Continuous chemical monitoring of volcanic gas in Izu-Oshima volcano, Japan

A new continuous monitoring system has been developed for the measurement of volcanic gas from the steam well located 3 km north from the summit of Izu-Oshima volcano, Japan. After removing the water vapor using three sequential dehydration methods, CO₂ and SO₂ contents are measured using IR sensors, and O₂ and H₂ using a zirconia sensor and a semiconductor sensor, respectively. This system has been in operation without any significant trouble for 3 years.The dehydrated volcanic gas from the well consists of a mixture of CO₂, O₂ and N₂. A decreasing trend of the CO₂ content was observed from 1995 to 1998 together with a decrease of volcanic activity. Seasonal changes have also been observed in CO₂ and O₂ contents, CO₂ being higher and O₂ lower in summer, which suggests larger contribution of magmatic components in summer. While changes in short-term variation in CO₂ and O₂ are influenced by atmospheric pressure changes; the CO₂ content correlates inversely with atmospheric pressure unlike O₂ with some hours delay. In contrast, the H₂ content increased intermittently up to 1200 ppm one to several hours after a sudden drop in the atmospheric pressure and without any apparent correlation with seasonal changes.This system allows us to study temporal variation in chemical composition of volcanic gas during quiescent periods of volcanic activity of Izu-Oshima volcano, and might help us detect anomalous changes before future eruptive events.