ABSTRACT

Remote measurements of chemical compositions of the Aso volcanic plume using an FT-IR spectral radiometer were carried out at the 1st crater of Mt. Nakadake, Aso volcano, Japan, for six times from 1996 to 2003. We have succeeded in detecting 6 volcanic gas species (SO₂, HCl, HF, CO, CO₂, COS) using the InSb detector. The equilibrium temperatures estimated from observed CO/CO₂ ratios have remained high at more than 700°C during the observation period. However, the temporal variation of the ratios between volcanic gas components showed significant scrubbing of HCl compared to other species by hydrothermal interaction in 1998. From the visual and FT-IR observations, we presume that SO₂ flux from the volcano was lower in 1998 than in other years. This decrease in flux was not due to hydrothermal scrubbing of SO₂ but was due to decrease in a total supply rate of gas from depth. This assumption is supported by relatively stable CO₂/SO₂ ratio which is probably reflecting stable gas chemistry at the gas source.