Using atmospheric chemistry and storm track information to explain the variation of nitrate stable isotopes in precipitation at a site in central Pennsylvania, USA

Stable isotopes of NO 3 − ( δ 15N–NO 3 − and δ 18O–NO 3 −) were monitored in precipitation at a central Pennsylvania site during six storm events in 2005 to determine whether information on atmospheric oxidants (e.g., O 3, NO 2, and NO x), and storm tracks (using the NOAA HYSPLIT model) were capable of explaining observed seasonal and within-storm isotopic variation. Results showed that δ 15N–NO 3 − and δ 18O–NO 3 − in precipitation varied significantly during individual storm events. Seasonally, δ 15N–NO 3 − and δ 18O–NO 3 − in precipitation followed a pattern of depletion during the summer months and enrichment during the winter months. NO 3 − precursor concentrations and atmospheric oxidants were useful for explaining the seasonal and within-storm variation of δ 15N–NO 3 − for all six storm events as evidenced by negative relationships with NO 2:NO x ratios and ozone (O 3). In comparison, δ 18O–NO 3 − was positively related to O 3 in three dormant season storms, which suggested that the O 3 oxidation pathway was important for producing the high δ 18O–NO 3 − observed in wintertime precipitation. Storm track information was especially useful for describing differences in δ 15N–NO 3 −. Cool-sector storms originating from the E/NE produced slightly negative δ 15N–NO 3 − values characteristic of automobile emissions, whereas warm-sector storms with tracks from the SW/S/SE produced slightly positive δ 15N–NO 3 − values characteristic of coal-fired emissions. Lightning also may have been an important source of atmospheric NO 3 − 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 NO 3 − stable isotopes in precipitation, and (2) knowledge of storm tracks (warm-sector versus cool-sector) may be important for determining sources of NO 3 − in wet deposition.