A Stable Isotope Investigation of Precipitation Nitrate
Cooney, Katherine Suzanne
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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 &#948;15N ranged from -5.1±0.5 to +5.9±0.5 (1&#963;), &#948;18O ranged from +42.6±0.5 to +81.9±0.5, and &#916;17O ranged from +15.33±0.05 to +31.71±0.05. Precipitation samples were re-analyzed for &#948;15N and &#948;18O 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 &#948;18O 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 &#948;15N, &#948;18O, and &#916;17O were highest in the winter and lowest in the summer. Ion concentrations, storm track data, and the &#948;34S of precipitation sulfate were used for interpretations. Most likely, &#948;15N varied due to seasonal changes in the NOx photo-stationary state, and &#948;18O and &#916;17O varied due to changes in oxidation chemistry.