Discharge rate of SO2from Unzen Volcano, Kyushu, Japan

The discharge rate of SO2from Unzen volcano, which started erupting in November 1990, has been measured with a correlation spectrometer (COSPEC) at a fixed ground‐based station about 2.3 km SSW from the summit lava dome. The discharge rate of SO2increased greatly after the explosive eruptions on June 8th and 11th, 1991. High discharge rates of SO2(an average of 180 ton/day) were observed from July 1991 to January 1992, followed by a general decrease to an average of 110 ton/day during March 1992 and May 1994. The discharge rate of SO2correlates with the extrusion rate of magma. Judging from the sulfur inventory, i.e., 130–450 g of discharged SO2per ton of erupted magma, and the sulfur content of the extruded lava, the proportion of sulfur degassed from the magma is estimated to be ca. 80%, suggesting an efficient volatile degassing. The discharge rate of the total volatiles is calculated to be 7,700 ton/day by combining the SO2discharge rate with the chemical composition of volcanic gases.