4:00 PM - 4:15 PM
[SVC35-08] Investigation of soil diffuse hydrogen sulfide flux measurement methods near fumarolic area.
Keywords:volcanic gas, soil difuse degssing, Hydrogen Sulfide
Volcanic gas emissions are generally considered to be dominated by plumes and fumaroles. However, the importance of diffuse emissions from volcanic soils has recently attracted attention. In particular, it has been revealed that the amount of carbon dioxide emitted by diffuse degassing is comparable to that emitted from plumes and fumaroles (Burton et al., 2013), through intensive surveys over the past 30 years. While carbon dioxide is the main target of soil diffuse emission surveys, hydrogen sulfide is also emitted near fumarolic areas. In this study, methods for measuring the soil diffuse degassing rate of hydrogen sulfide near fumarolic areas were re-investigated.
The accumulation chamber method (Parkinson, 1981; Chiodini et al., 1993) is widely used to measure carbon dioxide soil diffuse flux in volcanoes. In this method, a tub-shaped chamber is placed over the soil, and the soil diffuse emission is calculated from the rate of increase in the carbon dioxide concentration inside the chamber. The carbon dioxide concentration meter used in the chamber method is required to have a fast response (90% response in a few seconds), and if a carbon dioxide meter with a slow response is used, the diffuse flux may be underestimated. Therefore, an NDIR type carbon dioxide meter, in which gas is introduced into the gas cell from the outside by a pump, is often used for measuring carbon dioxide soil diffuse emission. On the other hand, a calculation method considering response characteristic of a CO2 sensor has been proposed to utilize CO2 sensors having a slow response (Mizoguchi and Ohtani, 2005). A general hydrogen sulfide sensor is a potentiostatic electrolytic type and takes 20 to 30 seconds for 90% response. To accurately measure the soil diffuse emission of hydrogen sulfide with such a sensor, it is necessary to perform processing that takes into account the response characteristics (Mizoguchi and Ohtani, 2005). However, in diffuse flux measurements in volcanic fumarolic areas, not only a high emission rate but also measurement in a short time is required, and such processing may be difficult in some cases. In this study, we investigated a method in which carbon dioxide and hydrogen sulfide are measured simultaneously. the carbon dioxide diffuse emission amount is obtained with a fast-response carbon dioxide meter, an accurate CO2/H2S ratio is obtained from the outputs of the carbon dioxide meter and the hydrogen sulfide meter, and this value is used in combination to obtain the hydrogen sulfide soil diffuse flux.
In soil diffuse emission measurements near fumarolic areas, hydrogen sulfide concentrations exceeding several hundred ppm may be observed. Therefore, a hydrogen sulfide sensor (EC4-1000-H2S, SGX Sensortech) capable of measuring up to 1000 ppm was embedded into a CO2 fluxmeter (Westsystems, CO2 sensor is LICOR LI-820). In order to obtain an accurate CO2/H2S ratio even in a short time measurement, analysis was performed taking into account the response characteristics of the hydrogen sulfide sensor. Based on the CO2 flux and CO2/H2S ratio obtained by the conventional chamber method, the hydrogen sulfide soil diffuse flux amount was calculated. In a test observation conducted near the Sesshogawara fumarolic area of Kusatsu-Shirane volcano, a value 0 to 40% larger than the hydrogen sulfide flux value obtained by the conventional method, which is considered to be underestimated, was obtained. In the presentation, we will summarize the hydrogen sulfide soil diffuse flux measurement around the fumarolic area and examine a analytical method suitable for measuring the hydrogen sulfide diffuse emission rate by analyzing the test observation data at Kusatsu-Shirane volcano.
The accumulation chamber method (Parkinson, 1981; Chiodini et al., 1993) is widely used to measure carbon dioxide soil diffuse flux in volcanoes. In this method, a tub-shaped chamber is placed over the soil, and the soil diffuse emission is calculated from the rate of increase in the carbon dioxide concentration inside the chamber. The carbon dioxide concentration meter used in the chamber method is required to have a fast response (90% response in a few seconds), and if a carbon dioxide meter with a slow response is used, the diffuse flux may be underestimated. Therefore, an NDIR type carbon dioxide meter, in which gas is introduced into the gas cell from the outside by a pump, is often used for measuring carbon dioxide soil diffuse emission. On the other hand, a calculation method considering response characteristic of a CO2 sensor has been proposed to utilize CO2 sensors having a slow response (Mizoguchi and Ohtani, 2005). A general hydrogen sulfide sensor is a potentiostatic electrolytic type and takes 20 to 30 seconds for 90% response. To accurately measure the soil diffuse emission of hydrogen sulfide with such a sensor, it is necessary to perform processing that takes into account the response characteristics (Mizoguchi and Ohtani, 2005). However, in diffuse flux measurements in volcanic fumarolic areas, not only a high emission rate but also measurement in a short time is required, and such processing may be difficult in some cases. In this study, we investigated a method in which carbon dioxide and hydrogen sulfide are measured simultaneously. the carbon dioxide diffuse emission amount is obtained with a fast-response carbon dioxide meter, an accurate CO2/H2S ratio is obtained from the outputs of the carbon dioxide meter and the hydrogen sulfide meter, and this value is used in combination to obtain the hydrogen sulfide soil diffuse flux.
In soil diffuse emission measurements near fumarolic areas, hydrogen sulfide concentrations exceeding several hundred ppm may be observed. Therefore, a hydrogen sulfide sensor (EC4-1000-H2S, SGX Sensortech) capable of measuring up to 1000 ppm was embedded into a CO2 fluxmeter (Westsystems, CO2 sensor is LICOR LI-820). In order to obtain an accurate CO2/H2S ratio even in a short time measurement, analysis was performed taking into account the response characteristics of the hydrogen sulfide sensor. Based on the CO2 flux and CO2/H2S ratio obtained by the conventional chamber method, the hydrogen sulfide soil diffuse flux amount was calculated. In a test observation conducted near the Sesshogawara fumarolic area of Kusatsu-Shirane volcano, a value 0 to 40% larger than the hydrogen sulfide flux value obtained by the conventional method, which is considered to be underestimated, was obtained. In the presentation, we will summarize the hydrogen sulfide soil diffuse flux measurement around the fumarolic area and examine a analytical method suitable for measuring the hydrogen sulfide diffuse emission rate by analyzing the test observation data at Kusatsu-Shirane volcano.