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 °C, concentration and isotopic composition of nitrate remained constant during the 16-day experiment. At −24 °C, 7.5% of nitrate was lost, synchronous with 1.5‰ decrease in δ¹⁸O and a constant δ¹⁵N. At −4 °C, 38% of nitrate was lost, and δ¹⁵N and δ¹⁸O decreased by 3.1 and 1.8‰, respectively. Results at −4 °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 °C, should be minor, but the isotopic effects at warmer sites should be considered in interpreting archived nitrate records.