Selective Collection of Particulate Ammonium for Nitrogen Isotopic Characterization Using a Denuder-Filter Pack...

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.