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

Abstract:

The stable isotopic composition of atmospheric nitrate (nitric acid (HNO_3(g)) + particulate nitrate (pNO₃^-)) has been used to track precursor emissions, oxidation chemistry, aerosol acidity, and depositional patterns. However, current collection methods have not been evaluated for their ability to accurately determine nitrogen (δ¹⁵N) and oxygen (δ¹⁸O and Δ¹⁷O) isotope compositions, while speciating between gaseous and particle phases of atmospheric nitrate. Suitability of a denuder-filter active 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 and Na₂CO₃ solutions were used to collect HNO_3(g). Laboratory experiments found that both coating solutions quantitatively collected HNO_3(g), with the Na₂CO₃ coating solution demonstrating a higher operative capacity (>1,640 µg HNO₃; n=25) compared to the NaCl solution (~750 µg HNO₃; n=25). The precision for laboratory-tested HNO_3(g) collections was ±0.6‰ and ±1.2‰ for δ¹⁵N and δ¹⁸O in the NaCl solution, and ±0.8‰ and ±1.2‰ for the Na₂CO₃ solution, respectively. Replicate environmental (urban) samples indicated that the Na₂CO₃ coating is significantly less selective for HNO_3(g) collection than the NaCl coating. Nylon filters were found to collect ammonium nitrate and sodium nitrate particles quantitatively; retain laboratory-generated ammonium nitrate particles; and yielded δ¹⁵N and δ¹⁸O maximum standard deviations of ±0.3‰ and ±0.3‰, respectively. Field replicates, while predictably more variable, also show consistency for δ¹⁵N and δ¹⁸O (±0.6‰ and ±1.3‰). Under laboratory or chamber-based conditions, where air composition can be controlled, either Na₂CO₃ or NaCl coatings can be utilized for HNO_3(g) collection. For field and chamber collections of speciated HNO_3(g) and pNO₃^- for isotopic measurements the NaCl coating and Nylon filters are recommended.