Study on the regional distribution of tropospheric methane using a rapidly and continuously measuring instrument

1991 Fiscal Year Final Research Report Summary

• Project/Area Number: 02402014
• Research Category: Grant-in-Aid for General Scientific Research (A)
• Allocation Type: Single-year Grants
• Research Field: 気象・海洋・陸水学
• Research Institution: University of Tokyo
• Principal Investigator: OGAWA Toshihiro University of Tokyo, Fac.of Science, Professor, 理学部, 教授 (70011616)
• Co-Investigator(Kenkyū-buntansha): KITA Kazuyuki University of Tokyo, Fac.of Science, assistant professor, 理学部, 助手 (30221914) ; IWAGAMI Naomoto University of Tokyo, Fac.of Science, assoc.professor, 理学部, 助教授 (30143374)
• Project Period (FY): 1990 – 1991
• Keywords: greenhouse effect / methane / regional distribution / emission sourse

Research Abstract

In 1990 modifying the methane instrument that we had developed, we designed and fabricated a new model to be adapted to a moving platform and made laboratory tests on its performance. A field test observation was made on the research vessel Shirase cruising from Tokyo to the Antarctica with respect to its performance aboard a vibrating and moving platform and the disturbance of atmospheric water vapor to the measurement. The observation exhibited a precision of about 5% with a measurement time constant of a few minutes when a statistical data processing was applied to remove periodic oscillations in the data due to the vessel engine. The processed data depicted the latitudinal distribution of tropospheric methane. In 1991 we made several performance tests of the instrument, to find that it was the stability of the laser that was limiting measurement precision. The temperature fluctuation was thought to be a major source to degrade the stability. We improved the instrument to keep tempe rature constant and spatially uniform around the laser oscillator tube.
After the end of the projected research period, in 1992 and 1993 we made further improvement of the instrument. A significant improvement was attained by adding a feedback roop to stabilize the laser output, in which the distance between the laser resonator mirrors was adjusted with a piezoelectric element. The improved instrument exhibited a precision of 1% with a measurement time constant of a few minutes, and was used in the observations on board an automobile and an airship with respect to the spatial distributions of atmospheric methane concentration over and around rice paddies in Ibaraki-ken and over the Tokyo metropolitan area. Applying a diffusive transport model to the methane concentration distributions observed around rice paddies, we were able to estimate the emission source strength of methane from rice paddies. The estimated value exhibited a reasonable agreement with those deduced from the chamber method and the gradient method, implying a possibility of estimating the methane emission flux in a horizontal scale of a few 100 m to a few km with the aid of the diffusive transport model. The airship observation over the Tokyo metropolitan area revealed an area of large methane mixing ratio exceeding 2 ppmv around Urayasu, suggesting a strong emission source of methane.