Investigation of coastal ammonium aerosol sources in the Northwest Pacific Ocean

Solid Phase Extraction Methodology for Robust Isotope Analysis of Atmospheric Ammonium

“…Once the parameters of the solid-phase extraction protocol (resin bead volume and NaCl eluent concentration) were optimized, the method was applied to environmental samples. The method was initially utilized to compare with previously reported δ 15 N-NH 4 + from aerosol samples collected from Changdao Island, China . These samples were previously analyzed for δ 15 N-NH 4 + without the use of the resin concentration method because these samples exhibited no matrix interference upon NH 4 + oxidation and had sufficiently elevated [NH 4 + (aq)] (i.e., typically greater than 10 μM) .…”

“…The method was initially utilized to compare with previously reported δ 15 N-NH 4 + from aerosol samples collected from Changdao Island, China . These samples were previously analyzed for δ 15 N-NH 4 + without the use of the resin concentration method because these samples exhibited no matrix interference upon NH 4 + oxidation and had sufficiently elevated [NH 4 + (aq)] (i.e., typically greater than 10 μM) . These samples were analyzed in three different ways: (1) no pretreatment such that samples were thawed and represented their initial matrix (Milli-Q water extracts from aerosol filters), (2) no resin treatment but with samples made up in a 4 M NaCl matrix (e.g., samples diluted with a 4 M NaCl solution instead of pure Milli-Q water), and (3) complete processing of samples using the described pretreatment resin method.…”

“…The stable nitrogen isotope composition of NH 4 + has been shown to be a useful tool for categorizing and detailing the sources and chemistry of reactive N in the environment. − This is because different sources of NH 4 + tend to have unique nitrogen isotope compositions. , The stable nitrogen isotope composition is typically reported using delta notation (δ 15 N) and is determined as follows: δ N 15 false( ‰ false) = false[ false( N 15 / N 14 ) sample / false( N 15 / N 14 ) reference − 1 false] × 1000 where the ( 15 N/ 14 N) reference is air-N 2 . Physical, chemical, and biological processes discern between the two isotopes ( 15 N and 14 N), which result in small but measurable differences in the ratio of 15 N: 14 N among the inorganic forms of N (e.g., NH) observed in the environment emitted by various sources .…”

“…Further, the chemical method for δ 15 N-NH 4 + determination was developed for and tested on freshwater and saltwater samples and was found to have no significant matrix interferences during the conversion of NH 4 + to NO 2 – to N 2 O 11 . Since this method’s development, it has been applied to a range of NH 4 + samples including precipitation, aerosols, and denuded gas. ,, Many studies assume successful NH 4 + oxidation to NO 2 – without direct confirmation, but when these checks are performed, quantitative oxidation is often not found. Further, incomplete oxidation of NH 4 + to NO 2 – can induce isotope fractionation resulting in a δ 15 N bias.…”

“…Here, we evaluated the resin methods ability to accurately reproduce previously reported δ 15 N-NH 4 + in marine aerosol samples from Changdao Island, China. 7 These samples, during and after the original analyses, were stored in a freezer to preserve chemical composition. When analyzing the four previously measured aerosol samples, all three methods were shown to determine δ 15 N-NH 4 + values with excellent comparison to their expected δ 15 N-NH 4 + values (Table 4).…”