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Snow nitrate is vulnerable to photolytic loss that causes isotopic alteration, and thus its isotopes can potentially track the extent of snow nitrate photolysis and its impacts in environments where loss is significant. Large increases in δ15N‐NO3− below the snow surface have been attributed to photolysis and this behavior is generally consistent amongst theoretical as well as lab and field studies. Oxygen isotope ratios are thought to be influenced by photolysis as well as secondary condensed‐phase chemistry, but the competing effects have yet to be reconciled. Here we use a model that simulates nitrate burial, photolytic fractionation, and re‐oxidation in snow to quantitatively assess these processes with the aim of developing a consistent framework for interpreting the photolytic effects of the complete nitrate isotopic composition (δ15N, δ18O, and Δ17O). This study reveals that isotopic effects of nitrate photolysis and aqueous‐phase re‐oxidation chemistry are important sources of uncertainties in modeling δ18O‐NO3−. Plain Language Summary: Nitrate is a major ion species in Antarctic snow, and often measured in snow and ice cores. Accurate interpretation of ice core nitrate records, however, remains challenging mostly due to uncertainties associated with nitrate photolysis, especially in low snow accumulation sites. Consequently, both nitrogen and oxygen stable isotopes of nitrate are altered via photolysis, but can serve as a useful diagnostic of these processes. Compared to nitrogen isotopes, which have been characterized in detail by both model simulations and observations, oxygen isotopic effects of nitrate photolysis are far from understood. Here, we present a model incorporating processes of snow burial, photolytic loss, and re‐oxidation to quantitatively assess the changes in the full isotopic parameters (δ15N, δ18O, and Δ17O) over the course of snow nitrate burial in central Antarctica. Model simulations show that about 25%–30% of the photoproducts will be re‐oxidized to form secondary nitrate within the snow. Major uncertainty of our model predicted δ18O is largely associated with limited knowledge on oxygen isotopic fractionation during nitrate photolysis and reformation in snow grains. This model will help diagnose the processes of nitrate burial, photolytic fractionation, and re‐oxidation in snow and allow for better interpretation of these tracers in snow and ice. Key Points: We developed a model to quantify changes in NO3− isotopes based on burial, photolytic fractionation, and re‐oxidation in East Antarctic snowThe observed snow Δ17O profile is constrained by photolysis‐driven aqueous‐phase formation of secondary NO3− on the order of ∼25–30%Isotopic effects of NO3− photolysis and aqueous‐phase re‐oxidation chemistry are important sources of uncertainties in modeling δ18O [ABSTRACT FROM AUTHOR]

Several postdepositional processes impact snow nitrate; however, only the isotopic effects of nitrate photolysis have been quantified. Here we discuss results from experiments in field Antarctic snow investigating isotopic fractionation of nitrate due to volatilization. At −35 °C, concentration and isotopic composition of nitrate remained constant during the 16‐day experiment. At −24 °C, 7.5% of nitrate was lost, synchronous with 1.5‰ decrease in δ18O and a constant δ15N. At −4 °C, 38% of nitrate was lost, and δ15N and δ18O decreased by 3.1 and 1.8‰, respectively. Results at −4 °C yield calculated fractionation constants close to theoretical estimates including equilibrium isotopic exchange between nitric acid and nitrate and the desorption of nitric acid from water in quasi‐liquid layers. This suggests that isotopic fractionation associated with nitrate volatilization across most of Antarctica, especially at sites with temperatures <−24 °C, should be minor, but the isotopic effects at warmer sites should be considered in interpreting archived nitrate records. Plain Language Summary: In polar regions, there is great interest in using nitrate archived in snow/ice to reconstruct aspects of atmospheric composition and the influence of natural versus man‐made emissions. However, nitrate can be lost from the snowpack, such that the archived signals do not directly reflect atmospheric loading. Stable isotopes of nitrate in snow/ice allow for tracking and understanding the origins of nitrate and how it might have changed after deposition. While the isotopic fractionation associated with photolytic loss of nitrate has been directly quantified, that of volatilization remains poorly understood. Thus, we completed field experiments in Antarctica to investigate the isotopic effects of nitrate volatilization from snow. Results show that nitrate volatilization loss is significant at −4 °C, resulting in an important change (depletion of heavy isotopes) in both nitrogen and oxygen in remaining snow nitrate. This can be largely explained by a combination of equilibrium isotopic exchange between nitric acid and nitrate and the desorption of nitric acid from water in ice surface layers. At lower temperatures (<−24 °C), nitrate loss is rather minor, and the isotopic fractionation is found to be negligible. The findings suggest that the isotopic effects of NO3− volatilization in snow are closely related to temperature. Key Points: Field experiments in Antarctica (over 16 days) were used to quantify the isotopic effects of NO3− volatilization from snowVolatilization at −4 °C depletes heavy isotopes of NO3− in remaining snow versus relatively constant isotopic ratios observed at −35 °CVolatilization can decrease δ15N in remaining snow NO3− in warm regions by several per mil, contrary to isotopic effects of photolysis [ABSTRACT FROM AUTHOR]

Soil emissions represent ~25% of the global annual atmospheric budget of nitrogen oxides (NOx). The nitrogen isotopic composition of soil NOx emissions (δ15N‐NOx) is potentially useful to track soil emission contributions to NOx budgets, yet its in situ variations with fertilizer management and meteorology are unknown. We quantify in situ δ15N‐NOx distributions from liquid dairy manure‐fertilized cropland in State College, Pennsylvania at hourly resolution during spring 2016 and 2017. δ15N‐NOx (n = 37) ranged from −44.2 to −14.0‰ and was distinct between injected (−32.2 ± 12.1‰) and broadcast manure without tillage (−23.4 ± 2.1‰). δ15N‐NOx was not correlated with order of magnitude emission variations due to large changes in soil moisture. δ15N‐NOx differences between treatments were associated with nitrification and NO consumption contributions. Our results suggest that δ15N‐NOx can be a valuable observational tracer of soil emissions and varies with fertilizer management practices. Plain Language Summary: Soil processes contribute about one quarter of all global emissions of nitrogen oxides, with important impacts on air quality and ecosystem health. The ratios of heavier to lighter nitrogen atoms (isotopes) are potentially useful to track soil emissions, but it is uncertain how they vary with agricultural management choices and weather conditions. We measure isotopes of soil nitrogen oxides emitted from manure‐fertilized cropland and find large differences based on whether manure was injected below or applied onto soil surfaces. Our results provide a new potential way to track agricultural emissions and their impacts on air pollution and ecosystem health. Key Points: Novel hourly, in situ N isotopic composition of soil NOx emission observations were performed across manure‐fertilized croplandsδ15N‐NOx in injected manure (−32.2 ± 12.1‰) was enriched over time, while that for broadcast manure (−23.4 ± 2.1‰) was less negativeδ15N‐NOx was insensitive to emission variations associated with soil moisture, and mainly linked with nitrification of soil NH4+ [ABSTRACT FROM AUTHOR]