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Variation Characteristics of Ultraviolet Radiation over the North China Plain


doi: 10.1007/s00376-013-2319-5

  • In situ measured data of broadband solar radiation (Rs) and ultraviolet (Uv) radiation were used to investigate the spatiotemporal variation properties of Uv radiation and the ratio of Uv radiation to Rs over the North China Plain (NCP). Based on the analysis, an empirical model for estimating Uv radiation under all weather conditions in this region was developed. The results showed that the annual Uv radiation over the NCP ranges from 0.38-0.52 MJ m-2 d-1. The highest value during the study period was recorded at the Changwu site, which is located near the margin of the Loess Plateau, while the lowest value appeared at the station in Beijing. The seasonal variation pattern of the ratio of Uv radiation to Rs is similar to that of Uv radiation; namely, the highest value appears in August and then decreases gradually until the lowest value appears in November. A small increasing trend in the Uv radiation levels and the ratio of Uv radiation to Rs was observed over the NCP. The evaluation results showed that the empirical estimation model can be widely used to estimate Uv radiation under all atmospheric conditions. The relative error between the modeled and measured daily values were within 15%.
    摘要: In situ measured data of broadband solar radiation (Rs) and ultraviolet (Uv) radiation were used to investigate the spatiotemporal variation properties of Uv radiation and the ratio of Uv radiation to Rs over the North China Plain (NCP). Based on the analysis, an empirical model for estimating Uv radiation under all weather conditions in this region was developed. The results showed that the annual Uv radiation over the NCP ranges from 0.38-0.52 MJ m?2 d?1. The highest value during the study period was recorded at the Changwu site, which is located near the margin of the Loess Plateau, while the lowest value appeared at the station in Beijing. The seasonal variation pattern of the ratio of Uv radiation to Rs is similar to that of Uv radiation; namely, the highest value appears in August and then decreases gradually until the lowest value appears in November. A small increasing trend in the Uv radiation levels and the ratio of Uv radiation to Rs was observed over the NCP. The evaluation results showed that the empirical estimation model can be widely used to estimate Uv radiation under all atmospheric conditions. The relative error between the modeled and measured daily values were within 15%.
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Manuscript received: 28 December 2012
Manuscript revised: 07 April 2013
通讯作者: 陈斌, bchen63@163.com
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Variation Characteristics of Ultraviolet Radiation over the North China Plain

  • 1. State Key Laboratory of Atmospheric Boundary Layer Physics and Atmospheric Chemistry, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100029

Abstract: In situ measured data of broadband solar radiation (Rs) and ultraviolet (Uv) radiation were used to investigate the spatiotemporal variation properties of Uv radiation and the ratio of Uv radiation to Rs over the North China Plain (NCP). Based on the analysis, an empirical model for estimating Uv radiation under all weather conditions in this region was developed. The results showed that the annual Uv radiation over the NCP ranges from 0.38-0.52 MJ m-2 d-1. The highest value during the study period was recorded at the Changwu site, which is located near the margin of the Loess Plateau, while the lowest value appeared at the station in Beijing. The seasonal variation pattern of the ratio of Uv radiation to Rs is similar to that of Uv radiation; namely, the highest value appears in August and then decreases gradually until the lowest value appears in November. A small increasing trend in the Uv radiation levels and the ratio of Uv radiation to Rs was observed over the NCP. The evaluation results showed that the empirical estimation model can be widely used to estimate Uv radiation under all atmospheric conditions. The relative error between the modeled and measured daily values were within 15%.

摘要: In situ measured data of broadband solar radiation (Rs) and ultraviolet (Uv) radiation were used to investigate the spatiotemporal variation properties of Uv radiation and the ratio of Uv radiation to Rs over the North China Plain (NCP). Based on the analysis, an empirical model for estimating Uv radiation under all weather conditions in this region was developed. The results showed that the annual Uv radiation over the NCP ranges from 0.38-0.52 MJ m?2 d?1. The highest value during the study period was recorded at the Changwu site, which is located near the margin of the Loess Plateau, while the lowest value appeared at the station in Beijing. The seasonal variation pattern of the ratio of Uv radiation to Rs is similar to that of Uv radiation; namely, the highest value appears in August and then decreases gradually until the lowest value appears in November. A small increasing trend in the Uv radiation levels and the ratio of Uv radiation to Rs was observed over the NCP. The evaluation results showed that the empirical estimation model can be widely used to estimate Uv radiation under all atmospheric conditions. The relative error between the modeled and measured daily values were within 15%.

