Isotope Fractionation of Nitrate During Volatilization in Snow: A Field Investigation in Antarctica

Several postdepositional processes impact snow nitrate; however, only the isotopic effects of nitrate photolysis have been quantified. Here we discuss results from experiments in field Antarctic snow investigating isotopic fractionation of nitrate due to volatilization. At -35 degrees C, concentration and isotopic composition of nitrate remained constant during the 16-day experiment. At -24 degrees C, 7.5% of nitrate was lost, synchronous with 1.5 decrease in delta O-18 and a constant delta N-15. At -4 degrees C, 38% of nitrate was lost, and delta N-15 and delta O-18 decreased by 3.1 and 1.8, respectively. Results at -4 degrees C yield calculated fractionation constants close to theoretical estimates including equilibrium isotopic exchange between nitric acid and nitrate and the desorption of nitric acid from water in quasi-liquid layers. This suggests that isotopic fractionation associated with nitrate volatilization across most of Antarctica, especially at sites with temperatures <-24 degrees C, should be minor, but the isotopic effects at warmer sites should be considered in interpreting archived nitrate records. Plain Language Summary In polar regions, there is great interest in using nitrate archived in snow/ice to reconstruct aspects of atmospheric composition and the influence of natural versus man-made emissions. However, nitrate can be lost from the snowpack, such that the archived signals do not directly reflect atmospheric loading. Stable isotopes of nitrate in snow/ice allow for tracking and understanding the origins of nitrate and how it might have changed after deposition. While the isotopic fractionation associated with photolytic loss of nitrate has been directly quantified, that of volatilization remains poorly understood. Thus, we completed field experiments in Antarctica to investigate the isotopic effects of nitrate volatilization from snow. Results show that nitrate volatilization loss is significant at -4 degrees C, resulting in an important change (depletion of heavy isotopes) in both nitrogen and oxygen in remaining snow nitrate. This can be largely explained by a combination of equilibrium isotopic exchange between nitric acid and nitrate and the desorption of nitric acid from water in ice surface layers. At lower temperatures (<-24 degrees C), nitrate loss is rather minor, and the isotopic fractionation is found to be negligible. The findings suggest that the isotopic effects of NO3- volatilization in snow are closely related to temperature.