## 1 Introduction  
Solar-induced chlorophyll fluorescence (SIF) has been proven to be an efficient indicator of vegetation photosynthesis [1]. In recent years, a series of algorithms has been developed for SIF retrieval from ground-based [2; 3; 4; 5; 6; 7; 8], airborne [9; 10; 11] and satellite [12; 13; 14; 15] observations. The satellite-based SIF products have shown great potential for global monitoring of vegetation gross primary productivity (GPP) [16; 17; 18; 19]. However, many uncertainties regarding the relationship between the SIF and GPP remain. The tower-based eddy covariance (EC) technique is widely used for the measurement of the carbon flux at the ecosystem scale [20]. To investigate the relationship between the SIF and GPP, tower-based, continuous spectral observations in conjunction with EC measurements are needed. In recent years, a number of tower-based spectral observation systems have been installed at different EC sites. Examples include the Multiplexer Radiometer Irradiometer (MRI) [7; 21], the NASA FUSION tower, the HyperSpectral Irradiometer (HSI) [22; 23; 24], the TriFLEX at Avignon in France [25], the UNIEDI System at the FluxNet Hyytiala site in Finland [26], the FluoSpec [6], the SIF-Sys [8], and the ChinaSpec [27]. The most commonly used methods of retrieving SIF from ground-based observations based on the Fraunhofer Line Depth (FLD) principle need measurements of both the upwelling radiance and downwelling irradiance at the top-of canopy (TOC) level. For tower-based SIF observations, the downwelling irradiance is usually measured using a cosine corrector with a hemispherical field of view in the nadir direction, and the upwelling radiance is usually measured using a sensor with a conical or hemispherical field of view. For most configurations of tower-based SIF observation systems, the sensors used for both the downwelling and upwelling radiation observations are mounted on the tower at a height of between 5 m and 50 m, depending on the ecosystem type. For tower-based SIF retrieval, the telluric oxygen absorption bands are most commonly used because these absorption features are relatively strong and broad. However, the depth of the oxygen absorption bands is sensitive to the sensor height. According to the study by [PERSON] et al. [28; 29], a bias of 10 m in the sensor height will lead to a bias of about 0.1 mW/m\({}^{2}\)/nm/sr in the retrieved SIF at the O\({}_{2}\)-A band. [PERSON] et al. [30] also reported that the 3 FLD method produced large underestimates of SIF values if no oxygen compensation was applied. Therefore, atmospheric correction of observations of both the downwelling irradiance and upwelling radiance is important for tower-based SIF observations, especially for the O\({}_{2}\)-A band due to the strong absorption effect. [PERSON] et al. [30] provided a comprehensive analysis of how atmospheric effects impact SIF estimates made using proximal sensing. The atmospheric absorption and scattering effects are mainly related to the radiative transfer path length (RTPL, determined by the sensor height, and the solar and view zenith angles), the aerosol optical depth, and the pressure and temperature. Although it is possible to measure these parameters for full-physics modelling of the atmospheric radiative transfer effects, it would be too time-consuming for long-term SIF observations, and the errors in the measured atmospheric parameters would accumulate. Therefore, operational and simple methods for estimating the atmosphere transmittance are needed for accurate tower-based SIF observations.