## 3 Field Measurements and Data Processing  
### Field Installation at EC Flux Sites  
Up to now, the SIFSpec systems have been in operation at 10 EC flux sites, including five cropland sites, two forest sites, and three grassland sites. Details of the sites, including the location, height of instrument above the ground, and observed ecosystem type, are listed in Table 1.  
The SIFSpec system is usually mounted on the eddy covariance (EC) tower. A 4 m long square iron welded pipe, which extends 3 m away from the boundary of the platform, is used to fix the two optical fibers and also to ensure that the cosine-corrector on the upward optical fiber points vertically upward. The pipe is fixed on the south side using several detachable U-shaped extending pieces of irons when loading and unloading the instruments. A specially designed stainless board is mounted at the end of the pipe to fix the angle of the optical fibers. This board has an aluminum alloy spirit level on the top to monitor the angle of the board and an angulometer welded on the upper right side to control the orientation of the down-welling fiber. The bare fiber points at the vegetation canopy with a view zenith angle of about 25\({}^{\circ}\) in order to avoid the heterogeneous or sparsely vegetated surface below the EC tower. As for sites where there is no EC tower, the design concept for the instrument installation is different. A customized double-layer shelf is employed to place the instrument. The height of the shelf is about 2 m and the SIFSpec is placed on the upper layer to avoid the influence of the ground moisture and crop irrigation. A manual bracket with cantilevers is used to mount the optical fibers. Both the telescopic pole and the telescopic arm have three sections, which are used to regulate the height and \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Ecosystem Type** & **Site Name** & **ID** & **Latitude** & **Longitude** & **Height** \\ \hline \multirow{4}{*}{Cropland} & XiaoTangshan & XTS & 40.1786 N & 116.4432 E & 4 m \\  & HuaiLai & HL & 40.3489 E & 115.7882 N & 4 m \\  & DaMan & DM & 38.8555 E & 100.3722 N & 25 m \\  & ShangQiu & SQ & 34.5870 N & 115.5753 E & 12 m \\  & JuRong & JR & 31.8068 N & 119.2173 E & 7 m \\ \hline \multirow{2}{*}{Forest} & QianYanzhou & QYZ & 26.7478 N & 115.0581 E & 32 m \\  & DingHushan & DHS & 23.1733 N & 112.5361 E & 36 m \\ \hline \multirow{2}{*}{Grassland} & XiLinhaote & XLHT & 43.5513 N & 116.6710 E & 2.5 m \\  & HongYuan & HY & 32.8404 N & 102.5775 E & 3 m \\  & ARou & AR & 38.0444 N & 100.4647 E & 25 m \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Details of the eddy covariance (EC) flux sites with solar-induced chlorophyll fluorescence (SIF) measurement (SIFSpec) mounted on the tower. the length. The telescopic arm is normally set to extend 3 m from the telescopic pole. The height of the telescopic pole set up is 2.5 m for wheat or grassland measurements and 4 m for maize observations. Meteorological and radiation sensors for long-term observations are deployed on the flux tower at each site in the ChinaSpec network. Meteorological data are obtained every 10 min and processed into quality-controlled 30 min and daily averages [34]. The surface soil heat fluxes; photosynthetically active radiation (PAR); broadband incoming shortwave radiation (R\({}_{in}\)); thermal radiation of the cropland; and meteorological parameters including air temperature (Ta), air pressure (p), vapor pressure deficit (VPD), air humidity (Rh), wind speed (u), four-component radiation, and turbulent fluxes are simultaneously measured using the specific instruments deployed on the tower at different heights [35]. The EC flux data, including the exchange of CO\({}_{2}\), energy, and H\({}_{2}\)O, are collected by the EC system with a sampling frequency of 10 Hz [36]. The raw EC data are processed to obtain the atmospheric CO\({}_{2}\) concentration (Ca), net ecosystem exchange of CO\({}_{2}\) (NEE), sensible heat flux (H), latent heat flux (LE), and friction velocity (u*) using the Edire software [36; 37].