# Retrieval of High Spatial Resolution Aerosol Optical Depth from HJ-1 A/B CCD Data
## 3 Results and Discussion
### Comparison with MODIS Aerosol Product  
The MODIS aerosol product (MOD04_3K) with a spatial resolution of 3 km was employed for comparison to the AODs retrieved from the HJ-1 A/B CCD data. Figure 12a,d shows the false color HJ-1 A/B CCD images (band 4 is displayed in a red color, band 3 is displayed in a green color, and band 2 is displayed in a blue color) observed on 26 July 2014 and 6 October 2012, respectively. Figure 12b,e shows the AOD dataset retrieved from the HJ-1 A/B CCD data, whereas Figure 12c,f shows the corresponding MODIS aerosol products with an overpass time within 1.5 h. The comparison results showed that the AOD retrieved from the HJ-1 A/B CCD data agreed well with the distribution range and trend of the visual interpretation and the MODIS aerosol product. From Figure 12c,f, a serious data-missing phenomenon can be observed in the MODIS aerosol product. In contrast, the AOD retrieved by the algorithm proposed in this study can avoid such a situation. Figure 13 shows the partially enlarged image of Figure 12a (upper-right corner) and the corresponding AOD map retrieved from the HJ-1 A/B CCD data and the MODIS aerosol product. From these figures, it can be seen that the AOD maps retrieved from the HJ-1 A/B CCD data had a high spatial resolution, making a remarkable improvement when compared to the image quality of the MODIS aerosol product. The comparison results with the MODIS aerosol product showed that the AOD map retrieved from HJ-1 A/B showed superior image quality to that of the MODIS aerosol product. The density plots of the AOD retrieved from the HJ-1 A/B CCD data versus the MODIS aerosol products are shown in Figure 14. The results showed that the AOD estimation results of the MODIS aerosol product were lower than the AODs retrieved from the HJ-1 A/B CCD data for a relatively low AOD case, with an \(R^{2}\) of 0.859 and an RMSE of 0.131 (Figure 14a). The estimation results of the MODIS aerosol product were 0.208 higher than the AODs retrieved from the HJ-1 A/B CCD data (Figure 14b) for a high AOD case, with an \(R^{2}\) of 0.768 and an RMSE of 0.145.  
Figure 11: Scatterplots of inversion results versus AERONET measurements. (**a**) Validation approach I: Retrieval without any prior knowledge; (**b**) validation approach II: Retrieval with the measurements of an AERONET site as prior knowledge.  
Figure 12: False color images and the AOD inversion results. (**a**,**d**) The false color images of the HJ-1 B and HJ-1 A CCD data, respectively; (**b**,**e**) the AODs retrieved from (**a**,**d**), respectively; (**c**,**f**) the corresponding MOD04_3K AOD products of (**a**,**d**) with an overpass time within 1.5 h, respectively.  
Figure 13: Partially enlarged image and retrieved AOD maps. (**a**) Geolocation of the partially enlarged image; (**b**) partially enlarged image of Figure 12a; (**c**) AOD map retrieved from the HJ-1 A/B CCD data; (**d**) Moderate Resolution Imaging Spectroradiometer (MODIS) aerosol product (MOD04_3K); (**e**) interband regression coefficient \(a\); (**f**) interband regression coefficient \(b\) (where the offset step \(i\) is 26).