Carbon isotope of atmospheric methane and its application to the clobal methane cycle

为了更好地了解地球表面甲烷的全球循环，大气甲烷的碳同位素比值数据是必不可少的。在本研究中，我们制作了一台专用仪器，用于精确分析大气甲烷中的碳同位素比值，并采集了少量的空气样品。在750ﾟC下，将空气样品中的甲烷转化为二氧化碳，与负载在氧化铝颗粒上的0.5%铂催化剂接触，测定甲烷中的碳同位素比率。结果表明，甲烷转化为二氧化碳的效率为99.9%，分析的总精度为0.07%。利用该仪器和质谱仪对俄罗斯对流层采集的空气样品进行了甲烷同位素比值的分析。这是首次尝试测量欧亚大陆北部大气甲烷的碳同位素比值。结果表明，利用自然湿地和化石燃料甲烷的碳同位素比值数据可以分别估算甲烷的含量。我们还分析了日本上空的对流层和平流层空气样本。通过测量发现，在对流层中碳同位素比值相当恒定，并且在平流层中随着高度的增加而增加。一维光化学扩散模型的模拟表明，甲烷与氯原子的反应对于测得的碳同位素比的垂直分布特别重要。我们还建立了甲烷浓度及其碳同位素率的二维输运模型，然后根据这两个组分的观测值估计全球甲烷源和汇。

For a better understanding of the global cycle of methane on the earth's surface, the data of carbon isotopic ratio in atmospheric methane are indispensable. In this study, we made a special instrument for precise analyzes of the carbon isotopic ratio in atmospheric methane with a small amount of air sample. Methane contained in the air samples was converted to carbon dioxide for determining the carbon isotopic rati in methane, by contacting it with 0.5% platinum catalyst supported on alumina pellets at 750ﾟC.It was found that the conversion efficiency of methane to carbon dioxide is 99.9% and an overall precision of our analyzes is 0.07%.Using this instrument and a mass spectrometer, we analyzed the isotopic ratio in methane of air samples collected in the troposhere over Russia. This is the first attempt of measuring the carbon isotopic ratio atmospheric methane in the northern Eurasian Continent. The results showed that methane from natural wetlands and fossil fuel can be estimated separately using its carbon isotopic ratio data. We also analyzed the tropospheric and stratospheric air samples over Japan. It was found from this measurement that the carbon isotopic ratio was fairly constant in the troposphere and increased with height in the stratosphere. Simulations with an one-dimensional photochemical-diffusion model suggested that the reaction of methane with chlorine atoms is especially important for the measured vertical profile of the carbon isotopic ratio. We also developed a two-dimensional transport model with the methane concentration and its carbon isotopic rati, and then estimate global mathane sources and sinks from the observed values of both components.