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

A new continuous monitoring system has been developed for the measurement o f volcanic gas from the steam well located 3 km north from the summit of Iz u-Oshima volcano, Japan. After removing the water vapor using three sequent ial dehydration methods, CO2 and SO2 contents are measured using TR sensors , and O-2 and H-2 using a zirconia sensor and a semiconductor sensor, respe ctively. 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, O-2 and N-2. A decreasing trend of the CO2 content was observed from 1995 to 1 998 together with a decrease of volcanic activity. Seasonal changes have al so been observed in CO2 and O-2 contents, CO2 being higher and O-2 lower in summer, which suggests larger contribution of magmatic components in summe r. While changes in short-term variation in CO2 and O-2 are influenced by a tmospheric pressure changes; the CO2 content correlates inversely with atmo spheric pressure unlike O-2 With some hours delay. In contrast, the H-2 con tent increased intermittently up to 1200 ppm one to several hours after a s udden drop in the atmospheric pressure and without any apparent correlation with seasonal changes. This system allows us to study temporal variation in chemical composition o f volcanic gas during quiescent periods of volcanic activity of Izu-Oshima volcano, and might help us detect anomalous changes before future eruptive events. (C) 2000 Elsevier Science B.V. All rights reserved.