Spatio-temporal variability in isotopic signatures of atmospheric NOx emissions from vehicles

Abstract:

Atmospheric nitrogen oxides (NO_x = NO + NO₂) play key roles in atmospheric chemistry and radiative forcing. Their oxidation products, nitric acid or nitrate, have significant contributions to nitrogen (N) deposition, with implications for ecosystem health. On-road vehicle NO_x sources currently dominate U.S. anthropogenic emission budgets, yet vehicle NO_x emissions contributions to local and regional N deposition patterns are highly uncertain. NO_x isotopic signatures offer a potentially valuable observational tool to trace source contributions to N deposition. We characterize the spatio-temporal variability of vehicle NO_x emission isotopic signatures with a field and laboratory-verified technique for actively capturing NO_x in solution to quantify the nitrogen isotopic composition (δ¹⁵N-NO_x) to within ±1.5‰ (1σ) precision. We present a novel combination of on-road mobile and stationary urban δ¹⁵N-NO_x measurements at minutes to hourly resolution along with NO_x and CO₂ concentration measurements. We evaluate spatial gradients of δ¹⁵N-NO_x on U.S. Northeast and Midwest highways, including six urban metropolitan areas and rural interstate highways during summer and autumn. We also assess on-road diurnal δ¹⁵N-NO_x variations over ~800 km driving distance in Providence, RI by targeting the upwind footprint of urban background measurements to distinguish background and source NO_x. We observe on-road δ¹⁵N-NO_x signatures range from -3 to -10‰ under different traffic conditions in the U.S. Northeast and Midwest. On-road δ¹⁵N-NO_x daytime variations from -3 to -6‰ agree well with simultaneous urban background sampling in Providence, RI, suggesting that vehicles dominate NO_x emissions in this region. We use these datasets to estimate the range of representative δ¹⁵N-NO_x source signatures for U.S. vehicle fleet-integrated emission plumes. Our novel approach suggests that previously reported isotopic signatures for vehicle NO_x are not necessarily representative. These results have implications for evaluating isotopic signature ranges for distinguishing vehicle NO_x sources and tracking NO_x emission contributions to local and regional N deposition.