XiangHe. However, the difference at Xinglong during winter is smallest. The largest difference at the Xinglong site occurred during the spring and summer and the difference value is 0.03, which is less than those at Beijing and XiangHe. These indicate that the M_VIIRS_EDR at Beijing and XiangHe have more errors than at Xinglong. To know how many aerosol types used in VIIRS_EDR are suitable, we calculated the accuracy rate of the M_VIIRS_EDR at the NCP three sites and the results are shown in Figure 3. The accuracy rates of the M_VIIRS_EDR over Beijing, XiangHe, and Xinglong are 0.24, 0.20, and 0.37, respectively (Figure 3a). The accuracy rate of the M_VIIRS_EDR over Xinglong is highest, which is consist with the lowest SSA difference between the VIIRS_EDR and CE318 at Xinglong. As for each model type, although the dust model is more used in the M_VIIRS_EDR (Figure 2), the accuracy rate of the dust model is very small at Beijing and XiangHe and even equal to zero at Xinglong site (Figure 3a,c). Figure 2: The frequencies of the aerosol model types used in the VIIRS_EDR at Beijing, XiangHe and Xinglong. Urban(P) is the polluted urban aerosol model, Urban(C) is the clean urban aerosol model, Smoke(LA) is the low absorption smoke aerosol model, Smoke(HA) is the high absorption smoke aerosol model, and the last aerosol model is for Dust. The accuracy rate of low absorption smoke is relatively higher than other models of M_VIIRS_EDR. The accuracy rate of the polluted urban aerosol model is practically zero because the frequency of polluted urban model occurred in the M_VIIRS_EDR over Beijing and XiangHe stations is very low (Figure 2) and it is different from the aerosol type of the CE318 of the matchup.