1 Introduction
  • Ultraviolet (Uv) radiation is defined as the sum of UVB and UVA radiation. The solar spectrum ranging from 280-315 nm is defined as UVB radiation and that from 315-400 nm is termed UVA radiation. Research indicates that Uv radiation received at the surface of the Earth is 10% UVB and 90% UVA. Even at the top of the atmosphere, the percentage of Uv radiation to broadband solar radiation (Rs) is no more than 10%. So, the proportion of Uv radiation to Rs that reaches the Earth’s surface is smaller than that at the top of the atmosphere. Many research results have shown that Uv radiation is an indispensable parameter in the study of environmental issues and ecological processes. Many adverse health and environmental effects can be caused by excessive Uv radiation doses [Slaper and Koskela, 1997; National Radiological Protection Board, 2002; United Nations Environment Programme (UNEP), 2003; Outer et al., 2005]. For instance, enhanced Uv radiation can delay the growth and development of plants. An especially excessive dose of UVB radiation can penetrate through the leaf epidermis, and thus may injure the photosynthetic system and cell membrane and disrupt phytohormones. Correspondingly, the photosynthetic capacity, biomolecules, biomass, and ecophysiological characteristics of plants and ecosystems may also be influenced by high UVB radiation doses. In general, excessive UVB radiation can modify DNA and protein structure, resulting in changes in physiological and biochemical processes. The properties of Uv radiation also need to be investigated to study tropospheric chemistry and oceanography, skin cancer and forest degradation (Grants and Heisler, 1997; McKenzie et al., 2001). Knowledge of long-term variation properties of Uv radiation is an important parameter for evaluating the health and environmental risks it causes (Slaper and Koskela, 1997; Cañda et al., 2000).

    Early research noted that solar zenith angles, altitude, cloud micro-properties, concentration of the ozone column and the physico-chemical characteristics of aerosols have a strong influence on the variation characteristics of Uv radiation (Piazena, 1996; Seckmeyer et al., 1997;McKenzie et al., 2001). However, to meet the demands of studies on health risks and environmental problems, more detailed knowledge of Uv radiation was required. Thus, numerous further studies on the variation characteristics of Uv radiation and the ratio of Uv to Rs have also been carried out, and still need to be carried out. Many such studies have reported a number of different estimation methods that can be used to calculate historic Uv radiation under clear sky conditions (Foyo-Moreno et al., 2003; Paulescu et al., 2010). For instance, in situ observed and reconstructed Uv radiation data have been used to investigate the long-term variation trends of Uv radiation (Kaurola et al., 2000; Fioletov et al., 2001; Lindfors et al., 2007; Feister et al., 2008; Hu et al., 2010). However, studies on the long-term variation characteristics of Uv radiation are very scarce, especially over the North China Plain (NCP).

    To further understand the mechanisms of interaction between enhanced Uv radiation and the ecosystem, and to better prevent damage to plants and ecosystems caused by Uv radiation, larger-scale and long-term field research on Uv radiation should be carried out. However, the observer networks for Uv radiation are extremely scarce, particularly in the NCP. Thus, investigating the spatiotemporal variation characteristics of Uv radiation has great significance for agriculture, as the NCP is one of the most important agricultural regions of China.

    In the above context, the goal of the present reported study—besides investigating the temporal variation characteristics of Uv and the ratio of Uv radiation to Rs—was to present an efficient and accurate parameterization method for estimating Uv radiation through more routinely measured Rs. The estimation model was validated, and the transferability of the model evaluated.

2 Materials and methods
  • The NCP is known as China’s “granary”, as more than 40% and 25% of China’s wheat and corn production is produced in this region, respectively (Zhang et al., 2010). The NCP is located in the Yellow River valley, and is the largest alluvial plain in eastern Asia. The northern and western parts of the NCP are surrounded by the Yanshan Mountains and the Taihang Mountains, respectively. In the southern part, the Yangtze Plain is situated adjacent to the NCP. The Bohai Sea, Shandong Peninsula and the Yellow Sea lie in the eastern part of the NCP. The vast majority of Henan, Hebei and Shandong provinces consist of the NCP. It covers an area of approximately 409 500 km2 and the elevation of the majority of the region is less than 50 m. The NCP formed the cradle of Chinese civilization, and the region is among the most densely populated in the world. Six radiation observation stations were involved in this study and are affiliated with the Chinese Ecosystem Research Network (CERN) (Fig. 1).

