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

We recently reported the first multiple oxygen isotope composition of nitrate (NO3−) 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 N16O3−, N16O217O−, and N16O218O− in ice. Here we report on 17O‐ and 18O‐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 NaNO3 (Δ17O = 21.0 ± 0.4‰) between −30° and 25°C. We found that Fisher KNO3 undergoes mass‐dependent O‐fractionation, i.e., a process that preserves Δ17O = 0. In contrast, Δ17O in USGS‐35 NaNO3 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 NO3− photodecomposition into (NO2 + OH) is due to extensive cage recombination of the primary photofragments rather than to intramolecular processes, the observed Δ17O decreases likely reflect competitive O‐isotope exchange of geminate OH‐radicals with H2O (Δ17O = 0) and escape from the solvent cage, in addition to residual O‐isotope mixing of the final photoproducts NO, NO2, NO2−, with H2O. 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.