講演情報
[AAS08-P10]Field measurements of methane flux using a portable OA-ICOS instrument in the Minami Kanto gas field
*村上 達希1、服部 将佳1、長野 馨子1、朱 徐達1、趙 天鴻1、蔡 添禹1、下茂 道人2、戸野倉 賢一1 (1.東京大学、2.公益財団法人深田地質研究所)
キーワード：
メタンフラックス、フィールド計測、可搬型吸収分光装置、チャンバー計測、OA-ICOS、地球温暖化
Methane is well known to be the second most potent greenhouse gas after carbon dioxide, and its quantitative evaluation is important for assessments of global warming. Natural methane emissions are reported to be comparable to anthropogenic emissions, which highlights the need for field measurements to quantitative evaluations of natural methane emissions. Source identification and quantitative evaluation of methane emissions remain insufficient, and uncertainties in natural emissions are still high.
A major natural source of methane emissions is natural gas seepage. Some methane seepage points have been reported in the Minami-Kanto gas field, which boasts the largest natural gas reserves in Japan. However, even in this area, uncertainties in natural emission amounts remain high due to the lack of field measurement data.
A portable absorption spectroscopic instrument and chamber measurement technique are widely used for field measurements of methane seepage. Absorption spectroscopic instruments enable both qualitative and quantitative measurements of low concentration target molecules in the atmosphere through light absorption, which depends on the molecular species and concentration. In recent years, highly sensitive measurement techniques, such as off-axis integrated cavity output spectroscopy (OA-ICOS), which utilize a Fabry–Perot cavity composed of two high-reflectivity concave mirrors facing each other, have been developed. OA-ICOS instruments achieve the detection of ultra-trace amounts at the level of several parts per billion. These spectroscopic instruments are now commercially available, so anyone can perform field measurements. The chamber measurement technique involves placing a small chamber above a methane seepage point and continuously measuring the methane concentration with an instrument while it is being collected. The chamber measurement technique allows evaluation of the temporal variation of methane concentration inside the chamber. The methane emission rate per unit area and per unit time is referred to as the methane flux, which is widely used as a quantitative indicator of methane seepages.
In this presentation, we report the result of field measurements of methane flux using the portable OA-ICOS instruments and chamber measurement technique in the Minami Kanto gas field, for the purpose of quantitatively evaluating methane emissions. Makamegawa General Park in Kujukuri town, Mobara area, Chiba prefecture was selected for measurement location. Although methane emissions have been reported in this area, their quantitative evaluation has not been sufficiently conducted.
We successfully measured the temporal variation of methane concentration caused by methane seepages. The temporal variations were well fitted linearly, indicating that the seepages are continuous, stable and high concentration. However, the methane fluxes calculated from the slopes of the temporal variations showed considerable spatial differences. We suggest that the differences are caused by the surrounding environment, such as the surface paving condition.
These results show that portable spectroscopic instruments, such as OA-ICOS and the chamber measurement technique enable the measurements of local methane flux, contributing to the quantitative evaluation of methane emissions and potentially to understanding the mechanisms of methane seepage. A small chamber and measurement instrument are portable, allowing anyone to perform measurements in the field, making them powerful tools for providing quantitative data for research on global warming.
A major natural source of methane emissions is natural gas seepage. Some methane seepage points have been reported in the Minami-Kanto gas field, which boasts the largest natural gas reserves in Japan. However, even in this area, uncertainties in natural emission amounts remain high due to the lack of field measurement data.
A portable absorption spectroscopic instrument and chamber measurement technique are widely used for field measurements of methane seepage. Absorption spectroscopic instruments enable both qualitative and quantitative measurements of low concentration target molecules in the atmosphere through light absorption, which depends on the molecular species and concentration. In recent years, highly sensitive measurement techniques, such as off-axis integrated cavity output spectroscopy (OA-ICOS), which utilize a Fabry–Perot cavity composed of two high-reflectivity concave mirrors facing each other, have been developed. OA-ICOS instruments achieve the detection of ultra-trace amounts at the level of several parts per billion. These spectroscopic instruments are now commercially available, so anyone can perform field measurements. The chamber measurement technique involves placing a small chamber above a methane seepage point and continuously measuring the methane concentration with an instrument while it is being collected. The chamber measurement technique allows evaluation of the temporal variation of methane concentration inside the chamber. The methane emission rate per unit area and per unit time is referred to as the methane flux, which is widely used as a quantitative indicator of methane seepages.
In this presentation, we report the result of field measurements of methane flux using the portable OA-ICOS instruments and chamber measurement technique in the Minami Kanto gas field, for the purpose of quantitatively evaluating methane emissions. Makamegawa General Park in Kujukuri town, Mobara area, Chiba prefecture was selected for measurement location. Although methane emissions have been reported in this area, their quantitative evaluation has not been sufficiently conducted.
We successfully measured the temporal variation of methane concentration caused by methane seepages. The temporal variations were well fitted linearly, indicating that the seepages are continuous, stable and high concentration. However, the methane fluxes calculated from the slopes of the temporal variations showed considerable spatial differences. We suggest that the differences are caused by the surrounding environment, such as the surface paving condition.
These results show that portable spectroscopic instruments, such as OA-ICOS and the chamber measurement technique enable the measurements of local methane flux, contributing to the quantitative evaluation of methane emissions and potentially to understanding the mechanisms of methane seepage. A small chamber and measurement instrument are portable, allowing anyone to perform measurements in the field, making them powerful tools for providing quantitative data for research on global warming.