Controls on concentrations and clumped isotopologues of vehicle exhaust methane

Methane emissions from vehicle exhaust, as a source of methane, are often overlooked. However, in areas with high vehicle activity, the emissions can be substantial. There is a notable lack of characterization regarding the variable concentrations and isotopic signatures of methane in vehicle exhaust. This gap in knowledge limits our understanding of the mechanisms of methane production in vehicles and the factors controlling concentration variations and isotopic fractionation, which also makes it difficult to identify and reduce methane emissions from vehicle exhaust. This study characterized the methane concentration ([CH4]), methane-to-ethane ratio (C2:C1), methane carbon and hydrogen isotopes (δ13C and δD), and methane clumped isotopologues (Δ13CH3D and Δ12CH2D2) of the vehicle exhaust methane endmember. [CH4] varied widely from below 1 ppm to more than 3000 ppm, potentially influenced by vehicle maintenance and operational phases. Ethane concentrations ([C2H6]) correlated with [CH4], yet C2:C1 varied significantly from 0.1% to 18.3%. The δ13C and δD values of exhaust methane were less negative than those of natural gas. A large portion of samples showed a positive linear relationship between [CH4], δ13C from -22‰ to -11‰, and δD values from -170‰ to -120‰, while their clumped isotopologues exhibit ~0.8‰ clumping in Δ13CH3D and ~-2.4‰ anti-clumping in Δ12CH2D2. A small portion of the samples exhibited distinct isotopic characteristics, with their δ13C and δD values either becoming significantly more positive or aligning closer to the composition of ambient air, while their Δ12CH2D2 values showed a marked increase, reaching between +25‰ to +33‰. These concentration and isotope characteristics show trends that can be explained by a combination of processes, including 1) methane formation in the engine, 2) methane combustion in the engine, 3) methane oxidation by the catalytic converter, and 4) mixing with air. The observed isotopic fractionation can be explained by thermo equilibrium and Rayleigh fractionations. These processes, elucidated through isotopic and clumped isotopologue analyses, underscore the intricate dynamics and controls of vehicular methane emissions.