**3.1 Time series of UV radiation and UV***/R***s derived from measurement data**

The measurement site is located between the northern Third and Fourth Ring Roads in the mega city of Beijing. This site belongs to the Institute of Atmospheric Physics (IAP), Chinese Academy of Sciences. There are domestic dwellings to the north and south of the site and a freeway close to its east side. Measurement instruments were installed on a flat platform on top of the roof (about 10 m) of a building. The Measurement's representation of this site was validated by comparing it to observations results from the China Meteorological Administration (CMA) site (54511) in southern region of Beijing. Figure 1 show the variation characteristics of measurement *R*s in IAP and CMA station from January 2005 to December 2008 used for the comparison. From this figure, we find that *R*s values at the IAP site were almost same as those at the CMA (54511) station; the largest deviation in average *R*s between the two sites was 4.3% and the average deviation was 1.5% (Fig. 1). These results indicated that the measurements at the IAP station are a good representation of the Beijing urban area.

Fig. 1. Comparison of *Rs* measurements at CMA with that IAP from January 2005 to December 2008 in Beijing.(Hu et al., 2010 a)

The seasonal variation characteristics of UV radiation averaged over 4 years are presented in Fig. 2. UV shows the same seasonal features as that of *R*s. The high values of UV and *R*s both appear in summer and the low values appear in winter; values in spring and autumn are intermediate. For the diurnal variations, the maximums of *Qp* and *R*s both appeared around noon each day. There was a good correlation between these two solar-radiation components, by which one of them can be estimated from the other. For the annual variations, one year's variation mode is same as another year's. The minimum monthly average of daily values of UV radiation indicates, in the cool-dry season (i.e., winter period) an increasing trend in the spring with peaks in May. UV radiation levels decrease gradually after the May peak, falling to minimum level in the winter. The annual mean daily values of UV radiation for 6-year period were 0.39 ± 0.16 MJ m-2 d-1 with the lowest and highest UV radiation values being 1.06 MJ m-2 d-1 and 0.01 MJ m-2 d-1, respectively.

Fig. 2. Variation of UV in Beijing from 2005 to 2010

Figure 3 displays box plots of the monthly statistics of the UV*/R*s level in Beijing. The different symbols represent the monthly mean, maximum, and minimum values. There is significant monthly variation in UV*/R*s levels. The monthly mean UV*/R*s level gradually increases from about 2.7% in November to about 3.7% in August after which it gradually decreases. This seasonal variation of UV*/R*s is influenced by seasonal variations in the water vapor content and cloud amount. From spring, the content of column-integrated water vapor generally reaches its maximum in summer, with abundant rainfall, and decreases from August. This variation characteristic is similar to that of UV*/R*s. The high values of *K*<sup>t</sup> (the ratio of *Rs* to extraterrestrial irradiance) occurred in autumn and the low values occurred in summer; the values in spring and winter are intermediate. This seasonal variation pattern of Kt is similar to of column-integrated water vapor content. Under humid and cloudy conditions, the absorption of solar radiation in the infrared region is enhanced because of the increased water vapour content, whereas absorption in the UV region does not vary significantly.

noon each day. There was a good correlation between these two solar-radiation components, by which one of them can be estimated from the other. For the annual variations, one year's variation mode is same as another year's. The minimum monthly average of daily values of UV radiation indicates, in the cool-dry season (i.e., winter period) an increasing trend in the spring with peaks in May. UV radiation levels decrease gradually after the May peak, falling to minimum level in the winter. The annual mean daily values of UV radiation for 6-year period were 0.39 ± 0.16 MJ m-2 d-1 with the lowest and highest UV

radiation values being 1.06 MJ m-2 d-1 and 0.01 MJ m-2 d-1, respectively.

2005-4-30 2005-8-28

0.0

not vary significantly.

0.2

0.4

UV (MJ m-2

d-1

)

0.6

0.8

1.0

2005-12-26

Fig. 2. Variation of UV in Beijing from 2005 to 2010

2006-4-25

2006-8-23

2006-12-21

2007-4-20

2007-8-18

2007-12-16

Figure 3 displays box plots of the monthly statistics of the UV*/R*s level in Beijing. The different symbols represent the monthly mean, maximum, and minimum values. There is significant monthly variation in UV*/R*s levels. The monthly mean UV*/R*s level gradually increases from about 2.7% in November to about 3.7% in August after which it gradually decreases. This seasonal variation of UV*/R*s is influenced by seasonal variations in the water vapor content and cloud amount. From spring, the content of column-integrated water vapor generally reaches its maximum in summer, with abundant rainfall, and decreases from August. This variation characteristic is similar to that of UV*/R*s. The high values of *K*<sup>t</sup> (the ratio of *Rs* to extraterrestrial irradiance) occurred in autumn and the low values occurred in summer; the values in spring and winter are intermediate. This seasonal variation pattern of Kt is similar to of column-integrated water vapor content. Under humid and cloudy conditions, the absorption of solar radiation in the infrared region is enhanced because of the increased water vapour content, whereas absorption in the UV region does

2008-4-14

2008-8-12

2008-12-10

2009-4-9

2009-8-7

2009-12-5

2010-4-4

2010-8-2

 UV Rs

2010-11-30

0

20

Rs (MJ m-2 d-1

)

40

Fig. 3. The graphs of of (a) the monthly ratio of Ultraviolet radiation to solar radiation (b) water vapor content and (c) clearness index. (Hu et al., 2010 b)

In Figure 3, the central bar is the median and the lower and upper limits are the maximum and minimum, respectively.

