Nitrogen Stable Isotope Composition (δ15N) of Vehicle-Emitted NOx

The nitrogen stable isotope ratio of NOx (delta N-15-NOx) has been proposed as a regional indicator for NOx source partitioning; however, knowledge of delta N-15 values from various NOx emission sources is limited. This study presents a detailed analysis of delta N-15-NOx emitted from vehicle exhaust, the largest source of anthropogenic NOx. To accomplish this, NOx was collected from 26 different vehicles, including gasoline and diesel-powered engines, using a modification of a NOx collection method used by the United States Environmental Protection Agency, and delta N-15-NOx was analyzed. The vehicles sampled in this study emitted delta N-15-NOx values ranging from -19.1 to 9.8 that negatively correlated with the emitted NOx concentrations (8.5 to 286 ppm) and vehicle run time because of kinetic isotope fractionation effects associated with the catalytic reduction of NOx. A model for determining the mass-weighted delta N-15-NOx from vehicle exhaust was constructed on the basis of average commute times, and the model estimates an average value of -2.5 +/- 1.5, with slight regional variations. As technology improvements in catalytic converters reduce cold-start emissions in the future, it is likely to increase current delta N-15-NOx values emitted from vehicles.