    Figure 1.  Geographical locations of the observation sites in the NCP.

    Rs,Uv radiation, photosynthetically active radiation, reflected radiation, net radiation and routine meteorological parameters are measured simultaneously by this observation network. The in situ measured Uv radiation and Rs data were used to investigate the variation characteristics of Uv radiation and the ratio of Uv radiation to Rs over the NCP region. Measured data from January 2005 to December 2011 were used in this study, with the exception of the Jiaozhouwan site due to its late installation.

  • A CM-11 radiometer (Kipp and Zonen, Delft, Netherlands) was used to measure Rs, and the uncertainty of the measured Rs values was less than 3%. A CUV3 (Delft, Netherlands) was used to measure the sum of direct and scatter Uv radiation with a relative error less than 5%. An HMP45D apparatus (Vaisala, Finland) was used to measure the relative humidity (RH) and temperature. The measured accuracies for RH and temperature were 3% and 0.1°C, respectively. The solar radiation and meteorological parameters were observed at 1-minute intervals. The hourly and daily values were integrated from the minute data. All instruments were calibrated every two years. In addition, daily maintenance work was performed to ensure the radiometers were clear, free of dirt and positioned horizontally. (Hu et al., 2010) reported a detailed description of the calibration method.

    Figure 2.  Box plots of monthly ultraviolet radiation over the NCP.

    The control criteria employed for the measured Uv radiation data were as follows. First, the measured values of Uv radiation could not be higher than the flux value of the Uv radiation at the top of the atmosphere in the same geographical coordinates. Second, the ratio of Uv radiation to Rs had to range from 0.02 to 0.08. Otherwise, the measured Uv radiation data were deleted from the measured dataset. The quality control method for Rs was similar to that for Uv radiation. First, the values of the measured Rs could not be higher than the values at the top of the atmosphere in the same geographical coordinates. Moreover, the minimum value of Rs that could be accepted had to be higher than the minimum value appearing in continuous overcast conditions; namely, the clearness index (the ratio of Rs to the extraterrestrial irradiance flux of Rs at the top of the atmosphere) had to be larger than 0.03. The value of 0.03 represents a continuous heavily overcast sky condition (Geiger et al., 2002). Otherwise, the measured Rs was deleted from the measured dataset. No more than 2% of the measured data were eliminated after the quality control procedure.

3 Results and discussion
  • Figure 2 shows box plots of the monthly statistics of Uv radiation over the NCP. We can see that the lowest value of Uv radiation appears in winter and the highest value in summer (June). An increasing trend of Uv radiation can be observed from winter to summer. The Uv radiation values then decrease gradually until reaching the minimum in winter. The monthly variation feature of the Uv radiation is similar to that of Rs. The variation characteristics of the solar zenith angle are mainly responsible for the type of monthly variation of the Uv radiation.

    The annual average values of Uv radiation are 0.38, 0.43, 0.47, 0.51, 0.52 and 0.46 MJ m-2 d-1 at Beijing, Luancheng, Yucheng, Jiaozhouwan, Changwu and Fengqiu, respectively. The measured Uv radiation at the Beijing site was the lowest Uv value in the NCP region.

    Beijing is capital of China and is one of the largest cities in the world. There are more than 14 million people in Beijing. The northern and western parts of Beijing are surrounding by the Yanshan Mountains and the Taihang Mountains, with elevations between 1000 and 3500 m above sea level. The southern and eastern parts of the city are adjacent to the NCP. There are many heavily industrialized areas from the southwest to the east (Streets et al., 2007). Air pollution is very severe in Beijing, especially atmospheric fine aerosol pollution. Rapid economic development, population expansion and urbanization are the main factors causing the heavy levels of air pollution in Beijing. Many research results have shown that fine aerosol particles are the main air pollutant. Recently, the concentration of PM2.5 has also been found to be very high in Beijing (Chan and Yao, 2008; Zhao et al., 2011). As is known, fine particles can have a strong extinction effect on Uv radiation through selective scattering and absorption (Eck et al., 2005). A higher concentration of fine particles causes a greater abatement of short-wave radiation by scattering and absorption effects, so the lowest Uv radiation values appear in Beijing. A high concentration of column-integrated water vapor will increase attenuation of solar radiation. In contrast, fewer fine particles and less water vapor result in relatively high Uv radiation. This is based on the principle that the absorption of water vapor to the solar band is stronger than to the Uv band. The effects of fine particles and water vapor lead to the highest Uv radiation being observed at Changwu, which is situated near the margin of the Loess Plateau, and where there are much lower fine aerosol concentrations (Wang et al., 2008) and lower water vapor.

