## 3 Method
### Cloud Mask  
The NIR CWV retrieval algorithm is only applicable to the pixels not affected by clouds, so it is necessary to remove the cloud-influenced part of the PSAC CWV retrieval results to ensure the quality of PSAC CWV data. Two methods are used to identify cloud pixels. If a pixel is identified as a cloud pixel by any of the methods, the pixel is removed as a cloud pixel.  
(1) The TOA reflectance of band 2 (443 nm) and band 7 (1380 nm) is used for the cloud identification [33; 34]. For a pixel that needs to be detected, if the TOA reflectance of band 2 of the pixel is greater than 0.4, the pixel would be regarded as a cloud pixel. Next, we identify cloud pixels based on the TOA reflectance of band 7. For a pixel that needs to be detected, it is first assumed that the pixel is not affected by clouds. Then we can calculate the CWV of the pixel based on Equation (5). Next, the water vapor absorption transmittance of band 7 can be calculated using the Second Simulation of the Satellite Signal in the Solar Spectrum (6 SV) model based on the calculated CWV [35]. Since the TOA reflectance of band 7 is less than 1 for cloud-free pixels when water vapor content is equal to 0, so the actual observed TOA reflectance of band 7 should be less than the value of 1 multiplied by  
Figure 3: Scatter plot of matching results between the slanted water vapor content in the path of the sun-ground-satellite (i.e., \(V_{CWW}*L\)) and the natural logarithm of the water vapor absorption transmittance of band 6 of PSAC (i.e., \(ln\left(T_{g}^{910\text{ nm}}\right)\)) in East Asia (18–53°N, 73–130°E). Each blue solid circle represents a matching result of AERONET data and PSAC data. The red solid line represents the fitting equation of the natural logarithm of the water vapor absorption transmittance of band 6 and the slanted water vapor content in the path of the sun-ground-satellite.  
the water vapor absorption transmittance calculated above. In other words, if a pixel is not affected by clouds, the actual observed TOA reflectance of band 7 of the pixel is less than the water vapor absorption transmittance of band 7 of the pixel calculated based on the above method. Therefore, if the actual observed TOA reflectance of band 7 of a pixel is not smaller than the water vapor absorption transmittance of band 7 of the pixel calculated based on the above method, the pixel would be regarded as a cloud pixel.  
(2) The TOA reflectance difference between adjacent pixels of band 2 and band 7 is used for the cloud identification [5; 36]. For a pixel that needs to be detected, the standard deviation of apparent reflectance of it and its nearest 8 pixels is used to characterize the difference. The specific calculation method is given in Equation (6). If the calculated standard deviation of band 2 is greater than 0.01, or the standard deviation of band 7 is greater than 0.005, the pixel would be considered as a cloud pixel.  
\[\text{STD}=\sqrt{\frac{\sum_{i=1}^{g}(R_{TOA}^{i}-R_{TOA}^{mean})^{2}}{9}}, \tag{6}\]  
where \(R_{TOA}^{i}\) is the TOA reflectance with a different serial number, \(R_{TOA}^{mean}\) is the average value of all 9 TOA reflectance participating in the calculation, and STD represents the calculated standard deviation.  
Note that the above thresholds (i.e., 0.4, 0.01, and 0.005) were set with reference to the threshold used by the cloud mask algorithms of the Advanced Himawari Imager (AHI) and MODIS that use similar bands to PSAC. Since the cloud mask is not the focus of this paper, the requirement for the cloud mask algorithm in this paper is that most cloud pixels can be detected. Examples of cloud mask results based on the above methods are shown in Figure 4. By analyzing Figure 4, it can be seen that the cloud mask methods used in this work are basically effective in identifying cloudy pixels.