TERRAPUB Earth, Planets and Space

Earth Planets Space, Vol. 65 (No. 1), pp. e1-e4, 2013


Temporal variation in the chemical composition (HCl/SO2) of volcanic gas associated with the volcanic activity of Aso Volcano, Japan

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)

Abstract: 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.

Corresponding author E-mail: mohno@scs.kyushu-u.ac.jp

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