Evaluation of the Partition of Global Solar Radiation into UVA, PAR, and NIR Components in a Rural Environment

Abstract

Highlights: What are the main findings? Eight years of high-quality spectral measurements in Lindenberg (Germany) show that global solar radiation is partitioned into ~5% UVA, ~43% PAR, and ~26% NIR, expressed as energy fractions relative to Rs, with strong correlations (R2 > 0.90) between each component and Rs. Clear seasonal cycles and long-term decreasing trends were identified in all spectral fractions, particularly for UVA/Rs (−15%), largely modulated by cloud cover and atmospheric conditions. What are the implications of the main findings? The spectral fractions of UVA, PAR, and NIR can be reliably estimated from global radiation (Rs) when direct spectral observations are unavailable. These results provide valuable inputs for climate and vegetation modeling, solar energy forecasting, and human exposure assessments, especially in underrepresented rural environments. Observational studies in several regions and our dataset indicate changes in global solar radiation (RS); here, we analyze how atmospheric conditions modulate its spectral composition. This study investigates the effects of atmospheric conditions on the spectral composition of global solar radiation (RS) across different wavelength ranges: ultraviolet A (UVA), photosynthetically active radiation (PAR), and near-infrared radiation (NIR), using the ratios UVA/RS, PAR/RS, and NIR/RS. A high-quality spectral irradiance dataset (300–1025 nm) covering eight years of observations from a representative rural site in Central Europe (Meteorological Observatory Lindenberg, Tauche, in North-East Germany) was used. The average values obtained for the ratios were 0.049 ± 0.010 for UVA/RS, 0.433 ± 0.044 for PAR/RS, and 0.259 ± 0.030 for NIR/RS. Thus, the UVA range contributed approximately 5% to global radiation, PAR 43%, and NIR 26%. Strong correlations were found between each spectral component and RS, with determination coefficients exceeding 0.90 in all cases, particularly for PAR. This suggests that, in the absence of direct spectral measurements, these components can be reliably estimated from RS. A seasonal pattern was also identified, with maximum values in warmer months and minimum values in colder ones, most notably for PAR/RS. In contrast, NIR/RS exhibited an inverse pattern, likely influenced by atmospheric water vapor. A long-term decreasing trend in these ratios was also identified, being most pronounced in the UVA/RS ratio. Additionally, atmospheric conditions significantly affected the spectral distribution of RS, with UVA and PAR proportions increasing under specific conditions, while NIR remained more stable. Under overcast conditions, the ratios for shorter wavelengths (UVA and PAR) increased, indicating higher scattering effects, while NIR was less affected.