# MODIS Retrieval of Aerosol Optical Depth over Turboid Coastal Water
## 4 Results
The percentage of collocated pairs within expected error envelope (+(0.04 + 10%), \(-(0.02 + 10\%)\), asymmetric) and correlation coefficient increase from 35.5% and 0.72 in MxD04 only situation (Figure 6a) to 39.3% and 0.94 in CW only situation (Figure 6b), respectively. In addition to the situations where only MxD04 or only CW retrievals are available within the 25-km radius proximity of AERONET site, there is a third one where some of the cloud-free pixels are retrieved through the operational MODIS algorithm while some are retrieved through the CW algorithm. Figure 6c,d are similar to Figure 6a,b, respectively, but both include the AOD retrievals from their respective counterparts of this third situation. Overall, for all possible retrievals by each algorithm, CW retrievals are comparable to MxD04 in quality, although the MODIS operational algorithm has slightly more samples. The basis of MODIS-AERONET collocation is that air masses are always in motion and the average of MODIS AOD retrievals in a certain area which encompass an AERONET site should be comparable to the temporal statistics of the AERONET measurements [16]. In the situation that both MxD04 and CW retrievals are available within the 25-km radius proximity of AERONET site, if only MxD04 or CW retrievals are collocated with AERONET observation, it will be biased comparison. Therefore, we further show the inter-comparison between either MxD04 or CW AOD with AERONET AOD at this situation (Figure 6f-h). The percentage of collocated pairs in the EE for (MxD04 + CW) combined vs. AERONET (53.1%, Figure 6h) is larger than that of MxD04-AERONET (49.2%, Figure 6f) and CW-AERONET (40.6%, Figure 6g). The RMSE of (MxD04 + CW)-AERONET (0.110) is smaller than that of MxD04-AERONET (0.114) and CW-AERONET (0.148). Inter-comparison of MxD04 and CW merged AOD with AERONET AOD is shown in Figure 6e; such merged AOD include all data points in Figure 6a,b,h. The number of collocated pairs increases from 190 (62 in MxD04 only situation plus 128 in the situation that both MxD04 and CW retrievals are available) to 218 (28 in CW only situation are added). In addition, the inter-comparison statistics (Figure 6e) is still comparable to or better than those retrieved from MxD04 (Figure 6a,c) in quality. Overall, CW retrievals supplement MODIS DT, and improve the AOD retrievals both spatially and temporally without degrading (and sometimes increasing) the DT-Ocean AOD retrieval quality. In addition to evaluation of MODIS AOD diagnostic error in Figure 6, prognostic error is presented in Figure 7. Two thirds of AERONET-MODIS collocations are within the expected error envelope (y = \(\pm\)(0.05 + 0.15x)) (Figure 7c), thus 0.05 + 0.15 (MODIS AOD) can be considered as its prognostic  
Figure 6: Scatter plots of 0.55 μm AOD retrievals from MODIS (MxD04 and/or CW) versus AERONET observations of 6 sites in one month. Venn diagram on the upper left represents number of collocated data sets for MxD04-AERONET (blue), CW-AERONET (red), (MxD04 + CW)-AERONET (purple). Color part of Venn diagram on the top of each scatter plot (**a**–**h**) represents which collocated data sets are plotted; see details in the text for further description of the Venn diagram. (**a**) is MxD04-AERONET in the situation that only MxD04 retrievals are available within the 25-km of AERONET site. (**b**) is CW-AERONET in the situation that only CW retrievals are available. (**c**,**d**) are similar to **(a,b**), respectively, but both additionally include AOD retrievals from their respective counterparts of the situation that both MxD04 and CW retrievals are available. (**e**) is (MxD04 + CW)-AERONET in all the three situations. (**f**–**h**) are MxD04-AERONET, CW-AERONET, (MxD04 + CW)-AERONET, respectively, in the situation that both MxD04 and CW retrievals are available. 1:1 lines and expected error (EE) envelopes (+(0.04 + 10%), \(-\)(0.02 + 10%), asymmetric) are plotted as dot and dashed lines.