PAR and UVA composition of global solar radiation at a high altitude Mediterranean forest site

Abstract

Photosynthetically active (PAR) and ultraviolet-A (UVA) radiation have important effects on plant processes, affecting both health and ecology of the natural ecosystems. Studies dealing with the composition of global solar radiation Rs in PAR and UVA wavelengths are thus important. The present work is aimed at the investigation of the relationships of PAR, UVA with Rs, assessing the key factors that affect the corresponding ratios PAR/Rs and UVA/Rs at a high-altitude mountainous environment. The study was conducted in a protected Natura-2000 forest in Greece at a high altitude (1896 m a.s.l.) site and covers a time period of about 5 years (2014–2019). The analysis was performed on a monthly, seasonal and annual basis using hourly data. Results show that PAR/Rs and UVA/Rs present annual values 0.438 ± 0.031 and 0.034 ± 0.007, respectively with low monthly and seasonal variability. Further, sky clearness was found to have a significant effect on the composition of incident Rs, with PAR/Rs values to increase from 0.417 ± 0.013 to 0.483 ± 0.037 as sky conditions change from clear to overcast. The respective UVA/Rs values were 0.031 ± 0.002 and 0.045 ± 0.011. The hourly ratios become minimum at noon, whereas the magnitude of Rs was found to have a negative effect. In addition, PAR/Rs and UVA/Rs were increasing with relative humidity, optical thickness and relative optical air mass and decreasing with saturation vapor pressure, vapor pressure deficit, and to a less degree with air temperature. Concluding, at the high altitude Mediterranean forest site, Rs composition to PAR and UVA appears to be affected mainly by atmospheric clearness and by the geometric characteristics of the incident radiation rays, whereas atmospheric clearness was affected mainly by air humidity.