Speciated Collection of Nitric Acid and Fine Particulate Nitrate for Nitrogen and Oxygen Stable Isotope Determination

Stable isotopic composition of atmospheric nitrate (nitric acid (HNO₃) + particulate nitrate (pNO₃^-)) provides a higher-order dimensional analysis of critical atmospheric components, enabling a process-level understanding of precursor emissions, oxidation chemistry, aerosol acidity, and depositional patterns. Current methods have not been evaluated for their ability to accurately speciate and determine nitrogen (δ¹⁵N) and oxygen (δ¹⁸O and Δ¹⁷O) isotope compositions for gaseous and particle phases. Suitability of a denuder-filter sampling system for the collection of speciated HNO_3(g) and pNO₃^- for off-line concentration and isotopic determination was tested using both laboratory and field collections. Honeycomb denuders coated with either NaCl or Na₂CO₃ solutions were used to collect HNO_3(g). Laboratory experiments found that both coating solutions quantitatively collected HNO_3(g), with the Na₂CO₃ solution demonstrating a higher operative capacity (>1470 μg of HNO₃; n = 25) compared to the NaCl solution (∼750 μg of HNO₃; n = 25). The precision values for laboratory-tested HNO_3(g) collections are ±0.6‰ and ±1.2‰ for δ¹⁵N and δ¹⁸O for the NaCl solution and ± 0.8‰ and ±1.2‰ for the Na₂CO₃ solution. Replicate (urban) samples indicate that the Na₂CO₃ solution is significantly less selective for HNO_3(g) collection than the NaCl solution. Nylon filters were found to collect efficiently and retain laboratory-generated NaNO₃ and NH₄NO₃ particles, with maximum standard deviations for δ¹⁵N and δ¹⁸O of ±0.3‰ and ±0.3‰, respectively. Field replicates, while predictably more variable, also show consistency for δ¹⁵N and δ¹⁸O of ±0.6‰ and ±1.3‰ for particulate species, respectively. Recommended methods for field collections of speciated HNO_3(g) and pNO₃^- for isotopic measurements would best utilize the NaCl solution and Nylon filters.