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High resolution, extreme isotopic variability of precipitation nitrate

Rose, Lucy A., Yu, Zhongjie, Bain, Daniel J., Elliott, Emily M.
Atmospheric environment 2019 v.207 pp. 63-74
air, atmospheric chemistry, atmospheric precipitation, ecosystems, emissions, forest canopy, forests, growing season, isotope fractionation, nitrates, nitric acid, nitric oxide, nitrogen, nitrogen dioxide, oxidation, oxygen, soil, stable isotopes, statistical models, storms, West Virginia
Deposition of atmospheric nitrate (NO3−) in precipitation can be an important source of reactive nitrogen (N) to ecosystems, particularly in regions with high nitrogen oxide (NOx = nitric oxide (NO)+nitrogen dioxide (NO2)) 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 NOx emissions on NO3− deposition, we calculated the total NOx 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 NOx 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 NOx oxidation pathways on measured δ15N-NO3- 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 NOx sources, oxidation pathways, and fractionation processes contributing to HNO3 formation. During every storm, precipitation δ15N-NO3- values were lower than those expected to result from predominant HNO3 formation pathways or oxidation of estimated NOx emissions along back trajectories, suggesting a systematic underestimation of NOx contributions to atmospheric HNO3 formation from isotopically depleted soil emissions. Together, these analyses represent the most comprehensive assessment to date relating high temporal resolution δ15N-NO3- observations to NOx 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 NOx emissions, forest canopy effects, and their role in atmospheric NO3− deposition and isotope dynamics in forests.