## 3 Results and Discussion
Following an identical procedure to that described in Section 2.2, [PERSON] et al. [32] came to the same conclusion for [PERSON] and [PERSON] regarding the purported one-to-one relationship between APE and spectral GTI. The present confirmation of the findings in [32] is likely related to the similarity between the GTI and DNI spectra under cloudless conditions, as mentioned above. Interestingly, CV values up to 6% and over 30% within the 350-400 nm and 900-1050 nm wavebands, respectively, were found in Japan [33]. However, despite the fact that such high CV values indicate a large scatter of \(R_{c}\) around its mean, [PERSON] et al. [33] claimed that APE is a unique characteristic of the GTI spectrum distribution. In any case, the results depicted in Figure 4 deserve some further discussion from both the experimental and theoretical standpoints. The expanded uncertainties (\(U_{95}\)) for the spectral measurements obtained with the present spectroradiometer used are \(\pm\)10.89%, \(\pm\)4.13%, and \(\pm\)4.06% for the 350-450 nm, 450-900 nm, and 900-1050 nm wavebands, respectively, according to the certificate of calibration issued by the manufacturer. No significant additional uncertainty resulting from actual outdoor laboratory measurement conditions is likely because the instrument was cleaned each working day. It was, however, not cleaned during a week in March 2013 and another week in August 2013. Given the relative short periods of time during which the instrument remained uncleaned, no significant differences were found between spectral data collected in such periods relative to those gleaned over the course of the entire experimental campaign. The dispersion noticed for \(R_{c}\) below 450 nm and the resulting higher values of CV may be explained in part by the higher values of \(U_{95}\) at these shorter wavelengths. As discussed previously [32], however, another contribution is likely related to the substantial daily variance in AOD, which has a large impact on the direct spectrum at short wavelengths [46]. In contrast, the lowest \(U_{95}\) values occur above 900 nm, where CV also peaks. This would seem paradoxical if \(U_{95}\) was the only driver of CV variations along the spectrum, which thus requires scrutiny. As mentioned by [PERSON] et al. [32], it should be borne in mind that the instrument undergoes calibration indoors in a laboratory at a constant ambient temperature (25 \({}^{\circ}\)C), thus avoiding the potential impact of large temperature excursions  
Figure 4: Coefficient of variation of \(R_{c}\) across all 50-nm bands for spectral measurements binned in 0.02-eV width APE intervals over the range 1.75–1.89 eV (central values), using experimental spectral measurements at Jaén. during sunny days. Those field conditions could influence the reading accuracy in the NIR part of the spectrum much more than at shorter wavelengths [47; 48]. This temperature effect should be small here, however, because the detector core of this specific instrument is temperature controlled and has a specified temperature range of \(-\)10 to 50 \({}^{\circ}\)C. A more likely explanation for the high CV in the NIR is that the spectral irradiance is weak there due to water vapor absorption. Therefore, measurements in that band are more vulnerable to the influence of noise, which increases the actual measurement uncertainty. Moreover, water vapor conditions (characterized by PW) are variable on a daily basis over Jaen, which creates strong variance in the 900-950 nm waveband, where water vapor absorption is the strongest relative to the whole 350-1050 nm range. The results shown in Figure 4 confirm previous theoretical findings obtained by means of synthetic spectra generated by the SMARTS radiation model for various ideal atmospheric conditions [35]. Both modeled and experimental spectra obtained under variable conditions show that AM and AOD influence APE in a way that longer wavelengths are enhanced when they increase (red shift). On the other hand, PW has a more limited but antagonistic effect (blue shift).