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

The nitrogen stable isotope ratio of NOx (δ15N-NOx) has been proposed as a regional indicator for NOx source partitioning; however, knowledge of δ15N values from various NOx emission sources is limited. This study presents a detailed analysis of δ15N-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 δ15N-NOx was analyzed. The vehicles sampled in this study emitted δ15N-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 δ15N-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 δ15N-NOx values emitted from vehicles. [ABSTRACT FROM AUTHOR]