Efficacy of passive sampler collection for atmospheric NO2 isotopes under simulated environmental conditions

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.

Isotopic advances in understanding reactive nitrogen deposition and atmospheric processing

Recent advances in stable isotope measurements now allow for detailed investigations of the sources, transformations, and deposition of reactive nitrogen (N) species. Stable isotopes show promise as a complementary tool for apportioning emissions sources that contribute to deposition and also for developing a more robust understanding of the transformations that can influence these isotope ratios. Methodological advances have facilitated the unprecedented examination of the isotopic composition of reactive N species in the atmosphere and in precipitation including nitrogen oxides (NO_x = nitric oxide (NO) + nitrogen dioxide (NO₂)), atmospheric nitrate (NO₃^-), nitric acid (HNO₃), ammonia (NH₃), and ammonium (NH₄⁺). This isotopic information provides new insight into the mechanisms of transformation and cycling of reactive N in the atmosphere and moreover helps resolve the contribution of multiple NO_x and NH₃ emission sources to deposition across landscapes, regions, and continents. Here, we highlight the current state of knowledge regarding the isotopic ratios of NO_x and NH₃ emission sources and chemical alterations of isotopic ratios during atmospheric transformations. We also highlight illustrative examples where isotopic approaches are used and review recent methodological advances. While these highlights are not an exhaustive review of the literature, we hope they provide a glimpse of the potential for these methods to help resolve knowledge gaps regarding total N deposition to Earth surfaces. We conclude with promising opportunities for future research in the short-, medium-, and long-term.