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

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 similar to 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 (similar to 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.