Performance of a Counter-Current Flow Tube Method for Gaseous NH3 Collection and Isotope Analysis

Ammonia (NH₃), the most abundant atmospheric alkaline trace gas, plays a crucial role in fine particulate matter formation and nitrogen cycling. While agriculture is the primary source, recent studies highlight significant contributions from non-agricultural urban sources. However, NH₃ spatiotemporal variability is complex and not fully understood, necessitating reliable and high time-resolution data. For more comprehensive source apportionment, combining such data with stable nitrogen isotope ratio (δ¹⁵N) analysis serves as a powerful approach.

To enable high time-resolution measurements, we previously developed a continuous measurement system using a counter-current flow tube (CCFT) technique (Huy et al., J. Atmos. Chem. 73, 223-240, 2016). Initially designed for ambient levels, its performance at elevated concentrations typical of emission sources has not yet been evaluated. In this study, we modified the CCFT measurement system to collect NH₃ as ammonium (NH₄⁺) in an aqueous solution for δ¹⁵N analysis. We evaluated its absorption efficiency and δ¹⁵N measurement accuracy across a wide range of NH₃ concentrations.

Gaseous NH₃ was captured in pure water using the modified CCFT sampling system (Huy et al., 2016). Sample air was drawn at 1.0 L min^-1 into a vertical tube, while pure water was introduced from the top at 0.12 mL min^-1. NH₃ was absorbed by diffusion and dissolution into the counter flowing solution. NH₄⁺ concentrations were determined by ion chromatography; δ¹⁵N was measured using the denitrifier method and a GasBench II system coupled to an isotope ratio mass spectrometer. Detailed procedures are provided in Kawashima et al. (Rapid Commun. Mass Spectrom. 35, e9027, 2021). To evaluate the collection efficiency and isotopic accuracy, the modified CCFT system was operated in parallel with a conventional boric acid (BA) trap system as a reference.

For concentrations measured by a BA trap ([NH₃]_BA) exceeding 600 μg m^-3, CCFT absorption efficiencies were clearly below 1.0, whereas efficiencies nearly reached 1.0 below 300 μg m^-3. The difference in δ¹⁵N of NH₃ between the CCFT and BA systems increased with [NH₃]_BA, reaching 12.17 ‰ at 1307.1 μg m^-3. This suggests that within the CCFT sampler, lighter ¹⁴NH₃ is less efficiently collected than heavier ¹⁵NH₃. The isotopic difference was particularly pronounced above 400 μg m^-3.