Long-term variations of ultraviolet radiation in China from measurements and model reconstructions

Measurements of ultraviolet (UV) radiation at 38 stations from Chinese Ecosystem Research Network during 2006–2012 were used for reconstructing the historical UV levels in China for the first time. UV models were introduced by analyzing the dependence of UV irradiation on clearness index (Kt) and cosine of solar zenith angle under any sky conditions in each station. Mean bias error (MBE), mean-absolute bias error (MABE) and root-mean-square error (RMSE) were used for assessing the model performance; relative differences between UV estimates and measurements were generally lower than 10% at most stations, which indicated that our all-sky UV models can produce acceptable estimates in China. Long-term UV values during 1961–2012 were then reconstructed for investigating the spatiotemporal characteristics of UV radiation in China based on daily global solar radiation (G) at 115 meteorological stations from China Meteorological Administration. Annual mean daily UV radiation ranged from 0.55 MJ m−2 d−1 to 0.65 MJ m−2 d−1 with average value being about 0.61 MJ m−2 d−1. It was also discovered that UV radiation decreased slightly at about −2.72 kJ m−2 d−1 per decade during the study period and there was an increasing trend since 1991 (0.7 kJ m−2 d−1 per year).