## 2 Materials and Methods
### OCO-2 SIF Data and Processing  
OCO-2 is a sun-synchronous satellite that has a 16-day repeat cycle, and its local overpass time is at 13:30. The OCO-2 SIF data are retrieved from three grating spectrometers using the O\({}_{2}\)-A band (FWHM of around 0.042 nm) based on the Fraunhofer line infilling at 757 and 771 nm [8], similar to the methods employed for GOSAT SIF retrievals [7]. Each spectrometer measures eight independent cross-track spectra, with each spectra having a spatial resolution of approximately \(1.29\times 2.25\) km\({}^{2}\) at Nadir, which together cover a maximum range ~10 km wide full swath [11]. In the repeat cycles of measurements, OCO-2 switches between Nadir and Glint observation modes, and Target viewing mode is opened when the satellite overpass the ground validation sites, acquiring thousands of observations with different viewing zenith angles (VZA). The Nadir mode follows the same ground track to provide a relatively reliable dataset for the detection of variations over time, while certain varying east-west offsets of the ground tracks exist in the Glint mode's repeat cycles. It was pointed out in [11] that, over land, Glint measurements are likely to underestimate SIF values compared with those by Nadir, given that a more shaded fraction of the canopy may be measured (denoted as a vegetation dark-spot). Therefore, compared with the Glint mode, the Nadir mode's SIF measurements generally provide a more consistent dataset over land, and the effects of viewing geometries of the Glint mode should be taken into consideration when combining them together [1,8,11].  
The OCO-2 v8 product (v.B8100r) that stores the daily line files in netcdf (network common data form) was used in this study. The SIF Lite files only include the soundings that passing the initial quality standard thresholds and contain a significant SIF calibration program which uses non-fluorescent surfaces, including oceans and deserts [7,8]. Unless otherwise specified, the mean values of SIF at 757 nm and 771 nm were used in this study, and the SIF value at 771 nm was multiplied by a factor of 1.5. [PERSON] et al. [11] suggests that the fundamental for averaging the two wavebands is to reduce the retrieval error (by about 1.4 times) through doubling the number of soundings and to improve the robustness of the data analysis, considering the retrievals of the two micro-windows are influenced by different uncertainties.  
Finally, the averaged OCO-2 SIF data on Glint mode were used to examine the influence of different observation geometry on the SIF values and the relationship between SIF and GPP, and then the data with those on Nadir mode were combined in the SIF-based model for comparison with the LUE-based models. The land cover of the Huang-Huai-Hai Plain where the flux tower is located is known to be one of the three largest croplands in China. Therefore, the SIF data retrieval radius from the flux tower was broadened to 150 km [36] to increase the number of available soundings over the study period. Additionally, only those SIF data with a cropland proportion greater than 80% were used to establish the GPP-SIF function. For the comparison of the SIF model and the LUE-based models at different temporal aggregation levels, the daily correction factor provided in the SIF Lite files was applied to convert the mid-day SIF to daily SIF, and then to aggregate to a monthly average.