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. Post-depositional 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 NO3, NO2O ̄ and NO2O ̄ in ice. Here we report on Oand O-fractionation in the 313 nm photolysis of 10 mM aqueous solutions of normal Fisher ΚΝΟ3 (i.e., ΔΟ = -0.2 ± 0.2 ‰) and O-enriched USGS-35 NaNO3 (ΔΟ = 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 ΔΟ = 0. In contrast, ΔΟ 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 ΔΟ decreases likely reflect competitive O-isotope exchange of geminate OH-radicals with H2O (ΔΟ = 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.