Toshiya Mori

Remote detection of HCl and SO2 in volcanic gas from Unzen Volcano, Japan

A telescope-attached FT-IR spectral radiometer was applied for remote measurement of chemical composition of the volcanic gas of Unzen volcano, Japan, which started erupting in November 1990. The observations were carried out at the site about 1.3 km away from the gas-emitting region of the volcano in March and July 1992. Absorption features of SO2 (Vl) and HCl were identified and column amounts of SO2 and HCl of the volcanic gas plume were estimated using a nonlinear least-squares method. The SO2/HCl ratio was in the range between 0.7 and 1.8 for both March and July observations. This is the first remote ground-based observation of HCl in volcanic gas.

Diffuse emission of CO2 from Miyakejima volcano, Japan

Abstract Two soil gas surveys were carried out in May and September 1998 at Miyakejima volcano, in the Izu Mariana arc, Japan. CO 2 flux values for May and September surveys ranged from 0.1 to 18,150 g m −2 day −1 and from 0.1 to 9685 g m −2 day −1 , respectively. Statistical graphical analysis showed three overlapping populations. The spatial distribution of these emissions correlated quite closely with the geothermal and geological characteristics of the studied area. The structure releasing higher CO 2 is the summit cone Oyama and surrounding areas, where the most obvious geothermal features occur. A total output about 100–150 t day −1 is estimated from this area. A good correlation was observed between soil CO 2 flux and soil temperature at the summit caldera indicating extensive condensation of fumarolic steam within the upper part of Miyakejima. Carbon isotopic analysis of selected samples inside the summit caldera ( δ 13 C–CO 2 =−0.90‰ to −5.70‰) suggests a mixing of carbon derived from marine limestone and magmatic CO 2 while a clear biogenic origin ( δ 13 C–CO 2 =−14.76‰ to −25.52‰) is observed for the diffuse degassing of CO 2 outside summit caldera.

Remote CO, COS, CO2, SO2, HCl detection and temperature estimation of volcanic gas

Volcanic gas from an inaccessible fumarole at Aso volcano, Japan, was recently measured with a Fourier Transform InfraRed (FT-IR) spectral radiometer. Absorption features of CO, COS, CO2, SO2, and HCl were identified in the volcanic plume spectra. This is the first time the five volcanic gas components are detected simultaneously by a remote technique. Based on the measured CO/CO2 ratio of 2.6 × 10−3, the equilibrium temperature of the volcanic gas is estimated to be 750° ± 120°C.

Effect of UV scattering on SO2 emission rate measurements

We report the quantitative evaluation of the UV scattering effect on the SO 2 emission rate measurement by the compact UV spectrometer system. Plume spectra were obtained simultaneously at three measuring points with different distance to the volcanic plume. The apparent absorbance decreases with increasing distance to the plume and the attenuation becomes stronger at shorter wavelength bands. In addition, the attenuation intensity depends on the SO 2 column concentration. The underestimation of the measured absorbance caused by the UV scattering leads to the underestimation of the SO 2 emission rate. The attenuation was not significant with any wavelength band (<±10%) at 0.6 km but was 35-50% with shorter wavelength band at 2.6 km distance. The UV scattering effect on the SO 2 emission rate estimation can be evaluated by the comparison of the emission rates calculated with different wavelength bands.

Global CO2 emission from volcanic lakes

The global CO 2 discharge from subaerial volcanism has been estimated at ~300 Mt yr –1 . However, estimates of CO 2 emissions from volcanic lakes have not been considered. In order to improve this information, extensive research on CO 2 emissions of volcanic lakes worldwide has been performed. The observed normalized average CO 2 emission rates increase from alkaline (5.5 t km –2 d –1 ), to neutral (201.2 t km –2 d –1 ), to acid (614.2 t km –2 d –1 ) in volcanic lakes. Taking into account (1) normalized CO 2 emission rates, (2) the number of volcanic lakes in the world (~769), and (3) the fraction and average areas of the investigated alkaline, neutral, and acid volcanic lakes, the estimated global CO 2 emission from volcanic lakes is 117 ± 19 Mt yr –1 , with 94 ± 17 Mt yr –1 as deep-seated CO 2 . This study highlights the importance of a revision of the actual global CO 2 discharge from subaerial volcanism.

Remote detection of fumarolic gas chemistry at Vulcano, Italy, using an FT-IR spectral radiometer

Abstract An infrared absorption spectroscopy remote sensing technique was used to determine the SO 2 /HCl ratio in fumarolic plumes at Vulcano, Italy. The measurements were made from the southern crater rim of Fossa Grande Crater, about 400 m from the fumarolic area in the crater. Infrared absorption spectra of HCl and SO 2 were observed for four fumaroles a few tens of metres apart using the hot fumarolic surface as an infrared light source. The measured SO 2 /HCl ratios in the FA, F47, FW and lower parti of the F21 fumaroles were 4.5–5.4, 3.5, 9.5–11.2 and 5.8 respectively. The SO 2 /HCl ratio of the FA fumarole was higher than that of the gas collected directly in the fumarolic vent (SO 2 /HCl ratio = 2.9), and was closer to the S (total) /HCl ratio (= 4.6) of the collected gas. Our results show that the SO 2 /HCl ratios of two fumaroles only a few tens of metres apart exhibits differences of about twofold. This suggests that this remote monitoring technique is capable of detecting spatial distribution in the SO 2 /HCl ratios of volcanic plumes. Because temporal variations in S/Cl ratios can provide precursory signals for volcanic eruptions [1–3], this remote sensing technique can used efficiently for evaluation of volcanic activity.

