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 NO3- (delta N-15-NO3- and delta O-18-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., O-3, 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 delta N-15-NO3- and delta O-18-NO3- in precipitation varied significantly during individual storm events. Seasonally, delta N-15-NO3- and delta O-18-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 delta N-15-NO3- for all six storm events as evidenced by negative relationships with NO2:NOx ratios and ozone (O-3). In comparison, delta O-18-NO3- 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 delta O-18-NO3- observed in wintertime precipitation. Storm track information was especially useful for describing differences in delta N-15-NO3-. Cool-sector storms originating from the E/NE produced slightly negative delta N-15-NO3- values characteristic of automobile emissions, whereas warm-sector storms with tracks from the SW/S/SE produced slightly positive delta N-15-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. (C) 2009 Elsevier Ltd. All rights reserved.