High resolution, extreme isotopic variability of precipitation nitrate

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 (NO_x = 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 NO_x emissions on NO₃⁻ 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-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 NO_x 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 NO_x emissions along back trajectories, suggesting a systematic underestimation of NO_x 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 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 NO₃⁻ deposition and isotope dynamics in forests.