    In the present study we focused on the station at Beijing due to the comprehensive measurements carried out there. As shown in Fig. 3, we can see that there is a smaller increasing trend. Moreover, there is a quasi-biennial oscillation of Uv radiation, mainly due to monsoon activity. The increasing trend of Uv radiation is mainly caused by the slightly decreasing amounts of ozone and fine aerosols in the recent period. The ozone data were obtained from TOMS (http://toms.gsfc.nasa.gov/). The total amount of ozone over

    Figure 3.  Annual variations of Uv vs. the total values of ozone and the concentration of PM2.5 at the Beijing site.

    Figure 4.  Variation characteristics of Uv/Rs vs. column-integrated water vapor during the 2005-11 period over the NCP: (a) annual and (b) departure variation.

    Beijing decreased from 330 DU in 2005 to 228 DU in 2011. This decreasing trend of total ozone can also be seen at all other observation sites in the NCP region. Furthermore, the concentration of fine aerosol (PM2.5) in Beijing decreased from 83 g m-3 in 2005 to 67 g m-3 in 2011. A lower extinction effect of ozone and aerosol on short wavelength solar radiation results in higher Uv radiation.

  • Many studies have been carried out to obtain more Uv radiation data based on the relationship between simultaneous observation values of Uv radiation and Rs (Barbero et al., 2006; Mateos-Vill´an et al., 2010). The climatic variation characteristics of Uv radiation and the effect of Uv radiation on ecology are investigated using these long-term Uv radiation data. The Uv radiation estimation method obtains the relationship between Uv radiation and Rs through a linear regression process. Based on this linear relationship, the history of Uv radiation is calculated from more routinely measured Rs data. However, the ratio of Uv to Rs must be recalibrated according to local climate conditions and geography (Udo, 2000). The spatiotemporal variation properties of the ratio of Uv radiation to Rs are very useful for studying the long-term variation characteristics of Uv radiation, especially for sites without in situ measurement data of Uv radiation. The monthly variation properties of the ratio of Uv radiation to Rs are shown in Fig. 4. From this figure, we can see there is an obvious seasonal variation feature of this ratio. There is a gradually increasing trend of this ratio from April until the highest values appear in summer (August). Then, there is a gradual decrease until the lowest values appear in winter. The seasonal variation trend of this ratio is caused by the seasonal variation feature of water vapor content (Fig. 4). As is well known, the NCP is mainly influenced by the East Asian monsoon. In general, water vapor increases from spring until a maximum value appears in summer caused by enriched rainfall in this period. From August, water vapor starts to decrease gradually, due to the withdrawal of the monsoon. In theory, water vapor has a stronger extinction effect on the solar band than the Uv band.

    The spatial distribution of the ratio of Uv radiation to Rs is shown in Table 1. The range of this ratio varies from 0.031 ± 0.004 to 0.42 ± 0.006 over the NCP. As expected, the highest values of this ratio were recorded at the Jiaozhouwang site due to the higher content of water vapor and low PM2.5 concentration in the atmosphere (Table 1). Results have indicated that there is heavy aerosol loading in coarse-mode particles due to the humidity-swelling of sea salt aerosols, so the abatement effect of PM2.5 on Uv radiation is relatively small (Wang et al., 2008). The Jiaozhouwan site is located on the Shandong Peninsula, and is well known for its monsoon-influenced climate. Temperature and precipitation measurements classify this site as “intermediate humid subtropical” and “humid continental”, respectively.

