Discharge rate of SO 2 from Unzen Volcano, Kyushu, Japan

The discharge rate of SO 2 from 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 SO 2 increased greatly after the explosive eruptions on June 8th and 11th, 1991. High discharge rates of SO 2 (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 SO 2 correlates with the extrusion rate of magma. Judging from the sulfur inventory, i.e., 130–450 g of discharged SO 2 per 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 SO 2 discharge rate with the chemical composition of volcanic gases.