High resolution, extreme isotopic variability of precipitation nitrate. (15th June 2019)

• Record Type: Journal Article
• Authors: Rose, Lucy A.; Yu, Zhongjie; Bain, Daniel J.; Elliott, Emily M.
• Is Part Of: Atmospheric environment. Volume 207(2019)
• Page Start: 63
• Page End: 74
• Publication Date: 2019-06-15
• Subjects: Nitrate -- Atmospheric deposition -- δ15N -- δ18O -- Δ -- 17O -- NOx; Air -- Pollution -- Periodicals; Air -- Pollution -- Meteorological aspects -- Periodicals; 551.51
• DOI: 10.1016/j.atmosenv.2019.03.012
• Languages: English
• ISSNs: 1352-2310

Abstract

Abstract: 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 15 N isotope values for NOx sources in a three end-member mixing model to predict source-based δ 15 N values of deposition reaching the study site on an hourly basis. To evaluate the effect of NOx oxidation pathways on measured δ 15 N-NO3 - values, we compared observed hourly isotope values to a coupled δ 15 N and Δ 17 O array representing N isotope exchange between atmospheric oxidized N molecules. Within individual events, δ 15 N, δ 18 O, and Δ 17 O 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 δ 15 N-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 δ 15 N-NO3 - observations to NOx emission sources, oxidation chemistry, and isotopic fractionation effects. We present the first observations of extreme intra-storm δ 15 N, δ 18 O, and Δ 17 O 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. Highlights: Triple NO3 − isotopes in wet deposition demonstrate extreme intra-storm variability. Anthropogenic and biogenic NOx sources contribute to precipitation NO3 − formation. Intra-system N recycling may be an important source of wet NO3 − deposition.