Equilibrium and kinetic nitrogen and oxygen isotope fractionations between dissolved and gaseous N 2O

Abstract

Experiments were performed to determine the equilibrium as well as kinetic stable nitrogen and oxygen isotope fractionations between aqueous dissolved and gaseous N 2O. The equilibrium fractionations, defined as the ratio of the isotopic abundance ratios ( 15 R and 18 R, respectively) of gaseous and dissolved N 2O were found to be 0.99925 for 15 N 14 N and 0.99894 for 18 O 16 O at 298.2 K. No temperature dependence could be observed over the temperature range of 273.2–317.7 K outside experimental error. At 298.2 K the kinetic fractionation factors during N 2O invasion ( 15α in = 15R ( invaded) 15R ( gas) , and 18 α in defined similarly) were 1.0000 and 0.9992, respectively, and during N 2O evasion ( 15α ev = 15R ( evaded) 15R ( aq.solution) and 18 α ev defined similarly) were 0.9993 and 0.9981, respectively. The resulting changes in δ-values during air to sea transfer of N 2O are 0.0 and −0.8‰, and for sea to air transfer −0.7 and −1.9‰, for N and O, respectively. These results are in agreement with the hypothesis that the oceans would be a large source for N 2O in the atmosphere considering the fact that 15δ and 18δ of dissolved N 2O were found to be slightly heavier than those in air.