ABSTRACT
The family of atmospheric oxides of nitrogen, NOy (e.g., nitrogen oxides (NOx) + nitric acid (HNO3) + nitrous acid (HONO) + peroxyacetyl nitrate (PAN) + particulate nitrate (pNO3-) + other), have an influential role in atmospheric chemistry, climate, and the environment. The nitrogen (δ15N) and oxygen (δ18O and Δ17O) stable isotopes of NOy are novel tools for potentially tracking emission sources and quantifying oxidation chemistry. However, there is a lack of well-established methods, particularly for speciated gas-phase components of NOy, to accurately quantify δ15N, δ18O, and Δ17O. This work presents controlled laboratory experiments and complex chamber α-pinene/NOx oxidation experiments of a sampling apparatus constructed for the simultaneous capture of multiple NOy species for isotope analysis using a series of coated denuders, with a focus on nitrogen dioxide (NO2â¢). The laboratory tests indicate complete NO2⢠capture for the targeted concentration of 15 ppbv for at least 24 h collections at 10 liters per minute, with δ15N and δ18O precisions of ±1.3 and 1.0, respectively, and minimal (2.2% ± 0.1%) NO2⢠collection on upstream denuders utilized for the capture of HNO3 and other acidic gases. The multispecies NOy collection system showed excellent concentration correlations with online instrumentation for both HNO3 and NO2⢠and isotope reproducibility of ±1.7, ±1.8, and ±0.7 for δ15N, δ18O, and Δ17O, respectively, for replicate experiments and highly time-resolved collections. This work demonstrates a new method that can enable the simultaneous collection of HNO3 and NO2⢠for accurate quantification of concentration and isotopic composition.
ABSTRACT
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 HNO3(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 HNO3(g). Laboratory experiments found that both coating solutions quantitatively collected HNO3(g), with the Na2CO3 solution demonstrating a higher operative capacity (>1470 µg of HNO3; n = 25) compared to the NaCl solution (â¼750 µg of HNO3; n = 25). The precision values for laboratory-tested HNO3(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 HNO3(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 HNO3(g) and pNO3- for isotopic measurements would best utilize the NaCl solution and Nylon filters.
ABSTRACT
Nitrogen stable isotope analysis (δ15N) of particulate ammonium (NH4+) may provide additional constraints on this critical component of fine particulate matter; however, no previous collection method has been verified for its ability to accurately and precisely characterize δ15N(NH4+). This is a critical point due to the difficulty of quantitative NH4+ collection and possible sampling artifacts. Here, we report on δ15N(NH4+) precision using an established denuder-filter pack combination with two filter configurations including (1) a nylon filter plus an acid-impregnated cellulose filter and (2) an acid-impregnated glass fiber filter for NH4+ collection in both laboratory-controlled environments and ambient air samples. Laboratory NH4+ were generated from the nebulization of ammonium salt solutions and collected using a filter pack sampling train for off-line concentration and isotopic measurement. Quantitative collection of NH4+ was achieved using both filter configurations in both laboratory and field collections. Laboratory experiments indicate a δ15N(NH4+) precision of ±0.9 (1σ; n = 24) and ±0.6 ( n = 9) for the nylon plus citric acid impregnated cellulose filter and for the citric acid impregnated glass fiber filter, respectively. Field sample reproducibility was assessed from 24 h collected side-by-side samples and indicated δ15N(NH4+) to be reproducible within 1.1, consistent with the laboratory findings. This work represents the first established method for speciated NH4+ collection for isotopic analysis with important implications for furthering our understanding of its atmospheric dynamics.
