## 3 Field Measurements and Data Processing
### Data Processing
According to the method proposed in the previous work [39], a simple and operational method based on look-up-tables (LUTs) can be used to estimate the atmosphere transmittance between the tower-based instrument and the top of the vegetation canopy for both the downward irradiance and upward radiance. Atmospheric transmittances at the O\({}_{2}\)-A and O\({}_{2}\)-B bands are mainly related to the aerosol optical depth (AOD), radiative transfer path length (RTPL), atmospheric pressure, and temperature [42]. To establish the look-up-tables for a specific site, the parameter dataset, which includes the height of the instrument, altitude, view zenith angle (VZA), and a range of solar zenith angles (SZA) and values of the AOD, is needed in simulations made using the MODerate resolution atmospheric TRANsmission 5 (MODTRAN 5) model. For in situ observations, AOD\({}_{550}\) is calculated using the ratio of the down-welling irradiance at 790 nm to that at 660 nm. In this study, the upward vegetation radiance was measured using a bare optical fiber; this radiance is different from the downward irradiance measured with the cosine-corrector. Thus, the look-up tables for the upward and downward channels were constructed separately; and the tower-based SIF retrievals at O\({}_{2}\)-A band before/after atmospheric correction are presented in Figure 6. It shows that the SIF results are obviously higher after atmospheric correction than the uncorrected SIF results, especially for noon time. For details, please refer to [PERSON] et al. (2019) [39]. Thirdly, several ground-based SIF retrieval algorithms based on the FLD principle are widely used. Four different single-band SIF retrieval results at the H\(\alpha\) (656 nm), O\({}_{2}\)-A (761 nm), and O\({}_{2}\)-B bands (688 nm) based on standard FLD (sFLD) [43], 3-band FLD (3 FLD) [44], improved FLD (iFLD) [45], and principal component analysis-based FLD (pFLD) [46] algorithms are simultaneously obtained for the processing of SIFSpec measurements. In addition, the full-spectrum spectral fitting method (F-SFM) [47] based on principal components analysis (PCA) is employed to acquire the full-spectrum SIF retrievals. An example of single-band diurnal SIF curves at the O\({}_{2}\)-A band (measured on 7 September 2017) is illustrated in Figure 7. This example shows that there is no obvious difference between the results obtained using the different SIF retrieval methods. The 3 FLD method is considered to be relatively simple and robust for ground-based measurements with a spectral resolution of 0.3 nm [40; 48]. Therefore, the 3 FLD algorithm was selected for the SIF retrieval in this study. Moreover, common VIs, including the Normalized Difference Vegetation Index (NDVI), the MERIS Terrestrial Chlorophyll Index (MTCI), and the NIR reflectance of vegetation (NIRv), were also simultaneously calculated using the measured reflectance data. Figure 6: The diurnal variations in SIF results at the O\({}_{2}\)–A band before/after atmospheric correction as measured on 7 September 2017 at the DM site (corrected: red; uncorrected: blue). The online tool available on the Max Planck Institute for Biogeochemistry (MPI-BGC) website ([[http://www.bgc-jena.mpg.de/](http://www.bgc-jena.mpg.de/)]([http://www.bgc-jena.mpg.de/](http://www.bgc-jena.mpg.de/))\(\sim\){}MDIwork/eddyproc/) can be used to calculate the GPP data using the input parameters Rin, Rh, LE, H, Ta, and u* based on the night time-based partitioning algorithm [49; 50]. GPP is calculated by separating the day-time NEE and the estimated respiration. The gap filling uses the light response function (Michaelis-Menten equation) and respiration function ([PERSON]n't Hoff equation) [49; 51]. GPP data with 30-minute time intervals are calculated for analysis in conjunction with the SIFSpec measurements. The daily average of GPP is also calculated using the 30-minute data.