Vehicle NO x emission plume isotopic signatures: Spatial variability across the eastern United States

• Published in: Journal of geophysical research. Atmospheres 2017-04, Vol.122 (8), p.4698-4717
• Main Authors: Miller, David J., Wojtal, Paul K., Clark, Sydney C., Hastings, Meredith G.
• Format: Article
• Language: English
• ISSN: 2169-897X; 2169-8996
• DOI: 10.1002/2016JD025877

On‐road vehicle nitrogen oxide (NO x ) sources currently dominate the U.S. anthropogenic emission budgets, yet vehicle NO x emissions have uncertain contributions to oxidized nitrogen (N) deposition patterns. Isotopic signatures serve as a potentially valuable observational tool to trace source contributions to NO x chemistry and N deposition, yet in situ emission signatures are underconstrained. We characterize the spatiotemporal variability of vehicle NO x emission isotopic signatures (δ 15 N‐NO x ) 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 NO x in solution to quantify δ 15 N‐NO x at hourly resolution. On‐road δ 15 N‐NO x upwind of Providence, RI, ranged from −7 to −3‰. Simultaneous urban background δ 15 N‐NO x observations showed comparable range and variations with on‐road measurements, suggesting that vehicles dominate NO x emissions in the Providence area. On‐road spatial δ 15 N‐NO x 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 δ 15 N‐NO x and NO x emission factor extremes associated with heavy diesel emitter contributions. Overall, these results constrain an isotopic signature of fleet‐integrated roadway NO x emission plumes, which have important implications for distinguishing vehicle NO x from other sources and tracking emission contributions to NO x chemistry and N deposition. New approach constrains hourly NO x isotopic signatures (δ 15 N‐NO x ) from on‐road, fleet‐integrated vehicle plumes to −4.7 ± 1.7‰ Urban background δ 15 N‐NO x observations in Providence, RI, suggest that vehicles are the dominant regional source Spatial δ 15 N‐NO x gradients correlate with high diesel NO x emission contributions