## 1 Introduction
According to Reference [27], SIF\({}_{\text{inst}}\) cannot be a substitute for daily SIF (SIF\({}_{\text{daily}}\)) in assessing the SIF-GPP relationship because the SIF\({}_{\text{inst}}\)-SIF\({}_{\text{daily}}\) correlation obviously changes with different seasons and latitudes. Considering that the diurnal variations in SIF are mainly driven by PAR [17; 20; 28; 29; 30], the changes in the SIF\({}_{\text{inst}}\)-SIF\({}_{\text{daily}}\) correlation can be attributed to the variations in the correlation between instantaneous PAR (PAR\({}_{\text{inst}}\)) and daily PAR (PAR\({}_{\text{daily}}\)) across time and space. These variations are mainly caused by three factors: latitude, season, and weather condition. The effects of the first two factors can be explained by the different instantaneous values of the cosine of the solar zenith angle (cos(SZA)) and the length of days under different latitudes and seasons (see Figure 1a,b). In addition, due to the cloud and atmospheric scattering effects on PAR, the PAR\({}_{\text{inst}}\)-PAR\({}_{\text{daily}}\) correlations are obviously not consistent among days with different weatherconditions (see Figure 1c). Thus, it is important to correct all of these factors when upscaling SIF from the instantaneous to daily scale. However, current studies (e.g., References [14; 27; 31]) have only considered the first two factors based on the assumption of sunny days and calculated the correction factor for SIF upscaling using the ratio of instantaneous \(\cos(\mathrm{SZA})\) to daily integrated \(\cos(\mathrm{SZA})\). This method does not reproduce the real SIF\({}_{\mathrm{daily}}\) because the diffuse component of the total shortwave radiation at the surface ranges nearly 15-80% across latitudes from 19 to 80\({}^{\circ}\)[32; 33]. It confirms the need for a more proper method for upscaling SIF from the instantaneous to daily scale and to compare the potential performance of SIF\({}_{\mathrm{daily}}\) with SIF\({}_{\mathrm{inst}}\) in constraining the estimates of GPP. This study focused on upscaling the SIF observations from the instantaneous scale (with a unit of mW/m\({}^{2}\)/nm/sr) to a daily integrated scale (with a unit of J/m\({}^{2}\)/nm/sr/d). The upscaling of SIF\({}_{\mathrm{daily}}\) in this study referred to adding the temporal daily information to the instantaneous in-situ measurements. Using the tower-based high-frequency time series of the measurements, we aim to (i) quantitatively evaluate the results of the SIF upscaling method using PAR as a driving variable (hereafter named the PAR-based method) compared with those using \(\cos(\mathrm{SZA})\) as a driving variable (hereafter named the cos-based method) and (ii) investigate whether the SIF-GPP correlation can be improved when comparing GPP\({}_{\mathrm{daily}}\) with the PAR-based upscaled SIF\({}_{\mathrm{daily}}\) instead of the SIF\({}_{\mathrm{inst}}\) or cos-based upscaled SIF\({}_{\mathrm{daily}}\). Our findings may shed light on the importance of taking the temporal scale of SIF into account when assessing SIF-GPP relationships and are expected to further serve as a reference for the temporal upscaling of space-based SIF products.