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) + particulate nitrate (pNO)) 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 (δN) and oxygen (δO and ΔO) isotope compositions for gaseous and particle phases. Suitability of a denuder-filter sampling system for the collection of speciated HNO and pNO for off-line concentration and isotopic determination was tested using both laboratory and field collections. Honeycomb denuders coated with either NaCl or NaCO solutions were used to collect HNO. Laboratory experiments found that both coating solutions quantitatively collected HNO, with the NaCO solution demonstrating a higher operative capacity (>1470 μg of HNO; = 25) compared to the NaCl solution (∼750 μg of HNO; = 25). The precision values for laboratory-tested HNO collections are ±0.6‰ and ±1.2‰ for δN and δO for the NaCl solution and ± 0.8‰ and ±1.2‰ for the NaCO solution. Replicate (urban) samples indicate that the NaCO solution is significantly less selective for HNO collection than the NaCl solution. Nylon filters were found to collect efficiently and retain laboratory-generated NaNO and NHNO particles, with maximum standard deviations for δN and δO of ±0.3‰ and ±0.3‰, respectively. Field replicates, while predictably more variable, also show consistency for δN and δO of ±0.6‰ and ±1.3‰ for particulate species, respectively. Recommended methods for field collections of speciated HNO and pNO for isotopic measurements would best utilize the NaCl solution and Nylon filters.