Most studies use measured values to determine the relationship between UV and *R*s and based on this, UV-estimating models are developed. The range of UV*/R*s levels must be recalibrated to account for local climatic and geographical differences as well as atmospheric conditions (Udo, 2000). Our study uses direct-measurement UV data to investigate UV radiation properties under various atmospheric conditions in Beijing.

The clearness index, *K*t, is defined as the ratio of the total irradiance to extraterrestrial solar irradiance, both defined on a horizontal surface. The *K*t ratio is a general indicator of scattering and absorption processes due to aerosols, gases, and clouds that may interrupt the transmission of irradiance through the atmosphere (Liu and Jordan, 1960; Elhadidy et al., 1990).

Fig. 4 depicts hourly UV radiation as a function of the cosine of the solar zenith angle (μ).

Fig. 4. UV as a function of the cosine of the solar zenith angleat different Kt. Different clearness index (*Kt*) values are represented by different colours. The values of UV radiation within a narrow range of clearness index values increase almost exponentially with the cosine of the solar zenith angle.

In Figure 3, the central bar is the median and the lower and upper limits are the maximum

Most studies use measured values to determine the relationship between UV and *R*s and based on this, UV-estimating models are developed. The range of UV*/R*s levels must be recalibrated to account for local climatic and geographical differences as well as atmospheric conditions (Udo, 2000). Our study uses direct-measurement UV data to investigate UV

The clearness index, *K*t, is defined as the ratio of the total irradiance to extraterrestrial solar irradiance, both defined on a horizontal surface. The *K*t ratio is a general indicator of scattering and absorption processes due to aerosols, gases, and clouds that may interrupt the transmission

Fig. 4 depicts hourly UV radiation as a function of the cosine of the solar zenith angle (μ).

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

cosine of the solar zenith angle

Fig. 4. UV as a function of the cosine of the solar zenith angleat different Kt. Different clearness index (*Kt*) values are represented by different colours. The values of UV radiation within a narrow range of clearness index values increase almost exponentially with the

radiation properties under various atmospheric conditions in Beijing.

of irradiance through the atmosphere (Liu and Jordan, 1960; Elhadidy et al., 1990).

and minimum, respectively.

0

cosine of the solar zenith angle.

5

10

15

20

UV (W m-2

)

25

30

35

*K*t < 0.2 0.2 ≤ *K*<sup>t</sup>

0.3 ≤ *K*<sup>t</sup>

0.4 ≤ *K*<sup>t</sup>

0.5 ≤ *K*<sup>t</sup>

0.6 ≤ *K*<sup>t</sup>

0.7 ≤ *K*<sup>t</sup>

0.8 ≤ *K*<sup>t</sup>

< 0.3

< 0.4

< 0.5

< 0.6

< 0.7

< 0.8

40

$$UV = UV\_{0m} \times \mu^b \tag{6}$$

where UV*0m* is the maximum value of UV radiation per unit of μ.

Base on this, hourly UV radiation was estimated using the following equation. The method of establishing an empirical model for UV computation is explained in detail in Hu et al (2010 b).

$$UV = (0.95 + 74\mathbf{K\_t} \cdot 71\mathbf{K\_t}^2 + \dots \\ \text{5.5 } \mathbf{K\_t}^3) \times \boldsymbol{\mu}^{1.031} \text{ .} \tag{7}$$

The daily values of *R*s at the Beijing site were measured by the National Meteorological Center, China Meteorological Administration (CMA). The hourly values of *R*s, however, were not possible to obtain, therefore, in order to obtain long-period trends in the variation of UV radiation in Beijing, equation 7 was modified as in equation 8 in order to use the daily values of *R*s. However, the daily values of *R*s could be used to compute daily values of UV using equation 4:

$$UV\_{\rm daily} = (8.4 + 3206.8 \overline{\text{K}}\_t \cdot 2210.7 \ \overline{\text{K}}\_t^{-2} + 2074.8 \ \overline{\text{K}}\_t^{3}) \times (\overline{\mu})^{1.031} \times t\_d \tag{8}$$

where *UV*daily is the daily amount of UV radiation, Kt is the ratio of daily *R*s to daily extraterrestrial solar irradiance, is the average of the cosine of the solar zenith angle from sunrise to sunset, and *t*d is the length of daytime in hour.
