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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]

The US Corn Belt is a global hotspot of atmospheric ammonia (NH3), a gas known to adversely impact the environment and human health. We combine hourly tall tower (100 m) measurements and bi‐weekly, spatially distributed, ground‐based observations from the Ammonia Monitoring Network with the US National Emissions Inventory (NEI) and WRF‐Chem simulations to constrain NH3 emissions from April to September 2017–2019. We show that: (1) NH3 emissions peaked from May to July and were 1.6–1.7 times the annual NEI average; (2) average growing season NH3 emissions from agricultural lands were remarkably similar across years (3.27–3.64 nmol m−2 s−1), yet showed substantial episodic variability driven by meteorology and land management; (3) dry deposition was 40% of gross emissions from agricultural lands and exceeded 100% of gross emissions in natural lands. Our findings provide an important benchmark for evaluating future NH3 emissions and mitigation efforts. Key Points: Rare multiyear tall tower and ground‐based ammonia measurements were combined with WRF‐Chem to constrain emissions from the US Corn BeltMonthly ammonia emissions peaked in May through July and were 1.6–1.7 times the annual NEI averageEpisodic low WRF‐Chem bias versus tall tower observations underscore the importance of weather and land management as emission drivers [ABSTRACT FROM AUTHOR]