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.

Atmospheric nitrate (NO3− = particulate NO3− + gas‐phase nitric acid [HNO3]) and sulfate (SO42−) are key molecules that play important roles in numerous atmospheric processes. Here, the seasonal cycles of NO3− and total suspended particulate sulfate (SO42−(TSP)) were evaluated at the South Pole from aerosol samples collected weekly for approximately 10 months (26 January to 25 October) in 2002 and analyzed for their concentration and isotopic compositions. Aerosol NO3− was largely affected by snowpack emissions in which [NO3−] and δ15N(NO3−) were highest (49.3 ± 21.4 ng/m3, n = 8) and lowest (−47.0 ± 11.7‰, n = 5), respectively, during periods of sunlight in the interior of Antarctica. The seasonal cycle of Δ17O(NO3−) reflected tropospheric chemistry year‐round with lower values observed during sunlight periods and higher values observed during dark periods, reflecting shifts from HOx‐ to O3‐dominated oxidation chemistry. SO42−(TSP) concentrations were highest during austral summer and fall (86.7 ± 73.7 ng/m3, n = 18) and are indicated to be derived from dimethyl sulfide (DMS) emissions, as δ34S(SO42−)(TSP) values (18.5 ± 1.0‰, n = 10) were similar to literature δ34S(DMS) values. The seasonal cycle of Δ17O(SO42−)(TSP) exhibited minima during austral summer (0.9 ± 0.1‰, n = 5) and maxima during austral fall (1.3 ± 0.3‰, n = 6) and austral spring (1.6 ± 0.1‰, n = 5), indicating a shift from HOx‐ to O3‐dominated chemistry in the atmospheric derived SO42− component. Overall, the budgets of NO3− and SO42−(TSP) at the South Pole were complex functions of transport, localized chemistry, biological activity, and meteorological conditions, and these results will be important for interpretations of oxyanions in ice core records in the interior of Antarctica.

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Nitrogen stable isotope analysis (δN) of ammonia (NH) has shown potential to be a useful tool for characterizing emission sources and sink processes. However, to properly evaluate NH emission sources and sink processes under ambient conditions, it is necessary to collect and characterize the chemical speciation between NH and particulate ammonium (p-NH), together referred to as NH . Current NH collection methods have not been verified for their ability to accurately characterize δN-NH and/or provide necessary chemical speciation (i.e., δN-NH and δN-NH). Here, we report on the suitability of an established collection device that can provide NH speciation, an acid-coated (2% citric acid (w/v) + 1% glycerol (w/v) in 80:20 methanol to water solution) honeycomb denuder (HCD) with a downstream filter pack housed in the ChemComb Speciation Cartridge (CCSC), for characterizing δN-NH under a variety of laboratory-controlled conditions and field collections. The collection method was tested under varying NH concentration, relative humidity, temperature, and collection time at a flow rate of 10 L per minute (LPM). The acid-coated HCD collection device and subsequent chemical processing for δN-NH analysis is found to have excellent accuracy and precision of ±1.6‰ (2σ), with an operative capacity of ∼400 μg of collected NH for concentrations ≤207 ppb. This work presents the first laboratory verified method for δN-NH analysis and will be useful in future air quality studies.