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

Stable isotopic composition of atmospheric nitrate (nitric acid (HNO ₃ ) + particulate nitrate (pNO ₃ ^- )) 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 (δ ¹⁵ N) and oxygen (δ ¹⁸ O and Δ ¹⁷ O) isotope compositions for gaseous and particle phases. Suitability of a denuder-filter sampling system for the collection of speciated HNO _3(g) and pNO ₃ ^- for off-line concentration and isotopic determination was tested using both laboratory and field collections. Honeycomb denuders coated with either NaCl or Na ₂ CO ₃ solutions were used to collect HNO _3(g) . Laboratory experiments found that both coating solutions quantitatively collected HNO _3(g) , with the Na ₂ CO ₃ solution demonstrating a higher operative capacity (>1470 μg of HNO ₃ ; n = 25) compared to the NaCl solution (∼750 μg of HNO ₃ ; n = 25). The precision values for laboratory-tested HNO _3(g) collections are ±0.6‰ and ±1.2‰ for δ ¹⁵ N and δ ¹⁸ O for the NaCl solution and ± 0.8‰ and ±1.2‰ for the Na ₂ CO ₃ solution. Replicate (urban) samples indicate that the Na ₂ CO ₃ 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 NaNO ₃ and NH ₄ NO ₃ particles, with maximum standard deviations for δ ¹⁵ N and δ ¹⁸ O of ±0.3‰ and ±0.3‰, respectively. Field replicates, while predictably more variable, also show consistency for δ ¹⁵ N and δ ¹⁸ O of ±0.6‰ and ±1.3‰ for particulate species, respectively. Recommended methods for field collections of speciated HNO _3(g) and pNO ₃ ^- for isotopic measurements would best utilize the NaCl solution and Nylon filters.