The characteristics of ultraviolet radiation in arid and semi-arid regions of China

Measurements of the broadband global solar radiation (R (S)) and total ultraviolet radiation (the sum of UV-A and UV-B) were conducted from 2005 to 2010 at 9 sites in arid and semi-arid regions of China. These data were used to determine the temporal variability of UV and UV/R (S) and their dependence on the water vapor content and clearness index. The dependence of UV/R (S) on aerosol optical depth (AOD) and water vapor content was also investigated. In addition, a simple and efficient empirically model suited for all-weather conditions was developed to estimate UV from R (s). The annual average daily UV level in arid and semi-arid areas is 0.61 and 0.59 MJ m(-2) d(-1), respectively. The highest value (0.66 +/- 0.25 MJ m(-2) d(-1)) was recorded at an arid area at Linze. The lowest value (0.53 +/- 0.22 MJ m(-2) d(-1)) was recorded at a semi-arid area at Ansai. The highest daily value of UV radiation was measured in May, whereas the lowest value was measured in December. The monthly variation of the UV/R (s) ratio ranged from 0.41 in Aksu to 0.35 in Qira. The monthly mean value of UV/R (s) gradually increased from November and then decreased in August. A small decreasing trend of UV/R (s) was observed in the arid and semi-arid regions due to recently increasing amounts of fine aerosol. A simple and efficient empirically model suit for all-weather condition was developed to estimate UV from R (s). The slope a and intercept b of the regression line between the estimated and measured values were close to 1 and zero, respectively. The relative error between the estimated and measured values was less than 11.5%. Application of the model to data collected from different locations in this region also resulted in reasonable estimates of UV.