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Published August 16, 2005 | Published + Supplemental Material
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Oxygen isotopic fractionation in the photochemistry of nitrate in water and ice


We recently reported the first multiple oxygen isotope composition of nitrate (NO_3^−) in ice cores (Alexander et al., 2004). Postdepositional photolysis and volatilization may alter the isotopic signatures of snowpack nitrate. Therefore the precise assessment of the geochemical/atmospheric significance of O-isotopic signatures requires information on the relative rates of photolysis (λ > 300 nm) of N^(16)O_3^−, N^(16)O_2^(17)O^−, and N^(16)O_2^(18)O^− in ice. Here we report on ^(17)O^- and ^(18)O^-fractionation in the 313-nm photolysis of 10-mM aqueous solutions of normal Fisher KNO3 (i.e., Δ17O = −0.2 ± 0.2‰) and 17O-enriched USGS-35 NaNO_3 (Δ^(17)O = 21.0 ± 0.4‰) between −30° and 25°C. We found that Fisher KNO_3 undergoes mass-dependent O-fractionation, i.e., a process that preserves Δ^(17)O = 0. In contrast, Δ^(17)O in USGS-35 NaNO_3 decreased by 1.6 ± 0.4‰ and 2.0 ± 0.4‰ at 25°C, 1.2 ± 0.4‰ and 1.3 ± 0.4‰ at −5°C, and 0.2 ± 0.4‰ and 1.1 ± 0.4‰ at −30°C, after 12 and 24 hours, respectively. Since the small quantum yield (∼0.2%) of NO_3^− photodecomposition into (NO_2 + OH) is due to extensive cage recombination of the primary photofragments rather than to intramolecular processes, the observed Δ^(17)O decreases likely reflect competitive O-isotope exchange of geminate OH-radicals with H_2O (Δ^(17)O = 0) and escape from the solvent cage, in addition to residual O-isotope mixing of the final photoproducts NO, NO_2, NO_2^−, with H_2O. At the prevailing low temperatures, photochemical processing will not impair the diagnostic value of O-isotopic signatures in tracing the chemical ancestry of nitrate in polar ice.

Additional Information

© 2005 American Geophysical Union. Received 1 October 2004; revised 16 March 2005; accepted 12 April 2005; published 13 August 2005. UCSD gratefully acknowledges the support of the National Science Foundation, Atmospheric Science and Polar Research. The manuscript also benefited from the comments of two anonymous reviewers and input from Greg Michalski, Larry Hernandez, and Subrata Chakraborty.

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