22 May 2023
 | 22 May 2023
Status: this preprint is open for discussion and under review for Atmospheric Measurement Techniques (AMT).

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

Alexis Lamothe, Joel Savarino, Patrick Ginot, Lison Soussaintjean, Elsa Gautier, Pete D. Akers, Nicolas Caillon, and Joseph Erbland

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 NH3atm 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.

Alexis Lamothe et al.

Status: open (until 26 Jun 2023)

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Alexis Lamothe et al.


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Short summary
Ammonia is a reactive gas in our atmosphere that is key in air quality issues. Assessing its emissions and how it reacts is a hot topic that can be answered from the past. Stable isotopes (the mass of the molecule) measured in ice cores (glacial archives) can teach us a lot for that. However, the concentrations in ice cores are very small. We propose a protocol to limit the contaminations and apply it to one ice core drilled in the Mont-Blanc describing the opportunities our method brings.