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Stable isotopes of NO3− (δ15N–NO3− and δ18O–NO3−) were monitored in precipitation at a central Pennsylvania site during six storm events in 2005 to determine whether information on atmospheric oxidants (e.g., O3, NO2, and NOx), and storm tracks (using the NOAA HYSPLIT model) were capable of explaining observed seasonal and within-storm isotopic variation. Results showed that δ15N–NO3− and δ18O–NO3− in precipitation varied significantly during individual storm events. Seasonally, δ15N–NO3− and δ18O–NO3− in precipitation followed a pattern of depletion during the summer months and enrichment during the winter months. NO3− precursor concentrations and atmospheric oxidants were useful for explaining the seasonal and within-storm variation of δ15N–NO3− for all six storm events as evidenced by negative relationships with NO2:NOx ratios and ozone (O3). In comparison, δ18O–NO3− was positively related to O3 in three dormant season storms, which suggested that the O3 oxidation pathway was important for producing the high δ18O–NO3− observed in wintertime precipitation. Storm track information was especially useful for describing differences in δ15N–NO3−. Cool-sector storms originating from the E/NE produced slightly negative δ15N–NO3− values characteristic of automobile emissions, whereas warm-sector storms with tracks from the SW/S/SE produced slightly positive δ15N–NO3− values characteristic of coal-fired emissions. Lightning also may have been an important source of atmospheric NO3− during two warm-sector thunderstorms. This study showed that (1) information about oxidant levels can be useful to predict the seasonal and within-storm variation of NO3− stable isotopes in precipitation, and (2) knowledge of storm tracks (warm-sector versus cool-sector) may be important for determining sources of NO3− in wet deposition. |
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