## 3 Field Measurements and Data Processing
### Data Processing  
The data processing chain for the radiance and irradiance measurements collected by the SIFSpec system includes quality filtering, atmospheric correction, and reflectance and the retrieval of the SIF. The down-welling irradiance and up-welling radiance spectra are collected approximately every three minutes, which results in a large volume of spectral measurements. Thus, automated processing is essential for efficient retrieval of the SIF results. Firstly, quality filtering of the raw data is carried out to provide a high-quality SIF dataset. Two main criteria--the stability of measurements between two irradiance observations and obvious SIF  
Figure 4: Variation with wavelength of the calibration coefficients of the upward and downward channels of the SIFSpec system at the DM site based on laboratory calibration. Figure 3: (**a**) Laboratory radiometric calibration of the SIFSpec system; (**b**) the radiance spectra of the light source used for calibrations. contributions to the apparent reflectance--are included in the quality control. The mismatch between the downward irradiance and upward radiance observation times is the primary problem that needs to be considered. The data will be rejected for use in SIF retrieval if the SZA is larger than 85\({}^{\circ}\), as a larger SZA will increase the risk of there being a larger variation in the down-welling irradiance. In addition, the difference between the first and second downward irradiance values found during one whole measurement is used as a quality indicator, with a threshold value of 10% being used to directly reject data where there is a time mismatch between the measurements made by the two channels. The peak in the apparent reflectance is useful for further data quality control. Data with an obvious peak in the apparent reflectance will be retained for subsequent SIF retrieval. Furthermore, the spectral shift problem between upward and downward channels can also be identified using the 'dip' peak in the apparent reflectance. The spectral shift will obviously influence SIF retrievals that are made using FLD-based methods [38]. Secondly, an automatic software module is set up to conduct atmospheric correction on the long-term SIF system observations. The O\({}_{2}\)-A and O\({}_{2}\)-B bands are commonly used to retrieve SIF results based on long-term tower-based measurements. In order to obtain accurate SIF retrievals at the top of the canopy, the up-welling radiance and down-welling irradiance collected at the flux tower should be corrected to those at the top of canopy, as shown in Figure 5. According to the analysis made by previous studies [39; 40], for tower-based measurements, if the height of the flux tower is greater than 10 m, an SIF retrieval bias of about 0.1 mW m\({}^{-2}\) sr\({}^{-1}\) nm\({}^{-1}\) at the O\({}_{2}\)-A band will be introduced as a result of the radiative absorption and scattering between the sensor and the canopy. Compared with the absolute intensity of the SIF signal (<2 mW m\({}^{-2}\) sr\({}^{-1}\) nm\({}^{-1}\)), this bias is not negligible. Thus, it is important to carry out atmospheric correction on both the tower-based down-welling irradiance and up-welling radiance before the SIF retrieval process, especially for the O\({}_{2}\)-A band [41], which has a lower transmittance. Figure 5: Schematic diagram of the atmospheric correction for tower-based SIFSpec system observations. \(E_{lower}\) and \(E_{canopy}\) are the downward irradiance at the sensor and top of the canopy; \(L_{lower}\) and \(L_{canopy}\) are the upward radiance arriving the sensor and at the top of the canopy; \(\Delta\)H is the height between the canopy and the sensor; \(T_{\uparrow}\) and \(T_{\downarrow}\) are the upward and downward atmosphere transmittances between the canopy and the sensor, respectively.