Variation characteristics of ultraviolet radiation derived from measurement and reconstruction in Beijing, China

Abstract

Four years measurements of broadband global solar radiation (R^S) and total ultraviolet (UV) radiation (the sum of UV-A and UV-B) were conducted from 2005 to 2008 in Beijing. These data were used to determine the temporal variability of UV/RS and its dependence on water vapour content and clearness index. A simple, efficient and empirically derived all-weather model is proposed to estimate UV from R_S. The estimated data were used to analyse the characteristics of long-term variations in UV radiation. Results revealed that annual mean UV was 0.37 ± 0.17 MJ m⁻² d⁻¹. The highest daily value of UV radiation was measured in May, while the lowest value occurred in December. The monthly mean value of UV/Rs gradually increased from 2.7% in November to 3.7% in August. The annual mean value of UV/Rs was 3.1%.

The annual mean daily UV radiation during 1958–2005 was 0.46MJm⁻² d⁻¹. The interannual variations and long-term patterns of UV radiation derived from reconstructed daily UV values are compatible with changes in measured RS. Annual mean UV radiation levels decreased from the early 1960s to the late 1990s but began to increase in the late 1990s. Over the latter half of the 20th century, decrease in UV radiation was 0.018 MJ m⁻² d⁻¹ per decade. The decrease trends in long-term annual mean UV radiation of −0.026 MJ m⁻² d⁻¹ per year for 1958–1997 and −0.0024 MJ m⁻² d⁻¹ per year for 1958–2005. The decreasing trend in UV radiation was sharpest in spring and summer but more gradual in autumn and winter.