Controls on the Nitrogen and Oxygen Isotopic Composition (δ 15N, δ 18O, δ 17O) of Atmospheric Nitrate in Princeton, NJ

The oxygen isotopic composition of atmospheric nitrate reflects the oxidative mechanisms that convert NO_x to HNO₃, while the nitrogen isotopic composition of atmospheric nitrate may reflect different NO_x source signatures and/or fractionations related to NO_x chemistry [Michalski et al., 2003; Hastings et al., 2003; Freyer et al., 1993]. New analysis techniques are capable of determining the ¹⁵N/¹⁴N, ¹⁸O/¹⁶O and ¹⁷O/¹⁶O isotope ratios in samples at the nanomolar level [Sigman et al., 2001; Casciotti et al., 2002; see Kaiser et al., session H38]. This allows for the analysis of short-term variations in the isotopes of HNO₃ with the potential to diagnose causal relationships by comparing the isotopic data with other features of atmospheric deposition. The ¹⁵N/¹⁴N, ¹⁸O/¹⁶O and ¹⁷O/¹⁶O of nitrate were analyzed from precipitation samples collected on an event-basis in Princeton, NJ between December 2002 and 2003. The nitrate concentration in Princeton rain ranges from 2.5 to 99.7 μ M (mean=21.1 μ M, n=61), similar to that found in other urban areas of New Jersey by the National Atmospheric Deposition Program. The isotopes of nitrate fall in the wide range reported for various environments with the δ ¹⁵N ranging from -4.0 to 9.5‰ (vs. air), and the δ ¹⁸O and δ ¹⁷O ranging from 57.2 to 90.5‰ and 50.7 to 77.8‰ (vs. VSMOW), respectively. The correlation between nitrate and sulfate concentration (R²=0.66) and the lack of a relationship between these major ions and the isotopes of nitrate supports the conclusion that below cloud scavenging is not the dominant control on the isotopic variations observed. Seasonal variations are observed in both the nitrogen and oxygen isotopes of nitrate. Overall the δ ¹⁵N is not correlated with either δ ¹⁸O or δ ¹⁷O, although both the δ ¹⁵N and δ ¹⁸O average lowest in the summer and highest in the winter. δ ¹⁸O is highly correlated with δ ¹⁷O of nitrate with anomalous enrichment in ¹⁷O relative to ¹⁸O (Δ ¹⁷O ranges from 19.7 to 30.8‰ ), as a result of the interaction of NO_x and ozone in the atmosphere prior to HNO₃ deposition. Comparison of δ ¹⁷O and δ ¹⁸O of nitrate show the data falling along a mixing line between the oxygen isotopic composition of tropospheric ozone and that of hydroxy/peroxy radicals (i.e. water), which has also been observed in nitrate aerosol samples collected in La Jolla, CA [Michalski et al., 2003]. Although there is considerable scatter in the isotopic time series, a distinctive increase in the oxygen isotope ratios occurs in mid-September and continues through the end of December 2003. This aspect of the time series will be discussed in the context of changes in atmospheric chemistry based on seasonal variations in atmospheric transport patterns, meteorology, and NO_x and ozone concentrations. In addition to these parameters, the nitrogen isotopic variations will also be interpreted in the context of changes in source contributions, e.g. coal burning in the Midwest, based on multiple chemical analyses including trace metals, mercury, and major ion concentrations.