Automobile exhaust as a source of 13C- and D-enriched atmospheric methane in urban areas

The stable carbon and hydrogen isotopic compositions (δ13C and δD, respectively) of methane (CH4) in automobile exhaust were determined in order to quantify by mass balance the contribution from this source to atmospheric CH4 in an urban area of Nagoya, Japan. Both δ13C and δD values of CH4 in automobile exhaust increased in accordance with the model year of the engine, probably as a result of isotope fractionation effects associated with oxidation over metal catalysts in the catalytic converter of modern vehicles. Thus, CH4 emissions from recent automobiles exhibit conspicuous 13C and D enrichment compared to the flux from other major anthropogenic sources, such as natural gas leakage, landfills and rice paddies. Using average δ13C and δD CH4 values estimated for local sources, automobile exhaust in Nagoya, Japan, was determined to contribute significant amounts (up to 30%) of CH4 to the troposphere in the study area.