# Retrieval of High Spatial Resolution Aerosol Optical Depth from HJ-1 A/B CCD Data
## 3 Results and Discussion  
### Sensitivity Analysis of Interband Regression Coefficients  
To evaluate the sensitivity of the interband regression coefficients, four scenarios were carried out for comparison in this study: (1) A constant regression coefficient that did not vary by solar/view angle and land cover type; (2) regression coefficients that varied with different solar/view angles; (3) regression coefficients that varied with different land cover types; (4) and regression coefficients that varied with different solar/view geometries and land cover types. In the case of using a constant regression coefficient (Figure 8a), the overall offset error was 0.05, and the difference between the maximum and minimum values over the semi-hemispherical space was about 0.013. In contrast, in the case of using regression coefficients varying with the solar/view angle, the difference between the maximum and minimum values was about 0.014 (Figure 8a). In the case of using regression coefficients varying by land cover type, the overall dispersion was smaller for most of the land cover types, with a standard deviation of 0.0048, and there was a difference between the maximum and minimum values over the semi-hemispherical space of about 0.015 (Figure 8c). However, grasslands, shrubs, and bare land, as well as snow and ice (corresponding to GLC2000 land cover types 13, 14, 19, and 21) had relatively large errors (Figure 8b). In the case of using regression coefficients varying by solar/view geometry and land cover type, the values of _Diff_ were around 0, with a mean of \(-0.00004\) and a standard deviation of 0.0031 (Figure 9). It can be seen that in the principal and the perpendicular planes, the influence of solar/view angle on the interband regression coefficients could be better corrected by using regression coefficients varying with different solar/view angles and land cover types.  
Significant variations existed in the values of interband regression coefficients \(a\) and \(b\) over the semi-hemispherical space, especially at the hotspot and at the large view zenith angles (Figure 10). The fitted \(R^{2}\) was relatively small and the RMSE was relatively large at the hotspot and large view zenith angles, with an overall \(R^{2}\) exceeding 0.91 and an overall RMSE below 0.023, indicating that the interband regression coefficients \(a\) and \(b\) had a good overall fitting effect for the HJ-1 A/B CCD data (with a maximum view zenith angle of 35\({}^{\circ}\)).  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Site**} & **Longitude** & **Latitude** & **Approach I** & \multicolumn{2}{c}{**Approach II**} \\  & **(Degree)** & **(Degree)** & **The Years Used** & **The Years Used** & **The Years Used for** \\  & & & **for Validation** & **As an Input** & **Validation** \\ \hline Beijing & 116.381 & 39.977 & 2013–2015 & 2014 & 2012, 2013, 2015 \\ Beijing\_CAMS & 116.317 & 39.933 & 2013–2015 & 2012, 2015 & 2013, 2014 \\ Beijing\_RADI & 116.379 & 40.005 & 2013–2015 & — & 2012, 2013, 2014, 2015 \\ XiangHe & 116.962 & 39.754 & 2013–2015 & 2013 & 2012, 2014, 2015 \\ \hline \hline \end{tabular}
\end{table}
Table 4: Summary of AERONET sites.  
Figure 8: Comparison of different scenarios of interband regression coefficients. (**a**) Comparison using a constant regression coefficient and regression coefficients varying by solar/view angle; (**b**) differences in the grassland, shrub, and bare land, as well as snow and ice land cover types; (**c**) differences in the other land cover types of GLC2000.  
Figure 9: Distribution of _Diff_ values obtained using the three scenarios of regression coefficients in the (**a**) principal plane and (**b**) perpendicular plane.