## 3 Results  
### Effects of Viewing Zenith Angle in OCO-2 Glint Mode on SIF Observations  
We intended to combine SIF values from Glint and Nadir observation modes to increase the number of available SIF observations, but the SIF retrievals on Glint mode were measured at different VZAs over time, which may affect the magnitude of SIF values and the relationship between SIF and GPP. Consequently, we first extracted the mid-day SIF data in Glint mode from a circular region with a radius of 150 km centering on the flux tower for the period from January 2015 to December 2016 (Table 1, sorted by ascending VZAs). The mid-day SIF averages of all soundings without VZA restriction and those with varying spacings of VZAs were further compared to explore how the VZA affected the SIF values, as well as the relationship between SIF and GPP (Figure 1). The VZA was separated into three intervals [1] (VZA \(\leq 20^{\circ}\); \(20^{\circ}\)\(<\) VZA \(\leq 40^{\circ}\); and VZA \(>40^{\circ}\)) and these three intervals accounted for 27.1%, 36.0%, and 36.9% of the soundings, respectively. The sensitivity analysis showed that differences of the SIF values only existed in the threshold value, which had few soundings (1 and 8), resulting in large uncertainties (bold in Table 1, green oval in Figure 1a). Accordingly, the differences were negligible between the averaged SIF retrievals with and without VZA restrictions (Figure 1a). In addition, we also found that the viewing geometries in Glint mode varied with seasons. For example, low SIF values with high VZA (\(>\)40\({}^{\circ}\)) occurred in the non-growing/cold season (Figure 1a, blue circle, from November to February) and high SIF values with low VZA (\(\leq\)20\({}^{\circ}\)) appeared in the late stages of winter wheat and the early growth period of summer maize (Figure 1a, red and black circles). The corresponding SIF values with VZA \(\leq 20^{\circ}\) exhibited slightly higher values compared with those VZA \(>40^{\circ}\). However, the GPP-SIF function was not greatly affected in terms of the R\({}^{2}\) of 0.90 (\(\pm\)0.01) and the fitted slope of 23.74 (\(\pm\)1.88) (Figure 1b). In this case, SIF retrievals and the GPP-SIF relationship are only slightly different with varying VZAs on Glint mode in this flux tower. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**No.** & **Year** & **Date** & \begin{tabular}{c} **Day of** \\ **Year** \\ \end{tabular} & \begin{tabular}{c} **Count of** \\ **Soundings** \\ \end{tabular} & \begin{tabular}{c} **Cropland LC** \\ **Percent (\%)** \\ \end{tabular} & \
\begin{tabular}{c} **Local Time of** \\ **Overpass** \\ \end{tabular} &
\begin{tabular}{c} **Viewing Zenith Angle** \\ **at Overpass (deg)** \\ \end{tabular} \\ \hline  
an & 1 & 97 & 97.0 & 13:09:23 & 48.01 \\ \hline \hline \end{tabular} \({}^{\text{1}}\) The threshold values between different VZAs intervals.  
\end{table} 
Table 1: Specific information of the 35 SIF (sun-induced chlorophyll fluorescence) measurements retrieved by OCO-2 (Orbiting Carbon Observatory-2) in Glint mode within a circular area of a 150 km radius from the Yucheng flux tower (from January 2015 to December 2016). Figure 1: Effects of varying view zenith angle (VZA) on OCO-2 Glint SIF observations and the gross primary production (GPP)-SIF correlation at the mid-day timescale: (**a**) the differences between SIF with and without VZA restrictions: VZA \(\leq\) 20\({}^{\circ}\); 20\({}^{\circ}\)\(<\) VZA \(\leq\) 40\({}^{\circ}\); and VZA \(>\) 40\({}^{\circ}\); (**b**) the relationships of ground GPP and SIF under various intervals of VZAs: VZA \(\leq\) 20\({}^{\circ}\); 20\({}^{\circ}\)\(<\) VZA \(\leq\) 40\({}^{\circ}\); VZA \(>\) 40\({}^{\circ}\); and no restriction (0\({}^{\circ}\)\(<\) VZA \(\leq\) 50\({}^{\circ}\)). The data points are averages of the sounding for each point in the time referring to Table 1 and the error bar represents \(\pm\)1\(\sigma\) statistical uncertainty estimations of OCO-2 SIF.