The threshold exposure time for synthesis of vitamin D was simulated by using a radiative transfer model considering variations in total ozone, cloud, and surface conditions. The prediction of total ozone took the form of an empirical linear regression with the variables of meteorological parameters in the upper troposphere and lower stratosphere and the climatology value of total ozone. Additionally, to consider cloud extinction after the estimation of clear-sky UV radiation using a radiative transfer model simulation, a cloud modification factor was applied. The UV irradiance was estimated at one-hour intervals, and then, to improve the temporal resolution of the exposure time simulation, it was interpolated to a one-minute resolution. Exposure times from the simulation clearly followed seasonal and diurnal cycles. However, upon comparison with observations, biases with large variations were found, and the discrepancy in the exposure time between the observations and simulations was higher in low UV irradiance conditions. The large deviations in the prediction errors for total ozone and the simplified assumption for the cloud modification factor contributed to the large deviations in exposure time differences between the model estimation and observations. To improve the accuracy of the simulated exposure time, improved predictions of total ozone with a more detailed cloud treatment will be essential.
Ultraviolet (UV; 100~400 nm) radiation, which is a short-wavelength part of the solar spectrum, has harmful effects on the human body, such as skin aging, sunburn, and skin cancer. Because overexposure to UV causes erythema in human skin, the UV erythemal irradiance (UVER) is provided to people as an index of the risks of UV irradiation. In this study, statistical empirical models for estimating surface UVER were developed and evaluated at Gangneung, Pohang, Mokpo, and Gosan sites in South Korea, where UVER and global horizontal irradiance (GHI) observation data have been collected for a long period. For the establishment of an empirical model, the clearness index of the GHI ( ), total column ozone, and solar zenith angle were used as independent variables. The underestimations of the empirical models were improved by grouping the dataset with . The improved models exhibited an average R 2 of 90% at the four sites during the validation period. The atmospheric transmittance of the UVER was reconstructed using the empirical model at the GHI observation network during past 5 year data in South Korea, and the monthly variation showed seasonality with high values in May-August and low values in December-January.
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