**3.2 Long-periodtrends of ultraviolet radiation in Beijing**

The all-weather empirical model was established and validated with measured data at Beijing. Long-term UV radiation data were computed from Beijing observation station, which is located in the southwest Fourth Ring Roads of Beijing. A data set of *R*s values collected from 1958–2005 by the National Meteorological Center (CMA) were used in this study.

The Time series of the annual and seasonal averages of UV radiation for spring (represented by April), summer (represented by July), autumn (represented by Oct.), winter (represented by Jan.) are computed for Beijing and presented in Figure 3. Annual mean UV radiation levels decreased from the early 1960s to late 1990s, but began to increase by the late 1990s. The annual mean daily value of UV radiation from 1958-2005 was 0.46 MJ m-2 at Beijing station. By the latter half of the 20th century, decreases in UV radiation (0.018 MJ m-2 d-1 per decade) have been observed. We observed the decrease trends in the long-term annual mean UV radiation as observed from its value of -0.026 MJ m-2 d-1 per year during the period 1958-1997 and its value of -0.0024 mJ m-2 d-1 per year during the period 1958-2005.

Fig. 5. Long-term variation characteristics of annual average of Ultraviolet radiation for 1958–2005. (Hu et al., 2010 b)

### **4. Conclusions**

The temporal variations in UV radiation using UV radiation and *Rs* data collected in Beijing over the period January 2005 to December 2010 were studied. The UV radiation levels increased gradually from spring reaching a peak in the summer, and gradually decreased to its lowest levels in winter. Its annual mean daily value was 0.39 ± 0.16 MJ m-2 d-1, and its lowest and highest daily values were 1.06 and 0.01 MJ m-2 d-1, respectively. Its highest daily value occurred in May and the lowest value occurred in December. The monthly mean daily value of the ratio, UV*/R*s, gradually increased from 2.7% in November to 3.7% in August, after which it gradually decreased. The annual mean daily value of UV*/R*s was 3.1%.

The annual mean daily value of UV radiation from 1958-2005 was 0.46 MJ m-2 d-1. Over the latter half of the 20th century, there have been significant decreases in UV radiation (0.018 MJ m-2 d-1 per decade).

### **5. Summary to the chapter**

Measurements of total Ultraviolet radiation (UV), broadband global solar radiation (*R*S)*,*  reflective radiation, net radiation, and Photosynthetically active radiation from 2005 to 2010 in Beijing were used to determine temporal variation characteristics of UV in Beijing. The UV radiation levels increased gradually from spring reaching a peak in the summer, and then gradually declined to its lowest levels in winter. The annual mean daily value of UV radiation was 0.39 ± 0.16 MJ m-2 d-1, and the lowest and highest daily average UV radiation levels were 1.06 and 0.01 MJ m-2 d-1, respectively. The highest daily values of UV radiation occurred in May while the lowest values occurred in December. The monthly mean daily UV/Rs value gradually increased from 2.7% in November to 3.7% in August, after which it gradually decreased. The annual mean daily UV/Rs value was 3.1%.

A simple, efficient, and empirically derived, all-weather, model is proposed to estimate UV from *R*S. The annual mean daily value of UV radiation from 1958-2005 was 0.46 MJ m-2 d-1. Over the latter half of the 20th century, there have been significant decreases in UV radiation (0.018 MJ m-2 d-1 per decade).
