Vehicle NOx emission plume isotopic signatures: Spatial variability across the eastern United States

On-road vehicle nitrogen oxide (NOx) sources currently dominate the U.S. anthropogenic emission budgets, yet vehicle NOx emissions have uncertain contributions to oxidized nitrogen (N) deposition patterns. Isotopic signatures serve as a potentially valuable observational tool to trace source contributions to NOx chemistry and N deposition, yet in situ emission signatures are underconstrained. We characterize the spatiotemporal variability of vehicle NOx emission isotopic signatures (N-15-NOx) representative of U.S. vehicle fleet-integrated emission plumes. A novel combination of on-road mobile and stationary urban measurements is performed using a field and laboratory-verified technique for actively capturing NOx in solution to quantify N-15-NOx at hourly resolution. On-road N-15-NOx upwind of Providence, RI, ranged from -7 to -3. Simultaneous urban background N-15-NOx observations showed comparable range and variations with on-road measurements, suggesting that vehicles dominate NOx emissions in the Providence area. On-road spatial N-15-NOx variations of -9 to -2 were observed under various driving conditions in six urban metropolitan areas and rural interstate highways during summer and autumn in the U.S. Northeast and Midwest. Although isotopic signatures were insensitive to on-road driving mode variations, statistically significant correlations were found between N-15-NOx and NOx emission factor extremes associated with heavy diesel emitter contributions. Overall, these results constrain an isotopic signature of fleet-integrated roadway NOx emission plumes, which have important implications for distinguishing vehicle NOx from other sources and tracking emission contributions to NOx chemistry and N deposition.