火山ガス連続測定装置の作製と伊豆大島火山における観測(火山活動,<特集>自然災害の地球化学)

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We have developed a new continuous monitoring system for volcanic gas chemistry and set up this system at 3 km north from the summit of Izu-Oshima volcano. For continuous monitoring of dry gas, we use three types of dehydration methods to remove water vapor. CO_2 and SO_2 contents were measured by IR sensors, O_2 by a zirconia sensor and H_2 by a semiconductor sensor. Our system has been in operation more than 6 months without significant troubles. The dry volcanic gas is a mixture of CO_2 of magmatic origin and air component. Temporal variation in chemical composition of the volcanic gas is related with that in atmospheric pressure. The CO_2 content is inversely-correlated and O_2 is correlated to atmospheric pressure with 8 hours' delay. In contrast, H_2 content increases intermittently up to 1000ppm 1 hour after sudden drop of atmospheric pressure. The N_2/O_2 ratio is higher than the air ratio (3.8), and changes in a similar manner to CO_2. This suggests that a part of N_2 is of magmatic origin like CO_2.
1997-05-31