Ultraviolet radiation over China: Spatial distribution and trends

An efficient model for estimating Ultraviolet (UV) radiation under various sky conditions was developed based on UV radiation measurements during 2005-2014 from the Chinese Ecosystem Research Network (CERN). The empirical UV estimation model was introduced by analyzing the dependence of UV irradiation on the clearness index (K-s, the ratio of the total solar irradiance on a horizontal surface to the extraterrestrial total irradiance on a horizontal surface) and the solar elevation angle under various sky conditions at each typical station. This model provides accurate UV radiation data, with an average root mean square error of 14.31%. We combined this estimation model with a hybrid model to reconstruct the historical dataset of daily UV radiation at 724 routine weather stations of the China Meteorological Administration (CMA) from 1961 to 2014. The hybrid model considered 6 attenuations in the solar radiation transfer process: Rayleigh scattering, aerosol extinction, ozone absorption, water vapor absorption, permanent gas absorption and cloud extinction. The average UV radiation level was 0.49 MJ m(-2) d(-1). The spatial distribution and temporal variation of the daily UV radiation in different climate zones were discussed based on the reconstructed historical dataset. Northern China had more UV radiation than southern China, and eastern China had less radiation than western China. The UV radiation on the Qinghai-Tibet Plateau was the greatest (0.66 MJ m(-2) d(-1)). The UV radiation on the Qinghai-Tibet Plateau increased from 1961 to 1984 and then changed minimally, which did not coincide with the overall trends of the entire country and other regions. In addition, the aerosol optical depth, ozone column concentration, cloud cover and water vapor content attenuated approximately 7.59%, 1.12%, 18.13%, and 6.20%, respectively, of the UV radiation that reached the Earth's surface without the attenuation of the four factors.