## 4 Results of AOD Inter-Comparison
### Evaluation of the VIIRS AOD at Red and Blue Bands  
While the AOD is retrieved at 550 nm, neither algorithm uses reflectance at 550 to derive AOD. This is because the Earth's surface tends to be brighter in green wavelengths (e.g., vegetation), and not suitable for aerosol retrieval. The VIIRS_EDR algorithm is based on the calculation of surface reflectance at blue (488 nm) and red (672 nm) and three other bands (412 nm, 445 nm and 2250 nm) [6]. As for the VIIRS_ML algorithm, AOD at 550 nm is inversed by using 488 nm, 672 nm and 2257 nm measured TOA reflectance to find a the optimal solution [8]. Hence, the VIIRS AOD spectral dependence between blue and red bands are also evaluated with the CE318 data. According to quality flag analysis in Sections 4.1 and 4.2, we choose the matchup data of QF > 1 for both the VIIRS_EDR-CE318 and the \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**VIIRS\_ML QF** & **Site** & **N** & **MB** & **RMSE** & **R** & **\%EE** & **Slope** & **y-int** \\ \hline \multirow{4}{*}{**QF>0**} & Beijing & 292 & 0.26 & 0.33 & 0.94 & 21.2 & 1.19 & 0.18 \\  & XiangHe & 340 & 0.09 & 0.20 & 0.97 & 70.6 & 1.26 & \(-\)0.03 \\  & Xinglong & 185 & 0.03 & 0.13 & 0.89 & 62.2 & 1.07 & 0.02 \\ \hline \multirow{4}{*}{**QF>1**} & Beijing & 263 & 0.26 & 0.33 & 0.91 & 22.8 & 1.14 & 0.20 \\  & XiangHe & 322 & 0.08 & 0.20 & 0.97 & 72.0 & 1.26 & \(-\)0.04 \\  & Xinglong & 170 & 0.03 & 0.12 & 0.92 & 64.1 & 1.19 & \(-\)0.01 \\ \hline \multirow{4}{*}{**QF=3**} & Beijing & 227 & 0.28 & 0.36 & 0.89 & 19.8 & 1.08 & 0.25 \\  & XiangHe & 302 & 0.07 & 0.19 & 0.97 & 73.2 & 1.26 & \(-\)0.04 \\ \cline{1-1}  & Xinglong & 154 & 0.01 & 0.11 & 0.92 & 66.9 & 1.15 & \(-\)0.02 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Statistics of the matchup of the CE318 and VIIRS_ML AOD at 550 nm over each site during 2 May 2012–31 March 2014. VIIRS_ML-CE318. Then, we selected the data of VIIRS_EDR, VIIRS_ML and CE318 with the dates in common. That was to select the matchup data of VIIRS_EDR, CE318 and VIIRS_ML. Figure 1 shows the average AODs of the matchup data between VIIRS_EDR, CE318 and VIIRS_ML at three wavelengths over the three sites. Compared to CE318 measurements, the VIIRS_EDR AOD at 488 nm overestimates over Beijing and Xinglong but performs well for the XiangHe site. At 672 nm, the VIIRS_EDR AOD also overestimates over Beijing, but slightly undervalues AOD at XiangHe. However, the VIIRS_ML AOD overestimates at all three wavelengths and over all the three sites, especially over Beijing. The biases of each wavelength can reach 0.2. One of the possible reasons for the larger bias over Beijing may be that the VIIRS_ML algorithm is actually the Dark-Target aerosol retrieval algorithm and this algorithm may overestimate the AOD values over bright surfaces such as urban centers by 0.2 [22; 37]. The large VIIRS_ML biases may also be related to the errors of aerosol model type (to be discussed in next section). We also calculate the aerosol Angstrom Exponent (AE) between 488 nm and 672 nm (\(\text{AE}=\log(\text{AOD}_{488\text{nm}}/\text{AOD}_{672\text{nm}})/\log(672 \text{nm}/488\text{nm})\)) to describe the AOD spectral dependence. AE is often used as an indicator of aerosol size distribution which is related to aerosol type: AE \(\sim\) 0 corresponds to large particles; and AE \(\sim\) 2 corresponds to small particles. The average AE values in Figure 1 are calculated from each matchup data set. The AE values of the VIIRS_EDR and the VIIRS_ML show large differences when comparing to that from CE318. AE biases of the VIIRS_EDR are 0.06 in Beijing, 0.31 in XiangHe and 0.55 in Xinglong, while the biases for the VIIRS_ML are \(-\)0.42 over Beijing, \(-\)0.17 in XiangHe and 0.20 over Xinglong. The AE of the VIIRS_EDR is larger than that from CE318 over all the three sites, but the AE of the VIIRS_ML is often lower than CE318 except for over Xinglong site.