Earth Planets Space, Vol. 65 (No. 1), pp. e1-e4, 2013
Masao Ohno1, Mitsuru Utsugi2, Toshiya Mori3, Itsuro Kita1, Tsuneomi Kagiyama2, and Yoshikazu Tanaka2
1Department of Environmental Changes, Faculty of Social and Cultural Studies, Kyushu University, Nishi-ku, Fukuoka 819-0395, Japan
2Aso Volcanological Laboratory, Graduate School of Science, Kyoto University, Kumamoto 869-1404, Japan
3Geochemical Research Center, Graduate School of Science, The University of Tokyo, Bunkyo-ku, Tokyo 113-0033, Japan
(Received February 16, 2012; Revised November 21, 2012; Accepted November 23, 2012; Online published January 31, 2013)
Continuous monitoring of volcanic gas at Aso volcano using volatile traps from 2002 to 2011 revealed that higher HCl/SO2 ratios are associated with increasing volcanic activities. The HCl/SO2 ratios obtained during the observation period can be categorized into three levels: high (≥0.2), intermediate (0.1∼0.2), and low (≤0.1). From March 2003 to August 2005, and from December 2008 to August 2011, the HCl/SO2 ratios were generally at intermediate levels, with sporadic high HCl/SO2 ratios associated with small ash emissions. During other periods with no ash emission, the HCl/SO2 ratios were constantly low (∼0.02 in average). The HCl/SO2 ratio and the water level in the crater lake are correlated; the lower the water level in the crater lake, the higher the HCl/SO2 ratio. The reduced scrubbing effect of HCl from the intensely-emitted volcanic gas by a smaller amount of water is likely to generate high HCl/SO2 ratios of volcanic gas. In addition, evaporation of HCl from the water of the crater lake with extreme acidity may contribute to high HCl/SO2 ratios.
Key words: Volcanic gas, volatile trap, ash emission, chloride, sulfate.