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

Identifying Ammonia Hotspots in China Using a National Observation Network

The limited availability of ammonia (NH₃) measurements is currently a barrier to understanding the vital role of NH₃ in secondary aerosol formation during haze pollution events and prevents a full assessment of the atmospheric deposition of reactive nitrogen. The observational gaps motivated us to design this study to investigate the spatial distributions and seasonal variations in atmospheric NH₃ on a national scale in China. On the basis of a 1-year observational campaign at 53 sites with uniform protocols, we confirm that abundant concentrations of NH₃ [1 to 23.9 μg m^–3] were identified in typical agricultural regions, especially over the North China Plain (NCP). The spatial pattern of the NH₃ surface concentration was generally similar to those of the satellite column concentrations as well as a bottom-up agriculture NH₃ emission inventory. However, the observed NH₃ concentrations at urban and desert sites were comparable with those from agricultural sites and 2–3 times those of mountainous/forest/grassland/waterbody sites. We also found that NH₃ deposition fluxes at urban sites account for only half of the emissions in the NCP, suggesting the transport of urban NH₃ emissions to downwind areas. This finding provides policy makers with insights into the potential mitigation of nonagricultural NH₃ sources in developed regions