An in situ method for measuring CO2 flux from volcanic-hydrothermal fumaroles

Abstract The theory of an in situ method to measure CO 2 flux from hydrothermal fumarolic vents using a tracer gas technique is established and applied to vents and wells at three sites: Izu-Oshima (Japan), Kirishima (Japan) and Teide (Canary Islands, Spain) volcanoes. The method is done by releasing the tracer gas into the vent at known flux and by measuring CO 2 to the tracer gas ratio in situ. In Izu-Oshima, we carried out flux measurements at a low-temperature steam well (90°C) with three different tracer gas species (H 2 , CH 4 and SF 6 ) and three methods (GC, portable analyzers, FTIR) to evaluate this method. The calculated CO 2 flux showed an average of 2.3 t/day. In the case of Kirishima volcanic area, the flux measurement was tested at two low-temperature fumaroles on Iwoyama cone ( 2 fluxes were 9×10 −2 and 2×10 0 kg/day, respectively. The total CO 2 flux from the cone including both fumarolic and ground CO 2 discharges was estimated to be less than several tons per day. In Teide volcano, CO 2 flux measurements were performed in the fumarolic system of the summit crater (85°C). CO 2 flux from one fumarole reached levels up to 30.2 kg/day. We estimated that the CO 2 discharge from all fumaroles is roughly 30 t/day or less at maximum. These results imply that the total output of CO 2 flux from Teides fumarolic activity is much lower than the observed levels of CO 2 diffuse degassing from the summit crater.

High SiF4/HF ratio detected in Satsuma-Iwojima volcano's plume by remote FT-IR observation

Remote FT-IR measurements of volcanic plume of Mt. Iwodake, Satsuma-Iwojima volcano, Japan, were carried out in October 1996 to understand plume chemistry of the volcano, especially characteristics of fluorine-bearing species in the plume. The SO2/HCl molar ratio in the plume was about 4, that is larger than the ratio in high-temperature gases. The high content of SO2 is suggested to be caused by sulfur combustion in the crater. An average SiF4/HF molar ratio of 0.57 was observed for the plume, which is about one order of magnitude higher than the previously reported ratio. The result shows that SiF4 is an important species and has similar to even larger contribution for fluorine output from Satsuma-Iwojima volcano than HF. The SiF4 flux of Satsuma-Iwojima volcano is about 13 t/d, that is the largest SiF4 flux from volcanoes in the world. The observed SiF4/HF ratio cannot be explained only by the high-temperature fumarolic composition of the volcano according to the thermodynamic calculations. The ratio can be explained if contribution of F-rich low-temperature fumaroles to the total gas flux of the volcano is as high as 40%.

Carbon isotope systematics of CO2, CO and CH4 in fumarolic gases from Satsuma-Iwojima volcanic island, Japan

Carbon isotopic composition (δ13C) of CO2, CO and CH4 were determined for fumarolic gases from Satsuma-Iwojima, to range from −3.1 to −2.6‰, from −11.6 to −2.8‰ and from −56 to −37‰, respectively. Carbon isotope exchange equilibrium temperature between CO2 and CO was calculated to be ranging from 896° to 950°C for high temperature (691° to 882°C) fumarolic gases, suggesting that CO2 and CO were in isotopic equilibrium near the vent temperature. However, low temperature (<650°C) gases were not in isotopic equilibrium between CO2 and CO under any realistic conditions. The correlations of δ13C(CO), CO/CO2 ratio and temperature can be explained in terms of kinetic conversion of CO to CO2 with decreasing temperature. The δ13C values of CH4 and CO2 revealed that the CH4 was not equilibrated in carbon isotope exchange reaction with CO2 even at fumarolic gas temperature near 900°C. Correlation of δ13C(CH4) and CH4/C2H6 ratio indicate that CH4 of the low CH4/CO2 ratio group (<10−5) is mainly originated from thermogenic decomposition of organic matter, while the high CH4/CO2 ratio group gas (7.7 × 10−4 to 7.4 × 10−3) is contaminated by the CH4-rich gas with δ13C value of about −40‰.

Pressure gradient measurements in volcanic diffuse gas emanations

We describe a differential pressure measurement device which allows one to estimate fluid pressure gradients ranging from 6.3 × 10−6 to 3.15 × 10−3 atm/m in soil diffuse volcanic gas emanations. The lower limit roughly corresponds to the atmospheric (static) pressure gradient due to gravity, and should represent the minimum required value for volcanic gases to reach the Earths surface by viscous flow. In the study of diffuse gas emanations, such as for the purpose of volcanic surveillance, such physical data supplements the geochemical monitoring of concentrations and fluxes of specific substances constituting the flowing gaseous mixture and may be used to infer volcanic mechanical states. We describe laboratory experiments with the device and report on a successful preliminary test with diffuse gas emanations on the Miharayama cone of the Izu-Oshima volcano (Japan), where the highest fluid pressure gradients have been found to be associated with the highest CO2 fluxes and lowest CO2 concentration gradients.