A Stable Isotope Investigation of Precipitation Nitrate

Abstract

The isotopic composition of precipitation nitrate reflects the processes involved in its formation. Silver nitrates prepared from Maryland precipitation were thermally decomposed to gases for mass spectrometric analyses. Nitrate δ15N ranged from -5.1±0.5 to +5.9±0.5 (1σ), δ18O ranged from +42.6±0.5 to +81.9±0.5, and Δ17O ranged from +15.33±0.05 to +31.71±0.05. Precipitation samples were re-analyzed for δ15N and δ18O using a method in which bacteria convert nitrate to N2O for analyses, and d15N ranged from -3.6±0.2 to +7.1±0.2 and δ18O ranged from +61.6±0.3 to +86.8±0.3. Differences between the methods were attributed to organic contaminants in the silver nitrates. Nitrate δ15N, δ18O, and Δ17O were highest in the winter and lowest in the summer. Ion concentrations, storm track data, and the δ34S of precipitation sulfate were used for interpretations. Most likely, δ15N varied due to seasonal changes in the NOx photo-stationary state, and δ18O and Δ17O varied due to changes in oxidation chemistry.