**1. Introduction**

22 Will-be-set-by-IN-TECH

98 Solar Radiation

Xie, P. & Arkin, P. (1996). Analysis of global monthly precipitation using gauge observations, satellite estimates, and numerical model predictions. *J. Clim.*, Vol. 9, 840-858. Yamanaka, G. (2008). Discrepancies between observed and ocean general circulation

doi:10.1029/2008GL034737.

model-simulated anomalies in recent SSTs of the tropical Indian Ocean caused by apparent trends in atmospheric reanalysis data. *Geophys. Res. Lett.*, Vol. 35,

> Global ultraviolet radiation (UV) is a small fraction of the total extraterrestrial solar radiation outside the atmosphere. The amount of UV radiation reaching the Earth's surface comprises only a small fraction of global radiation, about 6-7% of global radiation is in the UV-A (320-400 nm) range and less than 1% is in the UV-B (280-320 nm) range. However, UV radiation plays an important role in many biological and photochemical reactions. UV of certain doses could lead to a variety of adverse health and environmental effects (National Radiological Protection Board, 2002; United Nations Environment Programme (UNEP), 1998; Slaper and Koskela, 1997; Outer, 2005). UV radiation also has impacts on the photodegradation of plastics, colorants, paints, and artificial and natural fibers as well as the formation and decomposition of photosensitive urban and industrial contaminants (Caňada et al., 2000). Understanding the amount of UV received by human, plant and animal organisms on the earth's surface is important to a wide range of field such as cancer research, forestry, tropospheric chemistry, agriculture and oceanography (Grants and Heisler, 1997; McKenzie et al., 1991). An understanding of changes in longterm, ground-level UV radiation is required to support assessments of UV radiationinduced health and environmental risks (Slaper and Koskela, 1997). UV radiationmeasuring networks are extremely scarce, particularly in the arid and semi-arid, where the effects of UV radiation may be of great importance due to many hours of sunshine throughout the year. Despite its anthropogenic importance and impacts, concern about the amount of UV radiation reaching the Earth's surface has only recently developed, primarily as a result of the thinning ozone layer linked to the depletion of stratospheric ozone in the 1980s (Su et al., 2005).

> Numerous factors can influence UV radiation incidence, including cloud characteristics, solar zenith angles, total ozone, aerosol pollution, and surface albedo. Altitude has an important effect on UV radiation (Piazena, 1996; Seckmeyer et al., 1997; McKenzie et al., 2001).Clouds are known to affect the attenuation of UV radiation differently, based on their location, percentage cover, optical thickness, water content, and droplet size distribution. In order to obtain more UV data for other study, lots of studies have focused on quantitative

variations in UV radiation and the ratio of UV to global solar radiation (*R*s). Additional studies have addressed long-term trends in the variations of UV through reconstructions of past UV radiation based on ground-based and satellite data (Kaurola et al., 2000; Fioletov et al., 2001; Lindfors et al., 2007; Feister et al., 2008;Hu et al., 2010a). In the last few decades, there has been a progressive increase and great concern in the amount of UV reaching the Earth's surface as a consequence of the thinning of the stratospheric ozone. Despite its anthropogenic importance and impacts, concern about the amount of UV radiation reaching the Earth's surface has only recently been developed, primarily as a result of the thinning of ozone layer linked to the depletion of stratospheric ozone in the 1980s (Su et al., 2005). UV radiation-measuring networks are extremely scarce, particularly in China.

The objective of this chapter, apart from showing seasonal variations of UV and UV*/R*<sup>s</sup> values in Beijing, based on the reconstruction method, is to develop a long-term data set of UV radiation, and also study variation characteristics of UV in Beijing.
