Carbon isotopic analysis of atmospheric methane in urban and suburban areas: Fossil and non-fossil methane from local sources

The carbon isotopic composition (12C, 13C and 14C) of atmospheric CH4 was studied to estimate CH4 sources in local areas. In urban areas of Nagoya, Japan (35°10′N, 136°55′E), where there is much anthropogenic influence, evidence of methane release into the atmosphere has been reported. Variations in the carbon isotopic composition were analyzed with diurnal fluctuation of atmospheric CH4 concentration, since the diurnal increase in the CH4 concentration is attributed mainly to release from local sources. In November and December 1993, the contribution of fossil CH4 to local CH4 released from the urban area was calculated to be 102±8%, and its δ13C was −40.8±3.0‰. Leakage of supplied natural gas and exhaust gas from automobiles would be important CH4 sources in the urban area in late autumn to winter. In suburban areas in Japan there are many rice paddy fields, and these are one kind of major anthropogenic source of atmospheric CH4. In a suburban area of Nagoya fossil, CH4 contributed to less than 10% of local release in July and August 1994. The calculated value of δ13C for non-fossil CH4 was approximately −65‰, which is within the range of reported values of δ13C for CH4 derived from bacterial CH4 sources such as irrigated rice paddies.