## 6 Conclusions  
Using CE318 data at three sites in the NCP region to evaluate the VIIRS AOD, aerosol model types and aerosol optical properties used in the VIIRS_EDR and VIIRS_ML algorithms at three sites over the NCP, we conclude:  
1. The VIIRS_EDR AOD at 550 nm has a positive MB of 0.04-0.06 with R of 0.83-0.86. Among the three sites, the bias at Beijing is largest with MB of 0.10-0.15, RMSE of 0.27-0.30 and low %EE of 36.8-38.8%. The quality flags analysis shows that using the high quality products of  
Figure 4: The comparisons of normalized aerosol physical properties from CE318 and VIIRS_EDR at the three sites. Each parameter at each site is normalized between 0.1 and 1 to well show the difference of CE318 and VIIRS_EDR. The length of each radius in the circle equals to 1 and each radius direction stands for one aerosol parameter: 1, real part of refractive indices; 2, imaginary part of refractive indices; 3, volume mean radius of fine mode; 4, standard deviation of fine mode; 5, volume concentration of fine mode; 6, volume mean radius of coarse mode; 7, standard deviation of coarse mode; and 8, volume concentration of coarse mode, respectively. The values in last column in each site stand for the values of the eight aerosol properties at the radiuses of 0.1 and 1 positions. AOD at XiangHe and Xinglong are recommended but not at Beijing site. The VIIRS_ML AOD overestimates more over the NCP region with higher positive MB of 0.13-0.14 but shows higher correlation (0.93-0.94) with ground-based AOD. The results of evaluation of the VIIRS_ML for each site and quality flags analysis are found to be similar to that for the VIIRS_EDR. * The aerosol model types used in the VIIRS_EDR AOD retrieval in the three sites are mostly dust and clean urban aerosol models (the frequencies of these two models in three sites are all larger than 50%) with less frequency of polluted urban aerosol models used (less than 1%). The accuracy rates of the M_VIIRS_EDR over the Beijing, XiangHe and Xinglong sites (0.24, 0.20 and 0.37) are lower than that of the M_VIIRS_ML (0.47, 0.51 and 0.46) during the evaluation period. * The values of SSA440 nm from CE318 are less than the VIIRS_EDR for all modes and sites, with differences of 0.02-0.04 for fine mode, 0.06-0.12 for coarse mode and 0.03-0.05 for bi-modes. The overestimation of SSA but positive AOD mean bias of the VIIRS_EDR indicate that other factors (e.g., surface reflectance characterization or cloud contamination) are important error sources in the VIIRS_EDR algorithm, and their effects on aerosol retrievals override the effects from non-ideality in aerosol model types. The differences of SSA670 nm between VIIRS_ML and CE318 in the NCP are mostly less than 0.015 but high seasonal differences are also found. The undervaluation of SSA used in the VIIRS_ML algorithm over the NCP causes the overestimation of AOD, especially at Beijing site. We recommend that the aerosol model types and the microphysical properties in the VIIRS_EDR algorithm in NCP region are not representative and need to be refined. The AOD bias in Beijing is largest but we do not find the lowest accuracy rate of the M_VIIRS_EDR. In addition, the higher values of the VIIRS_EDR SSA versus CE318, which should lead to lower AODs from the satellite inversion, are inconsistent with the positive MBs of AOD in the NCP region. All these points indicate that there are other error sources that need to be examined in the AOD retrieval for the VIIRS_EDR algorithm, especially for Beijing site. Future studies should investigate these potential sources of error including the surface reflectance and cloud contamination. This research was supported by the national key research and development program of China (No.2016 YFC0200403), the Strategic Priority Research Program (A) of the Chinese Academy of Sciences (XDA05100301), the National Science Foundation of China (41175031, 41590874 & 41375153), National Key R&D Program Pilot Projects of China (2016 YFA0601901) and the CAMS Basis Research Project (2016Z001 & 2014R17).