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Oxygen isotopic fractionation in the photochemistry of nitrate in water and ice

McCabe, J. R. and Boxe, C. S. and Colussi, A. J. and Hoffmann, M. R. and Thiemens, M. H. (2005) Oxygen isotopic fractionation in the photochemistry of nitrate in water and ice. Journal of Geophysical Research D, 110 (D15). Art. No. D15310. ISSN 0148-0227. https://resolver.caltech.edu/CaltechAUTHORS:20150623-154755193

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Abstract

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.


Item Type:Article
Related URLs:
URLURL TypeDescription
http://dx.doi.org/10.1029/2004JD005484DOIArticle
http://onlinelibrary.wiley.com/doi/10.1029/2004JD005484/abstractPublisherArticle
ORCID:
AuthorORCID
Colussi, A. J.0000-0002-3400-4101
Hoffmann, M. R.0000-0001-6495-1946
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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Issue or Number:D15
Record Number:CaltechAUTHORS:20150623-154755193
Persistent URL:https://resolver.caltech.edu/CaltechAUTHORS:20150623-154755193
Official Citation:McCabe, J. R., C. S. Boxe, A. J. Colussi, M. R. Hoffman, and M. H. Thiemens (2005), Oxygen isotopic fractionation in the photochemistry of nitrate in water and ice, J. Geophys. Res., 110, D15310, doi:10.1029/2004JD005484.
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:58474
Collection:CaltechAUTHORS
Deposited By: George Porter
Deposited On:28 Jul 2015 21:46
Last Modified:03 Mar 2020 13:01

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