High precision measurements of carbon isotopic ratio of atmospheric methane using a continuous flow mass spectrometer

A high-precision measurement system for the carbon isotope ratio of atmospheric CH ₄ (δ ¹³CH ₄) was developed using a pre-concentration device for CH ₄, and a gas chromatograph-combustion-isotope ratio mass spectrometer (GC-C-IRMS). The measurement system required 100 m/ _STP of an atmospheric air sample, corresponding to approximately 0.18 μ l _STp of CH ₄ to determine the δ ¹³CH ₄ value with a reproducibility of 0.07%o. Replicated analyses of a CH ₄-in-air standard gas during the period from 2002 to 2008 indicated that the value of δ ^BCH ₄ measured by this system was consistent within the measurement reproducibility. To evaluate the δ ¹³CH ₄ measurement system, thus developed, diurnal variations of the atmospheric CH ₄ concentration and δ ¹³CH ₄ were observed in the northern part of the Tokyo metropolitan area. From the relationship between the CH ₄concentration and δ ¹³CH ₄ dominant sources of the observed CH ₄ fluctuations were identified.