HR: 11:30h
AN: H52B-05 INVITED     [Abstracts]
TI: Online Method for Oxygen Triple Isotope Analyses of Nitrate
AU: * Kaiser, J
EM: kaiser@princeton.edu
AF: Princeton University, Department of Geosciences Guyot Hall, Princeton, NJ 08544 United States
AU: * Kaiser, J
EM: kaiser@princeton.edu
AF: Max Planck Institute for Nuclear Physics, Atmospheric Physics Division Saupfercheckweg 1, Heidelberg, Ger 69117 Germany
AU: Hastings, M G
EM: mhasting@princeton.edu
AF: Princeton University, Department of Geosciences Guyot Hall, Princeton, NJ 08544 United States
AU: Houlton, B
EM: houlton@princeton.edu
AF: Princeton University, Department of Ecology and Evolutionary Biology 106A Guyot Hall, Princeton, NJ 08544 United States
AU: Roeckmann, T
EM: T.Roeckmann@mpi-hd.mpg.de
AF: Max Planck Institute for Nuclear Physics, Atmospheric Physics Division Saupfercheckweg 1, Heidelberg, Ger 69117 Germany
AU: Sigman, D M
EM: sigman@princeton.edu
AF: Princeton University, Department of Geosciences Guyot Hall, Princeton, NJ 08544 United States
AB: Combined $^{17}$O/$^{16}$O and $^{18}$O/$^{16}$O isotope ratio analyses of nitrate in ground and surface waters help to understand the partitioning between atmospheric and terrestrial nitrate sources because only terrestrial nitrate shows mass-dependent relative enrichments in $^{17}$O and $^{18}$O, whereas atmospheric nitrate displays an anomalous enrichment in $^{17}$O. The $^{17}$O isotope anomaly of nitrate is therefore a sensitive tracer of fresh water pollution. Furthermore, isotope measurements of atmospheric nitrate in aerosols and precipitation provide insight into the partitioning between atmospheric NO$_{x}$ cycling pathways via ozone or hydroxy/peroxy radicals because only ozone has a significant non-mass dependent enrichment in $^{17}$O. Previous methods to analyze the oxygen triple isotope composition of nitrate rely on offline thermal decomposition of AgNO$_{3}$ amounts in the $\micro$mol range. We have recently developed an online (coupled gas chromatography-mass spectrometry) method that requires two to three orders of magnitude less material to achieve essentially the same analytical precision: 30 nmol of nitrate give a 1$\sigma$ uncertainty of 1.0 $\permil$ for the $\delta$^{18}$O value and 0.3 $\permil$ for the $^{17}$O anomaly ($\Delta$^{17}$O). The method uses a strain of bacterial denitrifiers to convert nitrate to N$_{2}$O [Casciotti et al., 2002], which is then quantitatively converted to elemental nitrogen and oxygen in a gold furnace at 800$\deg$C. Both gases are separated on a molecular sieve capillary column and introduced into the isotope ratio mass spectrometer. There is no significant memory effect, but calibration via nitrate or N$_{2}O$ standards is required for scale normalization. This novel method was used to analyze nitrate isotopes in rain water and streams and, thanks to the low sample size requirements, will also be suitable for ice core samples, which have very low nitrate concentrations. A tight correlation between $\Delta^{17}$O and $\delta^{18}O$ in rain water was found with a slope of about 0.3 (R$^2$ = 0.86), which reflects the average isotopic composition of hydroxy/peroxy radicals and tropospheric ozone [cf. contribution by Hastings et al. in session H38]. Nitrate isotope measurements in ice core samples offer opportunities for paleoatmospheric studies. The same method can also be used to analyze N$_2$O itself or other oxy compounds of nitrogen that can be converted to nitrate.
DE: 1806 Chemistry of fresh water
DE: 1040 Isotopic composition/chemistry
DE: 0365 Troposphere--composition and chemistry
DE: 0394 Instruments and techniques
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting