Controls on the nitrogen and oxygen isotopic composition (delta-15N, delta-18O, delta-17O) of atmospheric nitrate in Princeton, NJ

Abstract

The oxygen isotopic composition of atmospheric nitrate reflects the oxidative mechanisms that convert NOx to HNO3, while the nitrogen isotopic composition of atmospheric nitrate may reflect different NOx source signatures and/or fractionations related to NOx 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 muM (mean=21.1 muM, 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 delta-15N ranging from -4.0 to 9.5permil (vs. air), and the delta-18O and delta-17O ranging from 57.2 to 90.5permil and 50.7 to 77.8permil (vs. VSMOW), respectively. The correlation between nitrate and sulfate concentration (R-2=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 delta-15N is not correlated with either delta-18O or delta-17O, although both the delta-15N and delta-18O average lowest in the summer and highest in the winter. delta-18O is highly correlated with delta-17O of nitrate with anomalous enrichment in -17O relative to -18O (Delta-17O ranges from 19.7 to 30.8permil), as a result of the interaction of NOXand ozone in the atmosphere prior to HNO3 deposition. Comparison of delta-17O and delta-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 NOx 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.