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 sub(4) (d super(13)CH sub(4)) was developed using a pre-concentration device for CH sub(4) and a gas chromatograph-combustion-isotope ratio mass spectrometer (GC-C-IRMS). The measurement system required 100 ml sub(STP) of an atmospheric air sample, corresponding to approximately 0.18 k l sub(STP) of CH sub(4), to determine the d super(13)CH sub(4) value with a reproducibility of 0.07ppt. Replicated analyses of a CH sub(4)-in-air standard gas during the period from 2002 to 2008 indicated that the value of d super(13)CH sub(4) measured by this system was consistent within the measurement reproducibility. To evaluate the d super(13)CH sub(4) measurement system, thus developed, diurnal variations of the atmospheric CH4 concentration and d super(13)CH sub(4) were observed in the northern part of the Tokyo metropolitan area. From the relationship between the CH sub(4) concentration and d super(13)CH sub(4), dominant sources of the observed CH sub(4) fluctuations were identified.