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Ultraviolet (UV) radiation and broadband solar radiation (R,) measured from January 2005 to June 2006 at 31 stations in Chinese Ecosystem Research Network (CERN) were used to investigate the spatio-temporal characteristics of UV radiation and UV fraction (the ratio of UV radiation to R,) in China. Results indicated that the seasonal variations of UV radiation and R, were consistent with the solar activities, which reached their lower values during winter period, and increased throughout the spring, peaking in June or July, in most sites. The Meiyu weather system and Southwest Monsoon produced different variation characteristics of UV radiation and R-s in subtropical and tropical regions. The UV fraction values showed a similar seasonal trend as that of UV radiation, which was mainly determined by the seasonal change of the aerosol optical depth (AOD) and water vapor content in atmosphere. The seasonal variations of UV fraction were much smoother in southern China due to high water vapor content over the whole year. The UV radiation showed an increasing trend from east to west in China. In the western area, a simple increasing trend was observed from north to south, with the largest annual mean daily UV value 0.91 MJ m(-2) appearing in the Qinghai-Tibet Plateau area. In east China there was a low center that appeared in the subtropical region due to high aerosol burden, with the lowest value 0.41 MJ m(-2) observed in Yanting site. Two high centers were located in the tropics with higher solar altitudes and the north desert region with low atmospheric attenuation. The largest values of UV fraction appeared in the tropical and subtropical regions due to higher relative humidity (RH) in these areas. However, the smallest value did not appear in the north desert region where the RH was the lowest, they were found at the Luancheng site which featured relatively low humidity and abundant fine aerosols. The variations in the UV fraction were not such distinctive as those of the UV radiation in China due to the more complex influences of aerosol and water vapor. (c) 2007 Elsevier Ltd. All rights reserved.