Investigating Nitrate Formation Pathways Using Isotope Analysis in Controlled Chamber Experiments

Atmospheric nitrate, comprising particulate NO₃^- and gas-phase nitric acid (HNO₃), is a highly significant product of the oxidation of NO_x (= NO + NO₂). Its isotopic composition (Δ¹⁷O, δ¹⁸O and δ¹⁵N) provides valuable information on NO_x sources and atmospheric oxidation pathways, making nitrate preserved in ice cores a proxy for past atmospheric chemical reactivity. However, the interpretation of these ice core records is currently limited by an incomplete understanding of both isotope fractionation and isotope clumping effects associated with different nitrate formation pathways.

To better constrain these effects, we conducted a series of atmospheric chamber experiments in CESAM to investigate atmospheric nitrate production via two major nocturnal formation pathways: N₂O₅ heterogeneous hydrolysis on aerosol particles and the oxidation of volatile organic compounds by the NO₃ radical. Reactant concentrations, temperature, and humidity were monitored and controlled throughout each chamber experiment, and the resulting nitrate was collected on filters for isotope analysis. In addition, particulate NO₃^- formation and aerosol chemical composition were quantified simultaneously using a high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS), providing process-level constraints on nitrate production for each formation pathway.

This study aims to investigate whether distinct isotopic signatures arise between nitrate produced via N₂O₅ hydrolysis and NO₃-VOC reactions. The isotopic composition of the produced nitrate (Δ¹⁷O, δ¹⁸O, and δ¹⁵N) will be analysed to quantify pathway-dependent isotope effects. In addition, a newly developed methodology using the ESI-Orbitrap mass spectrometer will be applied to measure clumped isotopes (i.e. Δ¹⁵N¹⁸O and Δ¹⁸O¹⁸O) in the produced nitrate, to evaluate whether clumped isotope signatures provide an additional constraint on nitrate formation mechanisms.