## Results
### Possible Reasons for the Differences Among the OCO-2, GOME-2 and Flux Tower GPPs
#### 3.3.2 Effects of the Viewing Zenith Angle (VZA) on the Relationship Between SIF and Tower GPP  
We compared the OCO-2 instantaneous SIF averages based on all soundings of the VZA intervals to investigate whether the VZA had an impact on the GPP-SIF relationship (Figure 8). Due to the sparse coverage of OCO-2, the number of OCO-2 SIF samples was limited, with 71 samples obtained for the Daman and A'rou sites combined. The VZA values were divided into two intervals: 0\({}^{\circ}\) < VZA \(\leq\) 20\({}^{\circ}\) (the number of OCO-2 SIF samples was 56; most of them were in the nadir mode); VZA > 20\({}^{\circ}\) (the number of OCO-2 SIF samples was 15; all were in the glint mode). The results showed that SIF\({}_{757}\) and SIF\({}_{771}\) in the interval of 0\({}^{\circ}\) < VZA \(\leq\) 20\({}^{\circ}\) showed a stronger correlation with the tower GPP than SIF\({}_{757}\) and SIF\({}_{771}\) in the interval of VZA > 20\({}^{\circ}\) at an instantaneous timescale. The averaged SIF and the slope with 0\({}^{\circ}\) < VZA \(\leq\)20\({}^{\circ}\) exhibited slightly higher values than those with VZA > 20\({}^{\circ}\), and the statistical significance was more evident in the interval of 0\({}^{\circ}\) < VZA \(\leq\) 20\({}^{\circ}\) than in the interval of VZA > 20\({}^{\circ}\) (Figure 8). OCO-2 SIF obtained by the sensor in the nadir mode performed better than that obtained in the glint mode. OCO-2 SIF was highly sensitive to the VZA, and the VZA might play an important role in the GPP-SIF relationship. Figure 6: Availability of OCO-2 overpasses and nearest GOME-2 SIF pixels within the 0.5\({}^{\circ}\) grid cell on the corresponding date at the (**a**) Daman and (**b**) A’rou sites. Figure 8: Effects of the VZA on the relationships between the OCO-2 SIF and tower GPP at an instantaneous timescale: (**a**) the relationship between the tower GPP and OCO-2 SIF in the interval of \(0^{\circ}<\) VZA \(\leq 20^{\circ}\) at the Daman and A’rou sites; (**b**) the relationship between the tower GPP and OCO-2 SIF in the interval of VZA \(>20^{\circ}\) at the Daman and A’rou sites. Figure 7: Relationships of the OCO-2 SIF with GOME-2 SIF within the overlaps of OCO-2 and GOME-2 SIF for \(0.5^{\circ}\) grid cells (i.e., the coordinates for each flux tower were taken as the center of the grid cell) at the Daman and A’rou sites. (**a**) And (**b**) relationships of OCO-2 SIF (i.e., all samples were averaged by using all soundings) with GOME-2 SIF. (**c**) And (**d**) relationships of OCO-2 SIF (i.e., the overlaps of certain samples with more than 70 soundings were selected) with GOME-2 SIF. 3.3. Relationships of the OCO-2 and GOME-2 SIF with APAR and APAR \(\times\) T\({}_{\rm scalar}\)\(\times\) W\({}_{\rm scalar}\)  
At the Daman site, APAR and APAR \(\times\) T\({}_{\rm scalar}\)\(\times\) W\({}_{\rm scalar}\) explained 65% and 68% of the variance in SIF\({}_{757}\) as well as 48% and 50% of the variance in SIF\({}_{771}\), respectively (Figure 9a,b). At the A'rou site, APAR and APAR \(\times\) T\({}_{\rm scalar}\)\(\times\) W\({}_{\rm scalar}\) explained 79% and 87% of the variance in SIF\({}_{757}\) as well as 71% and 74% of the variance in SIF\({}_{771}\), respectively (Figure 10c,d). The correlations were much higher at A'rou than at Daman. These results indicated that both SIF\({}_{757}\) and SIF\({}_{771}\) were mainly dependent on APAR. APAR and the two environmental scaling factors (APAR \(\times\) T\({}_{\rm scalar}\)\(\times\) W\({}_{\rm scalar}\)) accounted for more variance in SIF\({}_{757}\) and SIF\({}_{771}\) than APAR accounted for alone at both sites, which indicates that both SIF\({}_{757}\) and SIF\({}_{771}\) were also related to environmental stresses. At the Daman site, APAR and APAR \(\times\) T\({}_{\rm scalar}\)\(\times\) W\({}_{\rm scalar}\) explained 64% and 66% of the variance in SIF\({}_{740}\) as well as 65% and 68% of the variance in SIF\({}_{740}\) at the A'rou site, respectively (Figure 10).