## 3 Results  
### Evaluating the Accuracy of Upscaled SIF with Long-Term Measurements  
To quantitatively assess the accuracies of the PAR-based method and cos-based method for SIF upscaling from the instantaneous to daily scale, the tower-based SIF measurements at the two sites were regarded as the reference data for comparison with the upscaled SIF at half-hour and daily intervals.  
First, for each measurement day, we evaluated the accuracy of the predicted SIF\({}_{\text{T30}}\) at two sites. The predicted SIF values using the two upscaling methods were compared with the measured SIF values to investigate whether the predicted SIF reliably tracked the diurnal cycles of SIF measurements. We also calculated the corresponding relative root mean square error (RRMSE) values between the measured and predicted SIF values for each day. The measurement days were divided into two weather conditions, sunny and cloudy days, by judging whether the coefficient of determination (R\({}^{2}\)) between PAR and cos(SZA) was larger than 0.9. Figure 4 displays the results for the XTS site in 2018 as an example. On sunny days with clear sky conditions (labeled with solid bars in Figure 4b), the diurnal SIF values predicted by both methods were quite consistent with the measured SIF values: The RRMSE values of the two methods were comparable and less than 40%. However, on cloudy days with unstable weather conditions (labeled with hollow bars in Figure 4b), only the SIF values predicted by the PAR-based method can reliably track the diurnal cycles of the measured SIF. The RRMSE values of the SIF predicted by the cos-based method were larger than 60% for most cloudy days due to the effects of clouds and scattering light.  
Secondly, for all measurement days, we evaluated the accuracy of the predicted SIF at the two sites at both half-hour and daily intervals. Figure 5 shows the correlation analysis between the predicted SIF using the PAR-based and cos-based methods with the measured SIF at the two sites at two temporal scales: half-hour and daily. In general, the predicted SIF values using the PAR-based method at both half-hour and daily temporal scales match the reference values well: The points were located close to the 1:1 line, the R\({}^{2}\) values were higher than 0.9, and the RRMSE values were 25.6% at the half-hour interval and 13.3% at the daily interval. However, the predicted SIF values using the cos-based method were not as robust as those predicted by the PAR-based method, with significantly lower R\({}^{2}\) values and larger RRMSE values at both half-hour and daily intervals.  
From the accuracy assessment on SIF upscaling for both individual days and all measuring days, we can conclude that the PAR-based method can predict diurnal and SIF\({}_{\text{daily}}\) more accurately than the cos-based method. The main reason for this lies in the effects of large weather changes on cloudy days. For more accurate SIF upscaling from the instantaneous to daily scale, the diurnal changes in weather should be taken into account by integrating the continuous PAR data for SIF upscaling.  
Figure 4: Comparison of diurnal cycles (**a**) and relative root mean square error (RRMSE) values (**b**) between the measured SIF and predicted SIF using the cos-based and PAR-based methods at 30 min intervals on sunny and cloudy days in 2018 at the XTS site.  
Figure 5: Scatter diagrams between the measured SIF and predicted SIF using the cos-based and PAR-based methods at half-hour (**a**) and daily (**b**) intervals at two sites. The black dotted line represents the 1:1 line.