浅間山噴煙中の火山ガス化学組成の遠隔観測(<特集>2004年浅間山噴火(2))  [in Japanese] Remote FT-IR Measurements of Volcanic Gas Chemistry in the Plume of Asama Volcano(<Special Section>The 2004 Eruption of Asama Volcano(2))  [in Japanese]

Remote FT-IR measurements for volcanic gas chemistry were carried out twice before and four times after the 2004 Asama eruptions. In these measurements, solar infrared light scattered by higher clouds or plumes was used as a light source for the FT-IR absorption measurements. We have successfully detected 3 volcanic gas components, SO_2, HCl and HF, in the observed spectra. The HCl/SO_2 ratios observed after the onset of the 2004 Asama eruptions were between 0.17 and 0.20, which are probably reflecting the ratios not influenced by any hydrothermal interactions. In contrast, the HCl/SO_2 ratios before the eruptions were slightly lower than those after the eruptions. The slight increase in the ratio from pre-eruptive to post-eruptive periods suggests that the hydrothermal or groundwater impact to the volcanic gas emitting system was small even three years before the eruptions. The observed HF/HCl ratios before the eruptions and in March 2005 were about 0.1, whereas the ratios were higher, over 0.19, during the high activity period from mid-September to October 2004. During the continuous ash emitting eruptions on September 16, 2004, exceptionally low HF/HCl ratio of 0.03 was observed. The HF column amount on this day was probably under detection limit, possibly due to HF depletion caused by adsorption on dense ash in the plume.