An extraction method for nitrogen isotope measurement of ammonium in low concentrated environment

Abstract. Ammonia (NH₃) participates in nucleation and growth of aerosols and thus plays a major role in atmospheric transparency, pollution, health, and climate related issues. Understanding its emission sources through nitrogen stable isotopes is therefore a major focus of current work to mitigate the adverse effects of aerosol formation. Since ice cores can preserve the past chemical composition of the atmosphere for centuries, they are a top tool of choice for understanding past NH₃ emissions through ammonium (NH₄⁺), the form of NH₃ archived in ice. However, the remote or high-altitude sites where glaciers and ice sheets are typically localized have relatively low fluxes of atmospheric NH₄⁺ deposition which makes ice core samples very sensitive to laboratory NH₃ contamination. As a result, accurate techniques for identifying and tracking NH₃ emissions through concentration and isotopic measurements are highly sought to constrain uncertainties in NH₃ emission inventories and atmospheric reactivity unknowns. Here, we describe a solid phase extraction method for NH₄⁺ samples of low concentration that limits external contamination and produces precise isotopic results. By limiting NH_3atm exposure with a scavenging fume hood and concentrating the targeted NH₄⁺ through ion exchange resin, we successfully achieve isotopic analysis of 50 nmol NH₄⁺ samples with a 0.6 ‰ standard deviation. This extraction method is applied to an alpine glacier ice core from Col Du Dôme, Mont-Blanc, where we successfully demonstrate the analytical approach through the analysis of two replicate 8 m water equivalent ice cores representing 4 years of accumulation with a reproducibility of ± 2.1 ‰. Applying this methodology to other ice cores in alpine and polar environments will open new opportunities for understanding past changes in NH₃ emissions and atmospheric chemistry.