## 3 Results  
### Validation of the Atmospheric Correction Using Measurements on Non-Fluorescent Matter Canopy  
The performance of the atmospheric correction for tower-based SIF observations was evaluated using the field experiments introduced in Section 2.4. Figure 5 shows the SIF retrieval results at 760.6 nm from the observations of mature summer maize. As all the leaves had turned yellow, the SIF emission was expected to be zero. The SIF values retrieved from the tower-based observations without atmospheric correction form a \"U\" shape, decreasing from dawn to noon and increasing from noon to dusk. Most of the values are negative. After the atmospheric correction, the retrieved SIF values are still mostly negative (this is partly related to the so-called \"direct radiation infilling effect\" due to directional effect of the canopy reflectance, which is discussed in Section 4.1 but much closer to zero, which indicates that the atmospheric correction is effective. Figure 4: Illustration of the bands used in 3 FLD method for solar-induced chlorophyll fluorescence (SIF) retrieval in this study. The vertical dash lines shows the spectral position of the bands at the left of, inside, and at the right of the O\({}_{2}\)-A absorption feature. Figure 5: Results for SIF retrieved from tower-based spectral observations made on mature summer maize with no green leaves (measured on 27 October 2017, at Daman site). The blue line shows the results without atmospheric correction; the red line shows the results with atmospheric correction. Validation of the Atmospheric Correction Using Synchronous Tower-Based and Ground-Based Measurements  
Figure 6 shows the apparent reflectance spectra at the O\({}_{2}\)-A band measured on the ground and measured on the tower with/without atmospheric correction. The peaks of the spectra are mainly caused by the SIF in-filling effect on the O\({}_{2}\)-A absorption feature. As shown in Figure 6, the peak of the spectrum measured on the tower without atmospheric correction is lower than that of the spectrum measured on the ground. After atmospheric correction, the peak of the spectrum measured on the tower is close to that of the spectrum measured on the ground. The results indicate that the atmospheric correction for tower-based SIF measurements is necessary and efficient. Figure 7 shows the results from the observations of summer maize at the filling stage with green leaves. Observations were made synchronously from the tower and on the ground. Figure 7a shows that for the O\({}_{2}\)-A band, the SIF values retrieved from the tower-based observations without atmospheric correction are obviously lower than the values retrieved from the ground-based observations. After the atmospheric correction, the retrieved SIF at the O\({}_{2}\)-A band matches well with the ground-based observations. Figure 6: Comparison of the apparent reflectance spectra at O\({}_{2}\)-A band measured on the ground and on the tower (with/without atmospheric correction) at 13:01 on 7 September 2018, at the Daman site. Figure 7: Results of SIF retrieval from synchronous tower-based and ground-based spectral observations of summer maize at the filling stage (measured on 7 September 2018, at the Daman site). (**a**) Diurnal variation of SIF retrieved from tower-based observations before/after atmospheric correction, and from synchronous ground-based observations. (**b**) Relationship between the SIF values retrieved from synchronous ground-based observations and tower-based observations with and without atmospheric correction. For quantitative analysis, the synchronous tower-based and ground-based observations with time lags of less than one minute were selected for comparison, as shown in Figure 7b.