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

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.