## 4 Results of AOD Inter-Comparison  
### Evaluation of the VIIRS_EDR AOD at 550 nm  
Table 2 reports the validation results of the two VIIRS algorithms compared to the collocated ground CE318 observations, for the period of 2 May 2012-31 March 2014 over the NCP region. There are 860, 762 and 564 instantaneous VIIRS_EDR-CE318 matchups of QF \(>\) 0, QF \(>\) 1 and QF = 3 at the NCP sites during the period, respectively. Starting with gross statistics, the slope and intercept of the best-fit equation between the VIIRS_EDR and CE318 AOD are 0.85-0.91 and 0.05-0.07, respectively, with R ranging from 0.83-0.86. The VIIRS_EDR data are well correlated with CE318 observations. However, the VIIRS_EDR AOD showed a positive MB of 0.04-0.06 and a rather large RMSE of 0.22-0.24. Only 44.3-48.9% of the compared AODs meet the expected error envelope of 0.05 + 0.15 AOD. Filtering by quality flag (QF \(>\) 1), the comparison improves for all statistics, however constraining to only high quality flags (QF = 3) does not improve the overall agreement any further. These issues need to be studied at each site. The MB and RMSE in the NCP are both larger than the counterparts in the global assessment statistics [6; 9]. Table 3 presents the evaluation results of each NCP site, separately. Clearly, all properties (MB, RMSE, R and %EE) demonstrate the worst performance over Beijing. The MB in XiangHe site is (\(-\)0.02)-0.00 with high R of 0.89-0.92, which is more comparable to the global agreement [9]. Beijing is an urban site, while the VIIRS_EDR uses the global surface reflectance ratios as the expected spectral surface reflectance relationship, which may cause the largest error at the Beijing site [6]. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**QF** & **N** & **MB** & **RMSE** & **R** & **\%EE** & **Slope** & **y-int** \\ \hline \multicolumn{8}{c}{**VIIRS\_EDR vs. CE318**} \\ \hline QF \(>\) 0 & 860 & 0.05 & 0.24 & 0.83 & 44.3 & 0.85 & 0.07 \\ QF \(>\) 1 & 762 & 0.04 & 0.22 & 0.84 & 46.5 & 0.88 & 0.05 \\ QF \(=\) 3 & 564 & 0.06 & 0.23 & 0.86 & 48.9 & 0.91 & 0.07 \\ \hline \multicolumn{8}{c}{**VIIRS\_ML vs. CE318**} \\ \hline QF \(>\) 0 & 817 & 0.14 & 0.25 & 0.94 & 51.0 & 1.27 & 0.02 \\ QF \(>\) 1 & 755 & 0.13 & 0.24 & 0.94 & 53.1 & 1.26 & 0.01 \\ QF \(=\) 3 & 683 & 0.13 & 0.25 & 0.93 & 54.0 & 1.26 & \(-\)0.00 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Statistics of matchup of CE318 and VIIRS AOD at 550 nm in the NCP region during 2 May 2012–31 March 2014. N is the number of matchup data. MB is the mean bias. RMSE is root mean squared error. R is correlation coefficient. %EE stands the percentage of data within the expected error of 0.05 + 0.15 AOD. Slope and y-int are the slope and intercept at y-axis of the linear regression, respectively. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**VIIRS\_EDR QF** & **Site** & **N** & **MB** & **RMSE** & **R** & **\%EE** & **Slope** & **y-int** \\ \hline \multirow{4}{*}{**QF \(>\) 0**} & Beijing & 336 & 0.10 & 0.29 & 0.76 & 36.9 & 0.83 & 0.17 \\  & XiangHe & 323 & \(-\)0.02 & 0.21 & 0.89 & 48.3 & 0.88 & 0.04 \\  & Xinglong & 201 & 0.05 & 0.17 & 0.81 & 50.2 & 0.86 & 0.08 \\ \hline \multirow{4}{*}{**QF \(>\) 1**} & Beijing & 291 & 0.10 & 0.27 & 0.79 & 38.8 & 0.90 & 0.14 \\  & XiangHe & 289 & \(-\)0.02 & 0.19 & 0.90 & 50.2 & 0.90 & 0.02 \\  & Xinglong & 182 & 0.04 & 0.15 & 0.82 & 52.7 & 0.81 & 0.08 \\ \hline \multirow{4}{*}{**QF \(=\) 3**} & Beijing & 204 & 0.15 & 0.30 & 0.80 & 36.8 & 0.82 & 0.22 \\  & XiangHe & 220 & 0.00 & 0.18 & 0.92 & 53.6 & 0.89 & 0.05 \\ \cline{1-1}  & Xinglong & 140 & 0.02 & 0.14 & 0.83 & 59.3 & 0.81 & 0.06 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Statistics of the matchup between CE318 and VIIRS_EDR AOD at 550 nm over each site during 2 May 2012–31 March 2014. In the previous global evaluation for VIIRS_EDR products, it was recommended to use data with a higher QF [9]. In the NCP, comparisons at both XiangHe (suburban) and Xinglong (rural) support this recommendation (R and %EE increase and MB decreases with increasing QF).