The global solar UV index is an indicator for notifying the level of harmful solar ultraviolet radiation reaching the surface of the earth and the general public. It was proposed by the WHO/WMO/UNEP/ICNIRP and was standardized by the CIE in 2003. This index is derived from the product of the spectral solar UV irradiance from 250 to 400 nm and the CIE standard of reference erythema spectrum. For calculation of the UV index, the measurement of spectral solar UV irradiance is needed. Spectral radiometry is the best method of measurement of solar UV irradiance, however spectral radiometers are cost prohibitive. On the other hand, a narrow-band solar UV-B radiometer is widely used for measurement of solar UV-B irradiance in the world. The Tokai Solar Radiation Monitoring Network, and the UV Monitoring Network-Japan performed by the National Institute for Environmental Studies are two examples of monitoring networks using narrow-band solar UV-B radiometer in Japan. In this paper an estimation method of the UV index from the measured UV-B irradiance with the narrow-band UV-B radiometer.
Since 1990, global solar UV-B irradiance has been measured by a narrow-band solar UV-B radiometer at Hiratsuka, Japan. To detect long-term trend of global solar UV-B irradiance measured by the narrow-band solar UV-B radiometer, evaluation of a calibration constant of the radiometer for accurate measurement is essential. Thus, for estimating trend of global solar UV-B irradiance from 2000 to 2003, the calibration constant of the narrow-band solar UV-B radiometer was evaluated by applying three methods. The first method is based on a spectral irradiance standard lamp, which has been applied when evaluating long-term trend of global solar UV-B irradiance from October 1990 to September 2000. The second one is based on a reference narrow-band solar UV-B radiometer. Global solar UV-B irradiance measured by our narrow-band solar UV-B radiometer was compared with that of the reference narrow-band solar UV-B radiometer under natural sunlight. The third one is based on the total ozone amount. The results with three methods showed good agreement within 5%, leading to the conclusion that three methods are comparable in evaluating the calibration constant of the narrow-band solar UV-B radiometer.
Seasonal variation of global solar radiation: ultraviolet-B (UV-B: 290 - 320 nm), ultraviolet-A (UV-A: 320 - 400 nm) and Total (Total: 300 - 3000 nm) has been measured at the Shonan Campus of Tokai University (35°21’N, 139°16’E, in central Japan) since October 1990. To clarify regional difference of solar radiation in East Asia, the same type of radiometers were installed at Wakkanai (October 2000: 141°48’N, 145°21’E, in Hokkaido, at the northern end of Japan), Mashiki (June 1996: 32°50'N, 130°52'E, in Kumamoto, at the mid-west Japan) and Taketomi (July 1998: 24°20’N, 123°41'E, on Iriomote Island, in Okinawa, at the south western end of Japan). Observation data are transferred to the Shonan Campus every day via Internet, and the data were posted on our web site. Comparison of sensitivity among all radiometers used in this network was carried out from August 2002 to September 2002. Observed data were corrected based on the comparison results.
Solar radiation showed clear regional differences depending on the season. Global solar radiation in summer was almost the same among stations, but in winter, global radiation showed very significantly among the stations. The ratio of UV-B in winter to summer was 14 times at Wakkanai, 6 times at Shonan and Mashiki, and 2 times at Taketomi. The ratio of Total in winter to summer was 2 times at Shonan, Mashiki and Taketomi, and 6 times at Wakkanai. The seasonal variation of the ozone level corresponded the latitude with Taketomi registering the least amplitude.
Ground-based global solar ultraviolet-B (UVB) irradiance has been measured by a narrowband-spectral UVB radiometer at Tokai University, Hiratsuka (35 degree(s)21' N, 139 degree(s)16' E) Japan for a 10-year period from October 1990 to September 2000. A precise calibration of the UVB radiometer was periodically performed and the yearly decay sensitivity was found to be -3.7% of the initial sensitivity. Using this decay rate, the measured UVB irradiance was corrected, and the long-term trends of the UVB irradiance were estimated. An increase in the UVB irradiance was demonstrated to be 1.57% per year with a seasonal variation correction, and 1.22% per year with global total solar irradiance and the seasonal variation corrections in addition to quasi-biennial oscillation (QBO) correction. A significant, negative correlation was confirmed between the UVB irradiance normalized by the total solar irradiance and the effective ozone amount ((total ozone amount) x sec (theta) , where (theta) is the solar zenith angle) at Tsukuba, the closest ozone amount measuring site. These findings can be supportive evidence for a direct relationship between solar UVB irradiance and stratospheric ozone amount.
To evaluate regional differences of global ultraviolet-B (UVB: 290-315 nm) irradiance, eight band-spectral UVB radiometers (MS-210W, EKO), which were used in the UV monitoring network organized by five universities, were compared at the Shonan Campus of Tokai University, Japan from August 2-10, 1997. Reference irradiances were measured by a spectroradiometer (OL752, Optronic Lab.) and the reference band-spectral UVB radiometer (MS-210W, EKO). UVB radiometers participated were classified into three groups by the characteristics of the spectral response depending on the UVB irradiance. It suggests that characteristics of out of band is different from the reference radiometer. Furthermore, to access regional difference of UVB, it is necessary to use the calibration factor that is not constant but a function depending on output.