## 3 Results  
### The CVs of PAR during Different Sky Conditions  
The diurnal dynamics of high-frequency PAR and the three \(CV\)s during three typical days are shown in Figures 4 and 5. On DOY 224, the sky was clear for most of the day, and the diurnal variation curve of PAR was close to the cosine curve, with only a few sharp fluctuations (Figure 4a). On this day, most (~99%) of the \(CV_{E}\) and \(CV_{L}\) values (Figure 5a,b) were lower than 0.5%. The \(CV_{section}\) values were generally greater than the values of \(CV_{E}\) and \(CV_{L}\). However, most (~90%) of the \(CV_{section}\) values were still smaller than 0.5% (Figure 5c). On DOY 227, the illumination fluctuated rapidly and frequently (Figure 4b). On this day, the number of data points with \(CV\) values greater than 0.5% was significantly higher than that on DOY 224 (Figure 5d-f). On DOY 230, the sky was cloudy during most of the day, and the PAR was lower than that on the other days (Figure 4c). On this day, all the \(CV_{E}\) and \(CV_{L}\) values (Figure 5g,h) were lower than 0.5%, and most (~55%) of the \(CV_{section}\) values were still smaller than 0.5% (Figure 5i). The CVs of high-frequency PAR data can accurately depict the stability of the illumination during spectral measurements. DOY 224 was mostly clear throughout the day. There were only a few data (-12%) that were filtered out for 3 FLD, SFM, and SVD (Figure 6a-c). The \(R^{2}\) values between the PAR and SIF values obtained by all three SIF retrieval methods were all higher than 0.7 and did not change significantly after data filtering (Figure 7a-c). On DOY 227, the illumination fluctuated rapidly. This resulted in many outliers and even negative SIF values retrieved using the 3 FLD and SFM algorithms (Figure 6d,e). The \(R^{2}\) values between the SIF and PAR values were less than 0.5. After data filtering, these \(R^{2}\) values increased significantly to approximately 0.72 (Figure 7d,e). However, the SVD results were very different on this day from the 3 FLD and SFM results (Figures 6f and 7f). Before data filtering, the \(R^{2}\) value of SVD was approximately 0.6, which was obviously higher than those of 3 FLD and SFM. After data filtering, the \(R^{2}\) value of SVD only increased by a small margin to 0.67. This is primarily because on this day, only 23% of the data was filtered out for the SVD method, which was significantly lower than the proportions of 3 FLD and SFM (-65%). Figure 5: The diurnal dynamics of \(CV_{E}\), \(CV_{L}\), and \(CV_{section}\) under three sky condition days: (**a–c**) for the clear day (DOY 224); (**d–f**) for the rapidly fluctuating illumination condition day (DOY 227); and (**g–i**) for the cloudy day (DOY 230). The gray dots represent the \(CV\)s of downwelling irradiance measurements. The blue dots represent the \(CV\)s of upwelling radiance measurements. The black dots represent the \(CV\)s of the measurement sections. DOY 230 was cloudy. For most of the day, the light intensity fluctuated more slowly than it did on DOY 227 (Figure 4c). Therefore, the proportions of the filtered data of 3 FLD and SFM on this day were lower than the proportions of DOY 227 (Figure 6g,h). In addition, SVD data were entirely retained (Figure 6i). After filtering the data, the \(R^{2}\) values of the 3 FLD and SFM algorithms increased from \(\sim\)0.83 to \(\sim\)0.93 (Figure 7g,h). The \(R^{2}\) of SVD was \(\sim\)0.76 (Figure 7i), which was lower than the values of 3 FLD and SFM. We also utilized all observation data on the 17 days without rain from the measurement period (DOY 214-234) to verify the proposed method. After eliminating the data with unstable illumination conditions, the \(R^{2}\) values of the 3 FLD and SFM algorithms increased from \(\sim\)0.64 to \(\sim\)0.72 (Figure 8a,b). However, the \(R^{2}\) of SVD was slightly improved (Figure 8c) and lower than the \(R^{2}\) of 3 FLD or SFM. These results show that the data quality assessment method is significantly effective for the 3 FLD and SFM algorithms and slightly effective for the SVD algorithm.