Air samples were collected during the test and southbound flight (from Tokyo, Japan to Biak, Indonesia) of International Strato/Tropospheric Air Chemistry I in March 1989 (INSTAC-I). The air samples collected at 4 - 5 km height were subjected to CH
4 and CO
2 measurement at first, and then CO
2 was extracted to measure the carbon isotopic ratio. The concentration of CO
2 (ppm, molefraction in dry air) was high over and near the Japan Islands, and shows a little hump around 13°N. The relationship between carbon isotopic ratio and concentration of atmospheric CO
2 shows that the high concentration over and near the Japan Islands and a hump around 13°N are caused by the CO
2 released by the fossil fuel consumption and/or land plants respiration.
The concentration of CH
4 (ppm, mole fraction in dry air) was linearly proportional to that of CO
2, which suggests a fairly constant ratio between CH
4 flux and CO
2 flux south of 35°N.
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