An improved method for measurement of the hydrogen isotope ratio of atmospheric methane and its application to a Japanese urban atmosphere

Abstract

Measurement of the hydrogen isotope ratio (D/H ratio) in atmospheric methane by isotope ratio mass spectrometry has been less developed so far, compared to that of the carbon isotope ratio ( 13C/ 12C ratio). This is because of the requirement for a large sample size (about 100 l of atmosphere, assuming a methane concentration of ∼1.8 ppm) and the complicated and time-consuming method of sample preparation. In this study, we examined an on-line method for measurement of the D/H ratio of atmospheric methane by a gas chromatography/high-temperature conversion/isotope ratio mass spectrometry (GC/TC/IRMS) technique. This method is less laborious, more rapid (about 1 h per sample) and attains high precision (±3.1‰) using a much smaller sample (∼120–360 ml of atmosphere). Its application to isotopic characterization, and hence identification of source of methane and estimation of methane budgets, was demonstrated by examination of urban atmosphere samples collected in November 2001 at Yokohama, Japan. The D/H ratio of atmospheric methane in the urban area ranged throughout the day from −98‰ to −118‰, showing a fluctuation with time that correlated with that of the methane concentration. Assuming that the fluctuation was caused by local methane emissions from anthropogenic sources, i.e., vehicles, industries, and landfill sites, the contribution from each source to the local methane was estimated by a combination of the D/H ratio with the 13C/ 12C ratio of atmospheric methane. The estimation indicates that the contribution from each source varied considerably throughout the day and that the methane emitted from landfill sites averaged 70%.