The effect of acidic solutions on the determination of the natural abundance of nitrogen isotopes in ammonia

Atmospheric ammonia (NH3) contributes significantly to environmental pollution and human health once it reacts with atmospheric acidic species and deposits on soil and water surfaces. The natural abundance of nitrogen isotope (δ15N) can be used to trace the source of NH3 for emission control. NH3 is generally passively absorbed by acidic solutions, which can convert NH3 to ammonium (NH4+). However, it remains unclear whether the passive collection process and the acidic solutions may influence recovery at different NH4+ levels and introduce isotope fractionation. Here, the recovery rate of NH3 was evaluated using two acidic solutions, sulfuric, and boric acid. Then, these two solutions were used to passively absorb isotope reference materials to assess their effects on δ15N measurements. Results showed that NH3 absorbed by sulfuric acid solution achieved an average recovery of 95%, indicating that its influence on δ15N values could be ignored. In contrast, the recovery rate of the boric acid solution was below 90%, which may induce isotope fractionation during absorption. Although both solutions provided acceptable accuracy for δ15N determination, sulfuric acid was more suitable than boric acid, even at NH4+ concentrations as low as 20 μmol L−1. Finally, this method was adopted to observe the δ15N values of NH3 emitted from croplands, livestock, orchards, and vegetables, and significant differences were found among these four sources.