## Results
### Possible Reasons for the Differences Among the OCO-2, GOME-2 and Flux Tower GPPs  
3.1 Effects of Spatial Mismatches on the Differences Among the OCO-2 SIF, GOME-2 SIF, and Flux Tower GPPs  
In this study, we demonstrated the correlation between OCO-2 SIF and tower GPP was stronger than the correlation between GOME-2 SIF with tower GPP. One of the reasons for this discrepancy is the spatial mismatch between the GOME-2 and OCO-2 footprints (Figure 6). A 0.5\({}^{\circ}\) grid cell around the flux site was used to extract GOME-2 and OCO-2 data. Within the grid cell, OCO-2 data covered only a narrow strip, while GOME-2 pixels covered the majority of the grid cell and were more or less outside the grid cell. For OCO-2 or GOME-2, the pixels located within the 0.5\({}^{\circ}\) grid cell changed over time. Due to different revisit periods, the overlap area changed greatly with time and location. A spatial overlap was observed between the GOME-2 and OCO-2 footprints and a given 0.5\({}^{\circ}\) grid cell based on the coordinates of each flux tower for samples on the corresponding date. For certain days, the GOME-2 SIF had high spatial overlaps with the OCO-2 SIF, such as on 17 May 2015, 13 February 2016, 26 October 2016, and 5 March 2017 at the Daman site as well as 21 October 2014, 24 December 2014, 6 December 2014, 18 July 2017, and 16 November 2017 at the A'rou site. However, the size of the spatial overlap was very small on other dates, such as on 6 September 2015 and 26 September 2016 at Daman and 31 July 2016 and 5 March 2017 at A'rou (Figure 6, Figures S4 and S5). The correlation between GOME-2 SIF and OCO-2 SIF was dependent on the size of the overlap, which may result in differences between OCO-2 SIF and GOME-2 SIF.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline  & & \multicolumn{3}{c}{**OCO-2**} & \multicolumn{3}{c}{**GOME-2 (16-d)**} \\ \cline{3-7}  & & **GPP\_SIF\_757** & **GPP\_SIF\_71** & **GPP\_VPM (16-d)** & **GPP\_SIF\_740** & **GPP\_RSIF** \\ \hline \multirow{5}{*}{Daman} & R2 & 0.86 & 0.72 & 0.95 & 0.68 & 0.92 \\  & a & 0.86 & 0.73 & 0.63 & 0.68 & 0.92 \\  & b & 0.23 & 0.45 & \(-\)0.06 & 0.86 & 0.29 \\  & p & \textless{}0.001 & \textless{}0.001 & \textless{}0.001 & \textless{}0.001 & \textless{}0.001 \\  & RMSE & 1.29 & 1.79 & 1.81 & 2.13 & 1.46 \\ \hline \multirow{5}{*}{A’rou} & R2 & 0.91 & 0.76 & 0.94 & 0.69 & 0.95 \\  & a & 0.92 & 0.77 & 1.05 & 0.7 & 0.95 \\ \cline{1-1}  & b & 0.11 & 0.3 & \(-\)0.19 & 0.43 & 0.11 \\ \cline{1-1}  & p & \textless{}0.001 & \textless{}0.001 & \textless{}0.001 & \textless{}0.001 & \textless{}0.001 \\ \cline{1-1}  & RMSE & 0.52 & 0.88 & 0.54 & 0.98 & 0.67 \\ \hline \hline \end{tabular}  
* R2 indicates the determination coefficient, and a and b represent the slope and intercept of the linear regression.  
\end{table}
Table 2: Comparison between the tower GPP and model-predicted GPP.  
Figure 5: Relationships of the tower GPP with the GPP estimates from SIF\({}_{757}\), SIF\({}_{771}\), SIF\({}_{740}\), and VPM at the (a) Daman and (b) A’rou sites.  
The GOME-2 SIF was compared with the OCO-2 SIF based on all soundings and the soundings with numbers greater than 70 at a daily timescale; then, how the sizes of the spatial overlaps influenced the OCO-2 SIF and GOME-2 SIF relationship was examined (Figure 7, Figures S4 and S5). We found that the correlation coefficients between GOME-2 SIF and OCO-2 SIF based on samples with more than 70 soundings were stronger than the correlation coefficients between GOME-2 SIF and OCO-2 SIF based on all soundings at both the Daman and A'rou sites. Correspondingly, the slope also significantly varied with the sounding samples (i.e., the slopes increased obviously when the sample overlaps were based on more than 70 soundings). This analysis indicated that the robustness of the correlation coefficients between GOME-2 SIF and OCO-2 SIF was highly dependent on their spatial overlaps.