    The lowest value of the ratio of Uv radiation to Rs was observed at Beijing due to the highest fine aerosol concentrations and the relatively low RH. Beijing is located within the Beijing-Tianjin-Hebei (BTH) mega-city cluster. Air pollution in the BTH region has recently been very severe due to rapid economic development, population expansion and urbanization. Aerosol pollution—especially the concentration of PM2.5—is more serious (Parrish and Zhu, 2009) in Beijing. The annual average concentration of PM2.5 is higher than 101 mg m-3 (He et al., 2001; Zheng et al., 2005).

    The physical foundation is that the extinction of aerosol particles is negatively related with wavelength. So the extinction to Uv radiation is more than to Rs. The water vapor absorption of the Uv band is smaller than the solar band, so the observed Uv radiation is almost constant and Rs decreases with increasing water vapor. Figure 4 shows a small increasing trend of the ratio of Uv radiation to Rs in this region due to the decreasing fine aerosol and increasing water vapor concentration during the observation period.

  • Numerous factors can influence the amount of Uv radiation reaching the Earth’s surface, such as solar altitude, the ozone column, aerosol concentrations, and microphysical aerosol and cloud properties. Altitude is another factor that can greatly influence Uv radiation (Piazena, 1996; Seckmeyer et al., 1997; McKenzie et al., 2001).

    The extinction effect of clouds on Uv radiation depends on the microphysical characteristics of clouds, such as cloud cover, cloud level, cloud optical thickness, cloud water content, and cloud droplet spectra. To investigate the influence of clouds on Uv radiation, microphysical chemical parameters of clouds, such as cloud cover, level, type, and albedo, are necessary (Nemeth et al., 1996; D´ıaz et al., 2000; Foyo-Moreno et al., 2003). Unfortunately, these parameters are difficult to obtain due to the high degree of variability of clouds.

    Many studies have indicated that the clearness index (Ks) can be used as a general indicator for extinction effect processes caused by aerosols, gases and clouds during the transfer of solar radiation through the atmosphere (Liu and Jordan, 1960). Accordingly, this parameter was used instead of cloud parameters. Ks was also used to analyze the influence of clouds on the ratio of Uv radiation to Rs. The definition of Ks is the ratio of Rs that reaches the surface to the solar irradiance flux at the top of the atmosphere.

    Figure 5 shows opposite variation features of Ks compared with the ratio of Uv radiation to Rs. Minimum Ks values occur in the rainy season (i.e., the summer period), which indicates higher extinction causes higher water concentration; namely, more clouds appear at this time. In contrast, a high Ks indicates low fine aerosols and water vapor, resulting in a small ratio due to a small abatement effect on shortwave solar radiation; namely, Uv radiation.

  • As mentioned above, non-dimensional parameters are influenced less by local features, so a parameterization estimation model for Uv radiation was developed based on Ks and the solar zenith angle with high accuracy (Foyo-Moreno et al., 2003; Varo et al., 2005; Barbero et al., 2006; Hu et al., 2010; Mateos Vill´an et al., 2010). (Xia et al., 2008) reported that a power law equation can be used to describe the influence of the cosine of the solar zenith angle (μ) on Uv radiation. The power law equation for Uv radiation within a narrow range of Ks can be written as

    Uv = U’v0×μN (1)

    where U’v0 is the maximum value of Uv radiation per unit of μ. N determines how Uv changes with μ. (Xia et al., 2008) also reported that the relationship between Ks and the maximum value of Uv radiation can be described by a polynomial equation. (Hu et al., 2010) used measured data from 2005-08 in Beijing to develop a parametric equation to calculate the maximum value of Uv radiation based on Ks:

    U’v0=0.95+74Ks +55Ks3 (2)

    Figure 5.  Monthly daily average of Uv/Rs and Ks in the NCP region: (a) temporal variation and (b) scatter plot of Uv/Rs and Ks at Beijing Station.

    Figure 6.  Temporal variation characteristics of measured and model-predicted Uv radiation values in the NCP region.

    A parameterization model for estimating Uv radiation based on the maximum value of Uv radiation and μ can be obtained. (Xia et al., 2008) found that their equation [Eq. (1)] could obtain the Uv radiation highly accurately in Xianghe when parameter N is set as 1.06. Xianhe belongs to Hebei Province, which is located between two megacities, Beijing and Tianjin, 70 km to the northwest and to the southeast, respectively. According to (Xia et al., 2008), we set parameter N as 1.06 in the parameterization model for estimating Uv radiation developed for the Beijing site.

