[1] We report the quantitative evaluation of the UV scattering effect on the SO 2 emission rate measurement by the compact UV spectrometer system. Plume spectra were obtained simultaneously at three measuring points with different distance to the volcanic plume. The apparent absorbance decreases with increasing distance to the plume and the attenuation becomes stronger at shorter wavelength bands. In addition, the attenuation intensity depends on the SO 2 column concentration. The underestimation of the measured absorbance caused by the UV scattering leads to the underestimation of the SO 2 emission rate. The attenuation was not significant with any wavelength band (<±10%) at 0.6 km but was 35 -50% with shorter wavelength band at 2.6 km distance. The UV scattering effect on the SO 2 emission rate estimation can be evaluated by the comparison of the emission rates calculated with different wavelength bands. Citation: Mori, T

[1] All available SO 2 flux data for 32 years of Japanese volcanoes, accounting for about 10% of the world's arc volcanoes, were compiled to evaluate the temporal variation of the flux of each volcano and to estimate the time-averaged SO 2 flux. The compiled data revealed that 6 volcanoes (Tokachi, Asama, Aso, Sakurajima, Satsuma-Iwojima, and Suwanosejima volcanoes) out of 17 significantly degassing volcanoes usually contributed more than 94% of the total flux. The time-averaged annual flux was 2.2 Tg a À1, which includes intense degassing of Miyakejima volcano after 2000, which raised the figure from 1.4 Tg a À1, indicating that a single huge emitter is capable of significantly skewing regional time-averaged degassing totals and indicating that the time-averaged flux assessments for infrequent huge emitters are important for accurate estimation. The regional SO 2 flux distribution in cumulative frequency-flux plot does not obey a power law distribution. It shows a roll-off curve bending at about 500 t d À1, implying that it is misleading to assume the power law distribution for estimation of the global flux. Because the contribution of the major degassing volcanoes including the six volcanoes and additional sporadically degassing volcanoes during eruptive and posteruptive periods to the total flux is more than 95%, measurement of all large flux volcanoes can approximate the global flux.

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