## 3 Ground-Truth Data and Methods for Satellite-Sunphotometer Comparison  
### Sunphotometer Data  
The ground data used to evaluate the VIIRS aerosol products in this study consists of CE318 sunphotometer (CE318) observations and retrievals. The CE318 instrument performs direct sun extinction measurements at eight wavelengths ranging from 340 to 1020 nm and sky radiance measurements at four wavelengths, i.e., 440, 675, 870, and 1020 nm. The AOD data were calculated from direct sun observations with an accuracy of 0.01 to 0.02 [24; 25]. Refractive index, volume mean radius, volume concentration and single scattering albedo (SSA) retrieved from the CE318 sky measurements characterize the aerosol type. The uncertainties of refractive index are 30-50% for the imaginary part and 0.04 for the real part when AOD at 440 nm (AOD\({}_{440\text{nm}}\)) \(>\) 0.4 and solar zenith angle \(>\) 50\({}^{\circ}\), and the uncertainties increase for lower AODs [26; 27]. SSA uncertainty is estimated to be less than 0.03 for AOD\({}_{440\text{nm}}\)\(>\) 0.4 and the uncertainty increases for lower AODs [26; 27]. Note that inversion data (size/optics) are sparse compared to direct sun observations of spectral AOD. CE318 sunphotometer data (including the corresponding inversion products) over three sites in NCP region during the period of 2 May 2012-31 March 2014 were used. The location and description of the three CE318 sunphotometer sites are provided in Table 1. These three sites can be considered as representative of urban (Beijing), suburban (XiangHe) and regional background (Xinglong) environments, respectively. Affected by Asian monsoons, the NCP region has a moderate continental climate with cold winters and hot summers. Heavy anthropogenic pollution from urbanization, industrial, and agricultural activities mixed with coarse dust particles (most occurring in spring) result in a rather complex nature of aerosol physical and optical properties in the NCP [12]. Notably, the regional background station Xinglong is located at a mountain with the elevation of 970 m which is higher than the other two sites. However, even at this station, urban/industrial and dust aerosol could occur through aerosol regional transportation [28] and secondary aerosol formation. Therefore, as will be shown in our analysis, the complex features of aerosol properties may help explain some of the uncertainties in satellite retrievals of AOD in this region. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Station Name** & **Lon (\({}^{\circ}\))** & **Lat (\({}^{\circ}\))** & **Site Description** \\ \hline
**Beijing** & 116.381 E & 39.977 N & Urban station, 92 m a.s.l., located in urban area of Beijing \\
**XiangHe** & 116.962 E & 39.754 N & Suburban station, 36 m a.s.l., 50 km to the east of Beijing \\
**Xinglong** & 117.578 E & 40.396 N & Regional back-ground station, 970 m a.s.l., on the top of \\  & & a mountain, 100 km to the north of Beijing \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Site location and description of the CE318. Data at Beijing and XiangHe are downloaded from AERONET ([[http://aeronet.gsfc.nasa.gov/](http://aeronet.gsfc.nasa.gov/)]([http://aeronet.gsfc.nasa.gov/](http://aeronet.gsfc.nasa.gov/))) and the data at Xinglong are obtained from China Aerosol Remote Sensing Network (CARSNET) [(29)]. AERONET level 1.5 inversion data (from sky-light measurements) are used since the level 2 inversion data are very less frequent and unsuitable for data statistics. At the same time, we used the conditions of AOD\({}_{440\mathrm{nm}}\) > 0.4 and solar zenith angle > 50\({}^{\circ}\) to constrain the data quality according to [(26; 27)]. The AOD in these two networks are consistent with one another as the correlation coefficients are larger than 0.999 and have a 99.9% significance level [(29)]. The CARSNET calibration and comparison with AERONET are described in detail in other references [(29; 30; 31)].