    Uv=(0.95+74Ks-74 Ks2 +55Ks3) ×μ1.06 (3)

    The evaluated results indicate that this parameterization model can obtain a more accurate estimation of Uv radiation from more routinely measured Rs data. Only solar elevation angle and cloudiness index parameters are involved in this parameterization estimation model. Furthermore, these two parameters can be easily obtained through more routinely observed data.

    To test the transferability of the parameterization model, Eq. (3) was used to model hourly Uv radiation data through highly accurate measured Rs datasets collected at five other sites in the NCP. As shown in Table 2, the statistical parameters of the linear regression equation between the observed and modeled Uv radiation data show that this parameterization model can obtain accurate Uv radiation through the measured Rs datasets. The slope and intercept of the linear regression are close to 1 and 0, respectively. The relative deviations between the modeled and observed daily Uv radiation are less than 15%. The mean bias errors (MBEs) are -0.04, -0.02, 0.01, 0.04, and -0.01 MJ m-2 d-1, for Luancheng, Yuchen, Jiaozhouwan, Changwu, and Fengqiu, respectively. The root mean square errors (RMSEs) are 0.08, 0.06, 0.08, 0.08, and 0.07 MJ m-2 d-1, for Luancheng, Yuchen, Jiaozhouwan, Changwu, and Fengqiu, respectively. These results indicate that this parameterization model can be used to estimate Uv radiation at observation sites in the NCP region other than Beijing, where the estimation model was developed.

    The temporal variation characteristics of observed and modeled data are shown in Fig. 6. The same variation features of the modeled Uv radiation can be seen in the Uv radiation in Fig. 6, which indicates that the parameterization model can be used to calculate Uv radiation from the Rs dataset with high accuracy.

    The advantage of this parameterization model is that it involves the cosine of the solar zenith angle and the measurement data of Rs. These two parameters can be obtained more easily because Rs is a routinely measured parameter in weather observation stations. However, there is a weakness in the parameterization model when transferred to other locations, especially for the relative deviation parameter. The results of Zerefos (2002) indicate that the total ozone concentration has a stronger influence on UVB than UVA under clear skies, so the effect of ozone on Uv radiation is not involved in this parameterization model. However, the effect of ozone on Uv radiation should be considered in further studies. This parameterization model can be easily modified to calculate daily Uv radiation with daily Rs by using the length of daytime, the daily average cosine of the solar zenith angle, and Ks.

4 Conclusions
  • The spatiotemporal variation properties of Uv radiation and the ratio of Uv radiation to Rs were analyzed in this study using in situ measurement data of Uv radiation and Rs from 2005-11 at six observation stations located in the NCP. The results showed that Uv radiation presents a similar seasonal variation to that of solar activity; that is, the values of Uv radiation gradually increase from spring until the highest values appear in summer, after which they decrease until the lowest values appear in winter. The highest value of Uv radiation was found to be 0.52×0.20 MJ m-2 d-1, recorded at the Changwu site. The lowest column-integrated water vapor and fewer fine particles at this site are the main factors leading to the highest values. In contrast, the strong extinction effect caused by the high fine aerosol burden results in the lowest value (0.52×0.20 MJ m-2 d-1), as measured at the Beijing site.

    The average of the ratio of Uv radiation to Rs was found to range from 0.031±0.004 to 0.042±0.006 in the NCP region. The temporal variation characteristics of this ratio are the same as the Uv radiation. A high value appears in summer, and a low value appears in winter, with this variation pattern mainly being influenced by water vapor. The highest value of this ratio was observed at Jiaozhouwan due to the high water vapor and lower fine aerosol. A slight increasing trend of Uv radiation was discovered, and the ratio of Uv radiation to Rs in this region is caused by the decreasing trend of the concentration of fine aerosol and ozone and the increasing trend of water vapor.

    A parameterization model was developed through the measured dataset of the Beijing observation site. This model was evaluated using measured data collected at the Beijing site and five other stations in the NCP. The regression results showed that this parameterization model can be used to obtain more accurate Uv radiation data from routinely measured Rs data. These results can provide more information for studying Uv radiation and climate change in the NCP region. Moreover, the parameterization model can improve the accuracy of estimating Uv radiation from Rs on a larger scale than that of only using the relationship between Uv radiation and Rs.

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