## 3 Results
### Comparison of C5 and C6 CI Products
#### 3.1.2 Spatial Coverage of the Main Algorithm  
The quality flag data stored in the CI products indicate the retrieval status of each pixel. Comparing the quality flag data on the global scale can help us further understand the overall quality of the C5 and C6 CI products. Figure 4a-d display the global maps of CI QA data corresponding to the CI maps in Figure 3a-d. The quality flags of 0-1 indicate the main algorithm retrievals by using the best- and good-quality BRDF model parameters, respectively; flag 2 represents the backup algorithm retrievals obtained from CI-AFX LUT, while the main algorithm retrievals with good- and high-quality MCD43A1 data are out of the physical range of [0.33, 1.0]; flag 3 denotes pixels labeled in snow status according to the MCD43A2 data, and the backup algorithm retrievals are used to reduce the influence of cover-snow on the anisotropic reflectance [100]. Pixels of excluded non-vegetation, water, and missing values are the same as those on the CI maps, and the quality flag of the missing value refers to a lack of BRDF model parameters, which indicates a limited number of clear-sky observations for retrievals [21; 35]. Figure 3e,f show the latitudinal distribution of the retrieval rates of different algorithm paths, including the main algorithm, backup algorithm and non-retrievals (missing value). In each bar chart, the left panel is for the C5 CI, and the right panel is for the C6 CI. C5 and C6 show similar patterns for the latitudinal distribution of global CI QA, while more high-quality pixels are acquired in C6 on a global scale, indicating an improvement in the quality of C6 retrievals. The overall proportion of main algorithm retrievals is higher in July than January due to the large number of main retrievals in the Northern Hemisphere summer. The proportion of main algorithm retrievals is higher in the Southern Hemisphere in January and higher in the Northern Hemisphere in July. Pixels in mid and low latitudes (\(-\)30\({}^{\circ}\)-30\({}^{\circ}\)N) consist of relatively more missing values due to cloud and aerosol contamination. Pixels at high latitudes (60\({}^{\circ}\)-90\({}^{\circ}\)N) contain even more missing values due to their long-term snow or cloud cover and large solar zenith angle. Compared to C5, C6 shows a higher rate of main algorithm retrieval and a lower rate of missing value in the summer hemisphere (Southern Hemisphere in January and Northern Hemisphere in July) overall, which benefits from the improved algorithm of the C6 MOD43 data. In the Southern Hemisphere in January, the rates of the main algorithm retrievals and missing value are 89% and 9% for C6 CI data, and 86% and 12% for C5 CI data, respectively. In the Northern Hemisphere in July, the rates of the main algorithm retrievals and missing value are 89% and 8% for C6 CI data, and 89% and 10% for C5 CI data, respectively. The C6 MODIS BRDF/Albedo products use all available clear-sky observations, rather than the four observations per day used by C5 products for the limitation of storage processing volume; thus, the former acquires more high-quality retrievals. On the other hand, the improvement in the cloud detection algorithm in MODIS C6 products, combined with the updated threshold of the quality of directional observations (measured by the RMSE and the WOD), partly leads to a lower rate of the main algorithm retrieval of C6 at low latitudes (\(-\)10\({}^{\circ}\)-10\({}^{\circ}\)N) in July. Moreover, the C6 CI shows a higher proportion of missing values than the C5 CI in high latitudes (50\({}^{\circ}\)-70\({}^{\circ}\)) in the winter hemisphere (Northern Hemisphere in January and Southern Hemisphere in July). In addition, 42% of the C6 CI data fail to be retrieved for the winter hemisphere in both January and July, while the corresponding proportions of C5 CI data are approximately 25% and 33% in the winter hemisphere in January and July, respectively. This difference may be attributed to the improved threshold of snow, cloud, and cloud shadow detection in the C6 BRDF/Albedo product.