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

Nitrogen stable isotope analysis (δN) of particulate ammonium (NH) 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 δN(NH). This is a critical point due to the difficulty of quantitative NH collection and possible sampling artifacts. Here, we report on δN(NH) 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 NH collection in both laboratory-controlled environments and ambient air samples. Laboratory NH 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 NH was achieved using both filter configurations in both laboratory and field collections. Laboratory experiments indicate a δN(NH) 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 δN(NH) to be reproducible within 1.1‰, consistent with the laboratory findings. This work represents the first established method for speciated NH collection for isotopic analysis with important implications for furthering our understanding of its atmospheric dynamics.