Chemical Method for Nitrogen Isotopic Analysis of Ammonium at Natural Abundance

We report a new chemical method to determine the ¹⁵N natural abundance (δ¹⁵N) for ammonium (NH₄⁺) in freshwater (e.g., precipitation) and soil KCl extract. This method is based on the isotopic analysis of nitrous oxide (N₂O). Ammonium is initially oxidized to nitrite (NO₂^–) by hypobromite (BrO^–) using previously established procedures. NO₂^– is then quantitatively converted into N₂O by hydroxylamine (NH₂OH) under strongly acid conditions. The produced N₂O is analyzed by a commercially available purge and cryogenic trap system coupled to an isotope ratio mass spectrometer (PT-IRMS). On the basis of a typical analysis size of 4 mL, the standard deviation of δ¹⁵N measurements is less than 0.3‰ and often better than 0.1‰ (3 to 5 replicates). Compared to previous methods, the technique here has several advantages and the potential to be used as a routine method for ¹⁵N/¹⁴N analysis of NH₄⁺: (1) substantially simplified preparation procedures and reduced preparation time particularly compared to the methods in which diffusion or distillation is involved since all reactions occur in the same vial and separation of NH₄⁺ from solution is not required; (2) more suitability for low volume samples including those with low N concentration, having a blank size of 0.6 to 2 nmol; (3) elimination of the use of extremely toxic reagents (e.g., HN₃) and/or the use of specialized denitrifying bacterial cultures which may be impractical for many laboratories

Fossil Fuel Combustion-Related Emissions Dominate Atmospheric Ammonia Sources during Severe Haze Episodes: Evidence from ¹⁵N‑Stable Isotope in Size-Resolved Aerosol Ammonium

The reduction of ammonia (NH₃) emissions is urgently needed due to its role in aerosol nucleation and growth causing haze formation during its conversion into ammonium (NH₄⁺). However, the relative contributions of individual NH₃ sources are unclear, and debate remains over whether agricultural emissions dominate atmospheric NH₃ in urban areas. Based on the chemical and isotopic measurements of size-resolved aerosols in urban Beijing, China, we find that the natural abundance of ¹⁵N (expressed using δ¹⁵N values) of NH₄⁺ in fine particles varies with the development of haze episodes, ranging from −37.1‰ to −21.7‰ during clean/dusty days (relative humidity: ∼ 40%), to −13.1‰ to +5.8‰ during hazy days (relative humidity: 70–90%). After accounting for the isotope exchange between NH₃ gas and aerosol NH₄⁺, the δ¹⁵N value of the initial NH₃ during hazy days is found to be −14.5‰ to −1.6‰, which indicates fossil fuel-based emissions. These emissions contribute 90% of the total NH₃ during hazy days in urban Beijing. This work demonstrates the analysis of δ¹⁵N values of aerosol NH₄⁺ to be a promising new tool for partitioning atmospheric NH₃ sources, providing policy makers with insights into NH₃ emissions and secondary aerosols for regulation in urban environments

First Assessment of NO_<i>x</i> Sources at a Regional Background Site in North China Using Isotopic Analysis Linked with Modeling

Nitrogen oxides (NO_<i>x</i>, including NO and NO₂) play an important role in the formation of atmospheric particles. Thus, NO_<i>x</i> emission reduction is critical for improving air quality, especially in severely air-polluted regions (e.g., North China). In this study, the source of NO_<i>x</i> was investigated by the isotopic composition (δ¹⁵N) of particulate nitrate (p-NO₃^–) at Beihuangcheng Island (BH), a regional background site in North China. It was found that the δ¹⁵N-NO₃^– (<i>n</i> = 120) values varied between −1.7‰ and +24.0‰ and the δ¹⁸O-NO₃^– values ranged from 49.4‰ to 103.9‰. On the basis of the Bayesian mixing model, 27.78 ± 8.89%, 36.53 ± 6.66%, 22.01 ± 6.92%, and 13.68 ± 3.16% of annual NO_<i>x</i> could be attributed to biomass burning, coal combustion, mobile sources, and biogenic soil emissions, respectively. Seasonally, the four sources were similar in spring and fall. Biogenic soil emissions were augmented in summer in association with the hot and rainy weather. Coal combustion increased significantly in winter with other sources showing an obvious decline. This study confirmed that isotope-modeling by δ¹⁵N-NO₃^– is a promising tool for partitioning NO_<i>x</i> sources and provides guidance to policymakers with regard to options for NO_<i>x</i> reduction in North China

Plant productivity in Inner Mongolia grassland

Our data were collected from a 10-year manipulative experiment which simulated nitrogen deposition and water addition in field