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 Δ ¹⁷O = 0. In contrast, Δ ¹⁷O 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 Δ ¹⁷O decreases likely reflect competitive O-isotope exchange of geminate OH-radicals with H₂O (Δ ¹⁷O = 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.