Study of volcanic activity monitoring based on continuous measurement of vdcanic gas chemistry

火山气体化学监测只针对火山场的环境空气进行。到目前为止，直接监测火山（喷口）气体的试验很少，因为普通的气体传感器在酸性、潮湿和炎热的条件下（如火山气体）不能很好地工作。因此，我们开发了一种新的火山气体化学连续监测系统。采用三种脱水装置将火山气高温蒸汽完全冷却脱除后，用红外传感器（CO_2和SO_2）、半导体传感器（H_2）和氧化锆传感器（O_2）连续监测得到的干气的化学成分。该系统设置在大尾岛火山山顶火山口的人工喷气孔内，已经运行了2年多。该富马酸气体是岩浆源CO_2与空气组分CO_2的混合物，其混合比例在长、短时间范围内均随时间变化。CO_2呈夏季高、冬季低的年变化特征。1996年伊豆大岛火山大喷发后岩浆活动的减弱趋势与夏季CO_2峰值的总体下降趋势一致。气体化学的短期变化与大气压力的变化有关。CO_2与大气压力呈负相关，O_2与大气压力呈负相关。在恒定条件下，在大气压力突然下降1小时后，H2含量间歇性增加至1000ppm。CO_2和H_2的不同行为表明每种植物的起源不同。

Monitoring of volcanic gas chemistry has been carried out only for ambient air of fumarolic fields. Until now there have been few trial to monitor volcanic (vent) gas directly, because common gas sensors do not work well in acid, wet and hot conditions such as in volcanic gas. Therefore, we have deveroped a new continuous monitoring system for volcanic gas chemistry. After high temperature steam of volcanic gas is cooled and removed completely using three types of dehydration apparatus, the chemical compositions of thus obtained dry gas are monitored continuously using an IR sensor (CO_2 and SO_2), a semiconductor sensor (H_2)and a zirconia sensor (O_2).This system was set up at an artificial fumarole inside the summit caldera of Ozu-Oshima volcano and has been in oparation for more than 2 years. This fumarolic gas is a mixture of CO_2 of magmatic origin and air component, and the mixing ratio changes with time both in long and short time ranges. The CO_2 shows an annual variation with high content in summer and low in winter. Generally decreasing trend of summer peak values in CO_2 may be coincide with the decreasing trend in magmatic activity of Izu-Oshima volcano after big eruptions in 1996. A short-term temporal variation of the gas chemistry is related with that in atmospheric pressure. The CO_2 content is inversely-correlated and O_2 is correlated with atmospheric pressure with 8 hour's delay. In constant, the H2 content increases intermittently up to 1000ppm 1 hour after sudden drop of atmospheric pressure. The differnt behaviors of CO_2 and H_2 suggest differnt origins of each species.