Variability of solar UV-B irradiance: in situ monitoring and model calculation of the vitamin D synthetic capacity of sunlight

The prediction of the biological effects of solar radiation on human health, in particular involving terrestrial level of solar ultraviolet (UV) radiation, requires the development of an adequate methodological strategy for remote sensing. To date, it is well understood that, in appropriate doses, UV radiation is beneficial for people, specifically due to the production of vitamin D3 in the skin from its precursor 7-dehydrocholesterol. But as far as excessive UV exposure causes acute and chronic health effects, in most cases biological activity of solar UV radiation is calculated by weighting solar UV spectra with International Commission on Illumination (CIE) erythemal action spectrum. Yet the beneficial vitamin D synthetic capacity of sunlight cannot be correctly estimated in this way because of the significant difference between the erythemic and vitamin D synthesis action spectra. With due regard to the essential role of vitamin D3 for human health, in this article we examine the possibility of simplified estimation in situ of provitamin D3 photoconversion into previtamin D3 from the UV absorption spectra of an in vitro model of vitamin D synthesis upon exposure to sunlight. A large-scale linear correlation (R = 0.99) was found on a clear summer day between the concentration of accumulated previtamin D3 and maximum absorbance decline in the initial provitamin D3 absorption spectrum. However, long-term observations showed a poorer (R = 0.77) correlation, and a source of ambiguity of such indirect estimation of previtamin D3 concentration is discussed in detail. In addition, we propose a reliable algorithm for the direct calculation of previtamin D3 accumulation using solar UV spectra as input data to the reaction model of previtamin D photosynthesis and demonstrate the critical dependence of previtamin D3 accumulation on stratospheric ozone, season, latitude and cloudiness. The comparison of experimental and simulation data conforms to recent findings on Europe's darker atmosphere in the UV-B and implicates the practical certainty of the presented algorithm for the calculation of the vitamin D synthetic capacity from the remotely sensed solar spectra.