Increasing trend of biologically active solar ultraviolet-B irradiance in mid-latitude Japan in the 1990s

Abstract

Ground-based global solar ultraviolet-B (UV-B: 290 to 320 nm) irradiance has been measured by a narrow band UV-B radiometer at Tokai University, Hiratsuka, Japan (35°218N, 139°168E) for a 10-year period from October 1990 to September 2000. A precise calibration of the UV-B radiometer was periodically performed, and the yearly decay in sensitivity was found to be 23.7%. Using this decay rate, the measured UV-B irradiance was corrected, and the long-term trends of the UV-B irradiance were estimated. When the seasonal variation was eliminated by taking 12-month moving averages, an increasing trend in the UV-B irradiance was demonstrated to be 1.57 % per year. Moreover, to re- move quasibiennial oscillation (QBO), 26-month moving averages were applied to the UV-B irradiance normalized by the global total (300 to 3000 nm) solar irradiance. An increasing trend in the normalized UV-B irradiance was found to be 1.22 % per year. In winter, the clearly increas- ing trend of the UV-B irradiance was statistically significant, although the increasing tendency of the UV-B irradiance in other seasons (spring, summer, and fall) is not clear. A significant inverse correlation was con- firmed between the UV-B irradiance normalized by the global total solar irradiance and the effective ozone amount defined as total ozone amount 3secu, where u is the solar zenith angle. These findings give supporting evidence for a direct relationship between solar UV-B irradiance and the stratospheric ozone amount. In conclusion, the increasing trend of global solar UV-B irradiance, especially in winter, was confirmed in response to stratospheric ozone loss in mid-latitude Japan in the 1990s. © 2002 Society