## 3 Ground-Truth Data and Methods for Satellite-Sunphotometer Comparison
### Methods for Aerosol Model Evaluation and Aerosol Properties Analysis  
Due to constraints placed on the inversion of CE318 sky-radiance data (AOD\({}_{440\mathrm{nm}}\) > 0.4; solar zenith angle > 50\({}^{\circ}\), etc. ), statistics for aerosol optical properties are sparse. To collocate aerosol optical properties from sunphotometer with aerosol models assumed by either VIIRS retrieval algorithm, we require different averaging domains. Here, we use daily-averaged aerosol optical properties retrieved from CE318 sky radiance measurements. Since the aerosol model types used for satellite AOD retrievals may vary spatially, we select only the model type assumed at the pixel that includes the site. In the extraction of AOD from the VIIRS_EDR, those with quality QF < 1 retrievals are rejected (these with QF < 1 are not products and are mostly with cloud contamination and sunlight). Seasonal and total frequencies of each aerosol model type occurrence in the three sites are calculated to show the typical aerosol model types used in the VIIRS_EDR land algorithm over NCP sites. The aerosol model type evaluation of VIIRS (VIIRS_EDR and VIIRS_ML) is based on the SSA comparisons between VIIRS and CE318. The SSA values at four wavelengths (440 nm, 670 nm, 870 nm and 1020 nm) of the five aerosol model types used in VIIRS_EDR can be obtained from [PERSON] et al. [(27)]. The values of SSA at 670 nm (SSA\({}_{670\mathrm{nm}}\)) are 0.98, 0.84, 0.93, 0.97 and 0.88 for dust, high absorption smoke, low absorption smoke, clean urban and polluted urban aerosol model, respectively. Thus, the SSA values at the NCP sites in the VIIRS_EDR retrival can be derived from the aerosol model type assumed in the VIIRS_EDR pixel that includes the site. The VIIRS_ML aerosol model type is assumed globally based on the cluster analysis of \(\text{SSA}_{670\text{nm}}\) derived from all AERONET inversions and it is fixed in each season for the NCP region (i.e., weakly absorbing fine model for Summer and Autumn: \(\text{SSA}_{670\text{nm}}\sim 0.95\), moderately absorbing fine model for Spring and Winter: \(\text{SSA}_{670\text{nm}}\sim 0.90\)) [22; 23], so it is unnecessary to extract the aerosol model from VIIRS_ML pixel-by-pixel. Thus, we do the seasonal comparison; that is, the seasonal mean \(\text{SSA}_{670\text{nm}}\) values of CE318 inversion are calculated and compared with the seasonal \(\text{SSA}_{670\text{nm}}\) values of the VIIRS_EDR and VIIRS_ML. Only the VIIRS SSAs with the AOD\({}_{550\text{nm}}>0.25\) are used to meet the requirement of CE318 AOD\({}_{440\text{nm}}>0.4\)[23]. The SSA is also used to classify the aerosol type of the CE318 inversion, which is to evaluate the aerosol model type for each retrieval from the VIIRS algorithms. We firstly collocate the daily matchup data between the VIIRS_EDR and CE318 and between the VIIRS_ML and CE318. To evaluate the aerosol model type of VIIRS_EDR, the CE318 inversions are classified to the five aerosol types as the VIIRS_EDR. The CE318 inversion with Angstrom exponent \(<0.6\) and AOD at 1020 nm \(>0.3\) (according to [PERSON] et al. [27]) is classified as dust type. If not dust type, the CE318 inversion with \(\text{SSA}_{670\text{nm}}<0.86\) is the high absorption smoke, with \(0.86<\text{SSA}_{670\text{nm}}<0.905\) is polluted urban, with \(0.905<\text{SSA}_{670\text{nm}}<0.95\) is low absorption smoke, and with \(\text{SSA}_{670\text{nm}}>0.95\) is clean urban aerosol type. As for the evaluation of aerosol model type used in the VIIRS_ML, the CE318 inversions are classified to the four aerosol types as the VIIRS_ML. Use the same way to find out the CE318 data with coarse model (same as the dust). For the rest CE318 data, that with \(\text{SSA}_{670\text{nm}}<0.875\) is regarded as strong absorbing fine model, \(0.875<\text{SSA}_{670\text{nm}}<0.925\) is the moderately absorbing fine model, and \(\text{SSA}_{670\text{nm}}>0.925\) is the weakly absorbing fine model. The threshold values of the classification are based on the SSA values of the aerosol models used in the VIIRS algorithms.