Influence of volcanic sulfur dioxide on spectral UV irradiance as measured by Brewer spectrophotometers

Spectra of UV irradiance measured by Brewer spectrophotometers at 13 stations in Japan and Canada have been examined to determine the effect of absorption by sulfur dioxide (SO2). A simple algorithm to estimate the total column amount of SO2 from global UV irradiance measurements has been developed. The algorithm which overestimates SO2 amounts at low altitudes is useful for identifying large SO2 absorption and for establishing upper limits of the SO2 values. Except at one station, the overall occurrence of days with SO2 amounts greater than 10 Dobson Units (DU) was about 0.22%. The exception was the Kagoshima observatory located 10 km from the volcano, Sakurajima. Volcanic activity resulted in frequent observation of column SO2 amounts of more than 20 DU over Kagoshima and in extreme cases up to 100 DU. The reduction in the long-term erythemally weighted UV irradiation due to absorption by SO2 is between one and two percent at Kagoshima and negligible at the twelve other stations.