Ground-Based Observation of Biologically Active Solar Ultraviolet-B Irradiance at 35°N Latitude in Japan

A new waterproof broadband UVB radiometer was designed to monitor biologically active solar UVB (290-320 nm) irradiance at ground level. Using this radiometer, solar UVB irradiance has been measured, together with UVA (320-400 nm) and Total (300-3300 nm) solar irradiances, for two years from October 1990 to September 1992. The global UVB solar irradiance shows clear seasonal changes, i.e., approximately by factors of 4 and 2 for the ratios of summer to winter and equinox to winter, respectively. In contrast, the seasonal variations in global UVA and Total are very similar and moderate in amplitude, and the corresponding ratios are about 1.7 and 1.3, respectively. Moreover, it is shown that the seasonal variation in the daily global UVB and that in the killing efficiency of sunlight for bacteriophage T1 have a similar pattern. For UVB and Total irradiances, the diffuse component was separately recorded. The monthly average ratio of the diffuse to the global (direct + diffuse) in UVB is persistently high, being about 80% throughout the year. The monthly average ratio of the diffuse to the global in Total solar irradiance was about 50% for the same period. Based on the present observation we found the 24-month average ratios of monthly global UVB/UVA/Total solar irradiance to be 0.14/4.1/100. Increases in the diffuse Total irradiance and in the global UVB irradiance, coupled with a decrease in the global Total irradiance, observed from November 1991 to February 1992 are interpreted as being an influence of the June 1991 eruption of Mt. Pinatubo.