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

摘要

Stable isotopes of NO_3~- (δ~(15)N-NO_3~- and δ~(18)O-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~(15)N-NO_3~- and δ~(18)O-NO_3~- in precipitation varied significantly during individual storm events. Seasonally, δ~(15)N-NO_3~- and δ~(18)O-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~(15)N-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~(18)O-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~(18)O-NO_3~- observed in wintertime precipitation. Storm track information was especially useful for describing differences in δ~(15)N-NO_3~-. Cool-sector storms originating from the E/NE produced slightly negative δ~(15)N-NO_3~- values characteristic of automobile emissions, whereas warm-sector storms with tracks from the SW/S/SE produced slightly positive δ~(15)N-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.