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Abstract Deposition of atmospheric nitrate (NO 3 −) in precipitation can be an important source of reactive nitrogen (N) to ecosystems, particularly in regions with high nitrogen oxide (NO x = nitric oxide (NO)+nitrogen dioxide (NO 2)) emissions. However, high resolution deposition data are lacking for most systems. We conducted hourly precipitation sampling across six growing season storms in a forested area historically subjected to some of the highest levels of chronic N deposition in the United States. To characterize the influence of electricity generating unit (EGU), vehicle, and biogenic NO x emissions on NO 3 − deposition, we calculated the total NO x emitted from these sources within a 100 km radius of air mass back trajectories determined for Fernow Experimental Forest (West Virginia, USA). We combined these emissions estimates with established 15N isotope values for NO x sources in a three end-member mixing model to predict source-based δ15N values of deposition reaching the study site on an hourly basis. To evaluate the effect of NO x oxidation pathways on measured δ15N-NO 3 - values, we compared observed hourly isotope values to a coupled δ15N and Δ17O array representing N isotope exchange between atmospheric oxidized N molecules. Within individual events, δ15N, δ18O, and Δ17O values ranged by as much as 19.5‰, 28.9‰, and 13.8‰, respectively. This extreme short-term isotopic variability suggests a dynamic mix of NO x sources, oxidation pathways, and fractionation processes contributing to HNO 3 formation. During every storm, precipitation δ15N-NO 3 - values were lower than those expected to result from predominant HNO 3 formation pathways or oxidation of estimated NO x emissions along back trajectories, suggesting a systematic underestimation of NO x contributions to atmospheric HNO 3 formation from isotopically depleted soil emissions. Together, these analyses represent the most comprehensive assessment to date relating high temporal resolution δ15N-NO 3 - observations to NO x emission sources, oxidation chemistry, and isotopic fractionation effects. We present the first observations of extreme intra-storm δ15N, δ18O, and Δ17O variability, emphasizing the need for improved constraints on soil NO x emissions, forest canopy effects, and their role in atmospheric NO 3 − deposition and isotope dynamics in forests. Highlights • Triple NO 3 − isotopes in wet deposition demonstrate extreme intra-storm variability. • Anthropogenic and biogenic NO x sources contribute to precipitation NO 3 − formation. • Intra-system N recycling may be an important source of wet NO 3 − deposition. [ABSTRACT FROM AUTHOR]

Abstract Recent advances in stable isotope measurements now allow for detailed investigations of the sources, transformations, and deposition of reactive nitrogen (N) species. Stable isotopes show promise as a complementary tool for apportioning emissions sources that contribute to deposition and also for developing a more robust understanding of the transformations that can influence these isotope ratios. Methodological advances have facilitated the unprecedented examination of the isotopic composition of reactive N species in the atmosphere and in precipitation including nitrogen oxides (NO x = nitric oxide (NO) + nitrogen dioxide (NO 2)), atmospheric nitrate (NO 3 −), nitric acid (HNO 3), ammonia (NH 3), and ammonium (NH 4 +). This isotopic information provides new insight into the mechanisms of transformation and cycling of reactive N in the atmosphere and moreover helps resolve the contribution of multiple NO x and NH 3 emission sources to deposition across landscapes, regions, and continents. Here, we highlight the current state of knowledge regarding the isotopic ratios of NO x and NH 3 emission sources and chemical alterations of isotopic ratios during atmospheric transformations. We also highlight illustrative examples where isotopic approaches are used and review recent methodological advances. While these highlights are not an exhaustive review of the literature, we hope they provide a glimpse of the potential for these methods to help resolve knowledge gaps regarding total N deposition to Earth surfaces. We conclude with promising opportunities for future research in the short-, medium-, and long-term. Graphical abstract Unlabelled Image Highlights • Isotopes of reactive nitrogen species can aid in understanding emissions sources and chemical transformations that contribute to reactive nitrogen deposition. • Methodological advances have ushered in an unprecedented examination of reactive nitrogen isotope chemistry. • Here, we review advances in methods, applications, and gains in knowledge during the past decade. • We conclude with promising opportunities for future research in the short, medium-, and long-term. [ABSTRACT FROM AUTHOR]

Rationale Nitrogen oxides or NOx (NOx = NO + NO2) play an important role in air quality, atmospheric chemistry, and climate. The isotopic compositions of anthropogenic and natural NO2 sources are wide-ranging, and they can be used to constrain sources of ambient NO2 and associated atmospheric deposition of nitrogen compounds. While passive sample collection of NO2 isotopes has been used in field studies to determine NOx source influences on atmospheric deposition, this approach has not been evaluated for accuracy or precision under different environmental conditions. Methods The efficacy of NO2 passive sampler collection for NO2 isotopes was evaluated under varied temperature and relative humidity (RH) conditions in a dynamic flux chamber. The precision and accuracy of the filter NO2 collection as nitrite (NO2-) for isotopic analysis were determined using a reference NO2 gas tank and through inter-calibration with a modified EPA Method 7. The bacterial denitrifer method was used to convert 20 μM of collected NO2- or nitrate (NO3-) into N2O and was carried out on an Isoprime continuous flow isotope ratio mass spectrometer. Results δ15N-NO2 values determined from passive NO2 collection, in conditions of 11-34 °C, 1-78% RH, have an overall accuracy and precision of ±2.1 ‰, and individual run precision of ±0.6 ‰. δ18O-NO2 values obtained from passive NO2 sampler collection, under the same conditions, have an overall precision of ± 1.3 ‰. Conclusions Suitable conditions for passive sampler collection of NO2 isotopes are in environments ranging from 11 to 34 °C and 1 to 78% RH. The passive NO2 isotope measurement technique provides an accurate method to determine variations in atmospheric δ15N-NO2 values and a precise method for determining atmospheric δ18O-NO2 values. The ability to measure NO2 isotopes over spatial gradients at the same temporal resolution provides a unique perspective on the extent and seasonality of fluctuations in atmospheric NO2 isotopic compositions. Copyright © 2017 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]