## 4 Discussion  
### Effects of Viewing Zenith Angle in OCO-2 Glint Mode  
The robustness of the GPP-SIF relationships is affected by the quality of SIF retrievals, the accuracy of the carbon fluxes, as well as the homogeneity of the studied ecosystems. Because of OCO-2's high discrete coverage, the SIF data in this study were obtained within a radius of 150 km centered at the flux tower to increase the number of soundings. Although only the SIF data with a cropland fraction greater than 80% were used to establish the SIF-GPP function (Table 1), this could also cause a footprint mismatch between the OCO-2 SIF and tower observations to some extent. On the other hand, [PERSON] et al. (2018) has suggested that the effects of VZAs should be considered when using OCO-2 Glint or Target modes. We found that there is little difference between SIF values with and without VZA restriction except for two threshold values (Figure 1a), while the slope of the GPP-SIF relationship with VZA ranging between 20\({}^{\circ}\) and 40\({}^{\circ}\) exhibited slightly higher values than those with other VZAs (Figure 1b). The circumstances are not exactly the same as the directional effect on the SIF emission in Target mode that was reported by ([PERSON] et al., 2018). [PERSON] et al. (2018) showed that the averaged SIF values and GPP-SIF relationship were almost unaffected when VZA was less than 40\({}^{\circ}\), while the averaged SIF with VZA greater than 40\({}^{\circ}\) exhibited slightly higher values than those with all VZAs and \begin{table}
\begin{tabular}{c c c c c c c c c c c c} \hline \hline \multirow{2}{*}{**Models Scale**} & \multicolumn{3}{c}{**Mid-Day**} & \multicolumn{3}{c}{**Daily**} & \multicolumn{3}{c}{**Monthly**} \\ \cline{2-13}  & \(\mathbf{R^{2}}\) & **RMSE** & **RRMSEBias** & \(\mathbf{R^{2}}\) & **RMSE** & **NRMSEBias** & \(\mathbf{R^{2}}\) & **RMSE** & **RRMSEBias** \\ \hline
**SIF model** & **0.59** & **9.23** & 15.61 & 0.55 & **0.77** & **2.50** & 13.49 & 0.84 & **0.94** & **0.59** & 4.06 & 0.98 \\
**SL-LUE** & 0.42 & 11.32 & **19.15** & **0.31** & 0.71 & 2.82 & **15.21** & **0.63** & 0.84 & 1.10 & **7.57** & **0.63** \\
**TL-LUE** & **0.68** & **9.03** & 15.28 & 0.50 & **0.83** & **2.51** & 13.54 & 0.75 & **0.98** & **0.65** & 4.47 & 0.91 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: R\({}^{2}\), bias (observations minus simulations), root-mean-square error (RMSE), and relative/normalized RMSE (RRMSE) of the mid-day, daily, and monthly GPP (in gC m\({}^{-2}\) d\({}^{-1}\)) in 2016 for the SIF model, SL-LUE model, and TL-LUE model, respectively. The units of bias and RMSE are both gC m\({}^{-2}\) d\({}^{-1}\). The unit of relative RMSE is %. Figure 6: Validation of the SIF model, the SL-LUE model, and the TL-LUE model on the mid-day, daily, and monthly timescales in 2016, respectively. The panels (**a–c**): the seasonal variations of simulated and tower observed GPP. The panels (**d–f**): comparison of the GPP simulated by the SIF model (red lines), SL-LUE (green lines), and TL-LUE (blue lines). the resulting GPP-SIF relationships were slightly different from those with smaller VZAs. This might be due to the VZA changes in different ways between the Target mode and Glint mode. In the Target mode, a great amount of measurements at different VZA values was acquired when the satellite focused on the ground sites during the overpass time. However, the viewing geometry of the Glint mode changed as the position of the land dark-spot varies over the season from east to west. That is, the average SIF with \(20^{\circ}<\) VZA \(\leq 40^{\circ}\) occurred in the growing season of winter wheat, while the SIF with \(0^{\circ}<\) VZA \(\leq 20^{\circ}\) appeared at the ripening stage of winter wheat and the early stage of summer maize, and the SIF with VZA \(>40^{\circ}\) appeared at the seedling stage of winter wheat.