The nitrogen stable isotope ratio of NOx (δ(15)N-NOx) has been proposed as a regional indicator for NOx source partitioning; however, knowledge of δ(15)N values from various NOx emission sources is limited. This study presents a detailed analysis of δ(15)N-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 δ(15)N-NOx was analyzed. The vehicles sampled in this study emitted δ(15)N-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 δ(15)N-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 δ(15)N-NOx values emitted from vehicles.

10 The oxygen stable isotope composition (δ 18 O) of nitrogen oxides [NO x = nitric oxide 11 (NO) + nitrogen dioxide (NO 2 )] and their oxidation products (NO y = NO x + nitric acid (HNO 3 ) + 12 particulate nitrate (p-NO 3 -) + nitrate radical (NO 3 ) + dinitrogen pentoxide (N 2 O 5 ) + nitrous acid 13 (HONO) + …) have been shown to be a useful tool for inferring the proportion of NO x that is 14 oxidized by ozone (O 3 ). However, isotopic fractionation processes may have an influence on 15 δ 18 O of various NO y molecules and other atmospheric O-bearing molecules pertinent to NO x 16 oxidation chemistry. Here we have evaluated the impacts of O isotopic exchange involving NO y 17 molecules, the hydroxyl radical (•OH), and water (H 2 O) using reduced partition function ratios 18 ( x β) calculated by hybrid density functional theory. Assuming atmospheric isotopic equilibrium 19 is achieved between NO and NO 2 during the daytime, and NO 2 , NO 3 , and N 2 O 5 during the 20 © 2016. This manuscript version is made available under the Elsevier user license http://www.elsevier.com/open-access/userlicense/1.0/ 2 nighttime, δ 18 O-δ 15 N compositions were predicted for the major atmospheric nitrate formation 21 pathways using our calculated exchange fractionation factors and isotopic mass-balance. Our 22 equilibrium model predicts that various atmospheric nitrate formation pathways, including NO 2 23 + •OH → HNO 3 , N 2 O 5 + H 2 O + surface → 2HNO 3 , and NO 3 + R → HNO 3 + R• will yield 24 distinctive δ 18 O-δ 15 N compositions. Our calculated δ 18 O-δ 15 N compositions match well with 25 previous atmospheric nitrate measurements, and will potentially help better understand the role 26 oxidation chemistry plays on the N and O isotopic composition of atmospheric nitrate.

The nitrogen stable isotope composition of NOx (δ(15)N-NOx) may be a useful indicator for NOx source partitioning, which would help constrain NOx source contributions in nitrogen deposition studies. However, there is large uncertainty in the δ(15)N-NOx values for anthropogenic sources other than on-road vehicles and coal-fired energy generating units. To this end, this study presents a broad analysis of δ(15)N-NOx from several fossil-fuel combustion sources that includes: airplanes, gasoline-powered vehicles not equipped with a three-way catalytic converter, lawn equipment, utility vehicles, urban buses, semitrucks, residential gas furnaces, and natural-gas-fired power plants. A relatively large range of δ(15)N-NOx values was measured from -28.1‰ to 8.5‰ for individual exhaust/flue samples that generally tended to be negative due to the kinetic isotope effect associated with thermal NOx production. A negative correlation between NOx concentrations and δ(15)N-NOx for fossil-fuel combustion sources equipped with selective catalytic reducers was observed, suggesting that the catalytic reduction of NOx increases δ(15)N-NOx values relative to the NOx produced through fossil-fuel combustion processes. Combining the δ(15)N-NOx measured in this study with previous published values, a δ(15)N-NOx regional and seasonal isoscape was constructed for the contiguous U.S., which demonstrates seasonal and regional importance of various NOx sources.

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Nitrogen (N) equilibrium isotope fractionation ( 15 α) involving gaseous, dissolved, and solid phases of ammonia (NH 3 ) and ammonium (NH 4 + ) (e.g., NH 3(g) −NH 3(aq) −NH 4 + (aq) −NH 4 + (s) ) represents a fundamental chemical process that has important implications for understanding the environmental dynamics involving NH x (NH 3 + NH 4 + ). However, recent literature disagrees with early experimental results from Urey and co-workers, suggesting the need for an update on theoretical estimates. Here, we have calculated theoretical 15 α values for NH 4 + (g) / NH 3(g) , NH 3(aq) /NH 3(g) , NH 4 + (aq) /NH 3(g) , NH 4 + (aq) /NH 3(aq) , and NH 4 + (s) / NH 3(g) using HF/6-31G(d) and B3LYP/6-31G(d) levels of theory. Overall, our theoretical calculated values matched experimental data reported by Urey and co-workers, with best agreement obtained at the HF/ 6-31G(d) level of theory with solvent effect accounted for using water cluster calculations. Our calculated results have important implications for tracing NH 3 gas-to-particle phase conversions that may have distinctive isotopic separation factors (Δ 15 δ NH4+/NH3 = δ 15 N(NH 4 + ) − δ 15 N(NH 3 )) between N isotopic compositions (δ 15 N) of NH 4+ and NH 3 depending on its conversion mechanism. While further experimental work is necessary to validate our predicted isotope effects over the considered temperature range, this work demonstrates the potential of N isotopic measurements of phase-resolved NH x to better understand its dynamics in the atmosphere.