High resolution, extreme isotopic variability of precipitation nitrate.

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]