## 3 Results  
### Spatiotemporal Distributions of Annual Radiation in China  
The spatial distributions of the average annual and standard deviation of \(R_{\rm g}\), \(R_{\rm d}\), and diffuse radiation fraction in China over the period 1981-2012 are shown in Figure 2. The \(R_{\rm g}\) (Figure 2a) was unevenly distributed across China, with a range of 3255-7154 MJ m\({}^{-2}\) y\({}^{-1}\) and a national average of 5258 MJ m\({}^{-2}\) y\({}^{-1}\) (Figure 2a). The standard deviation of \(R_{\rm g}\) ranged from 65 to 359 MJ m\({}^{-2}\) y\({}^{-1}\) (Figure 2b). Generally, \(R_{\rm g}\) values on the western plateau were higher than those on the eastern plain. The highest \(R_{\rm g}\) values were recorded on the southern part of the Qinghai-Tibet Plateau (>6500 MJ m\({}^{-2}\) y\({}^{-1}\)) because of the thin air density in this region. The northeastern part of the Sichuan Basin and the Yunnan-Guizhou Plateau had the lowest annual \(R_{\rm g}\) values (<3800 MJ m\({}^{-2}\) y\({}^{-1}\)). Values of \(R_{\rm d}\) (Figure 2c) ranged from 1989 to 3007 MJ m\({}^{-2}\) y\({}^{-1}\) and showed a national average of 2483 MJ m\({}^{-2}\) y\({}^{-1}\), which is close to the value of 2477 MJ m\({}^{-2}\) y\({}^{-1}\) estimated by [PERSON] et al. (2015, 2016). The standard deviation range of \(R_{\rm g}\) is 15-101 MJ m\({}^{-2}\) y\({}^{-1}\) with a national average of 33 MJ m\({}^{-2}\) y\({}^{-1}\) (Figure 2d). In general, \(R_{\rm d}\) was high in the south and west of China and low in the north. The highest values of \(R_{\rm d}\) were on the Qinghai-Tibet Plateau and on the southwestern part of the Yunnan-Guizhou Plateau, with values higher than 2800 MJ m\({}^{-2}\) y\({}^{-1}\). The lowest values were found for Northeast China, Inner Mongolia, and Northwest China, with values of less than 2300 MJ m\({}^{-2}\) y\({}^{-1}\). Values of diffuse radiation fraction ranged from 0.34 to 0.83 and showed a national average of 0.52 (Figure 2e), and the diffuse radiation fraction was highest in southern China, especially within the regions of Sichuan  
Figure 2: Spatial distribution of the mean (**a**) and standard deviation (**b**) of global radiation (unit: MJ m\({}^{-2}\) y\({}^{-1}\)), the annual mean (**c**) and standard deviation (**d**) of diffuse radiation (unit: MJ m\({}^{-2}\) y\({}^{-1}\)), and the mean (**e**) and standard deviation (**f**) of diffuse radiation fraction in China for the period 1981–2012 with a spatial resolution of 0.0727\({}^{\circ}\). and Guizhou. The standard deviation of diffuse radiation fraction ranged from 0.007 to 0.043 with a national average of 0.016 (Figure 2f). The \(R_{\rm g}\) exhibited broad variability during 1981-2012 but generally showed a significant (\(p<0.05\)) negative trend of 2.18 MJ m\({}^{-2}\) y\({}^{-1}\), yielding a rate of decrease of 1.3% y\({}^{-1}\) (Figure 3a). Specifically, \(R_{\rm g}\) decreased markedly and then increased slightly during the interval 1980-1990. During the interval 1991-1993, \(R_{\rm g}\) declined rapidly, possibly because of the volcanic eruption of Mount Pinatubo in 1991. This volcanic eruption led to the release of about 20 million tonnes of sulfide gas into the stratosphere, producing a reduction in \(R_{\rm g}\). The \(R_{\rm g}\) values were gradually restored after 1994 [9; 25; 30]. Values of \(R_{\rm g}\) also decreased slightly after 1999, consistent with the findings of [PERSON] et al. [22] and [PERSON] et al. [17]. Figure 3: Linear trends in annual mean (**a**) global radiation, (**b**) diffuse radiation, and (**c**) diffuse radiation fraction across China for the period 1981–2012. The simple linear trend in the average annual \(R_{\rm d}\) in China was positive from 1981 to 2012, although this trend was not significant (Figure 3b). The \(R_{\rm d}\) decreased during the interval 1981-1990 but increased and then decreased rapidly during the interval 1991-1994 because of the eruption of Mount Pinatubo. From the mid-1990s, \(R_{\rm d}\) increased slightly and then decreased again.