## 5 Conclusions  
Tower-based SIF observations are important in the field of SIF retrieval and applications, especially for a better understanding of the relationship between SIF and GPP. However, the strong absorption effect at the oxygen absorption bands has an obvious influence on the SIF retrieval. This means that atmospheric correction is required, even for tower-based SIF observations made from a sensor at a height of tens of meters.  
In this study, we analyzed the effects of atmospheric radiative transfer on tower-based SIF observations for both downwelling irradiance and upwelling radiance, and proposed an operational solution for atmospheric correction of tower-based SIF observations at the O\({}_{2}\)-A band. The aerosol optical depth (AOD) and radiative transfer path length (RTPL) were found to be the main factors influencing the transmittance at the oxygen absorption band. Consequently, LUTs based on MODTRAN simulations were established for the estimating the upward and downward atmosphere transmitters from the TOC to the height of the tower-based sensor. The RTPL (the ratio of the irradiance at 790 nm to that at 660 nm), the temperature, and pressure were required input parameters.  
According to the results of the field experiments, the atmospheric correction based on LUTs produced more accurate tower-based SIF retrieval at the O\({}_{2}\)-A band. For observations of a mature summer maize canopy with no SIF emission, the retrieved SIF values after atmospheric correction were closer to zero than those without atmospheric correction. The negative SIF values found were partly due to the so-called \"direct radiation infilling\" effect. For observations of a green summer maize canopy, the SIF values retrieved from tower-based observations with atmospheric correction matched well with the synchronous retrievals from ground-based observations. The RMSE between the SIF values retrieved from tower-based and ground-based observations decreased from 0.221 to 0.078 mW/m\({}^{2}\)/nm/sr after the atmospheric correction. The results also indicated that the atmospheric correction for tower-based SIF measurements is important for the O\({}_{2}\)-A band due to the strong oxygen absorption effect.  
Conceptualization, X.L. and L.L.; methodology, X.L. and J.H.; software, J.G.; validation, J.G. and J.H.; formal analysis, X.L., J.G., and J.H.; investigation, X.L. and J.G.; resources, J.G.; data curation, J.G.; writing--original draft preparation, X.L.; writing--review and editing, L.L.; visualization, [PERSON]; supervision, L.L.  
This research was funded by the National Key Research and Development Program of China, grant number 2017 YFC0503803, and the National Natural Science Foundation of China, grant number 41701396 and 41871239.  
The authors declare no conflict of interest.  
Figure 11: Upward atmospheric transmittances at 760.6 nm from MODTRAN simulations with different FWHM. The red points (\(T_{\uparrow}\)) are transmittances calculated as \(<\)_L\({}_{sensor}\)/\(L_{canopy}\)\(>\)_, and the blue points (\(T_{\uparrow}^{\prime}\) ) are transmittances calculated as \(<\)_L\({}_{sensor}\)\(>\)/\(<\)_L\({}_{canopy}\)\(>\)_. \(<\)\(>\) represents convolution with ISRF. The original FWHM of MODTRAN simulation is 1 cm\({}^{-1}\) (about 0.06 nm at 760 nm), and the ISRF were modelled using Gaussian functions with different FWHM. The atmospheric profile is mid-latitude summer, AOD is 0.1, VZA is 25\({}^{\circ}\), sensor’s height is 25 m, and surface altitude is 1.5 km.