Contribution of mass-dependent fractionation to the oxygen isotope anomaly of atmospheric nitrous oxide

Similar to other oxygen-bearing atmospheric compounds, lower-stratospheric and tropospheric nitrous oxide (N2O) show an oxygen isotope anomaly. This anomaly can be explained by in situ atmospheric chemical sources that transfer the well-known oxygen isotope anomaly of ozone to N2O. The isotope anomaly of ozone, in turn, is caused by non-mass-dependent fractionation during its formation. Nevertheless, recent work claimed that photodissociation of stratospheric N2O could account for up to half of the observed anomaly in N2O without having to invoke chemical N2O sources. It is shown that this prediction is due to the choice of inadequate parameters in the specific underlying physicochemical model of isotopic fractionation in N2O photolysis. Budget calculations based on experimentally measured fractionation factors at stratospherically relevant wavelengths show only negligible contributions of N2O photolysis to the observed oxygen isotope anomaly. However, biological sources at the Earth's surface, which are usually considered to produce mass-dependently fractionated N2O, may actually be responsible for part of the observed anomaly. This is as a consequence of slight variations in the mass-dependent relationships between 17O and 18O isotope effects and the relationship assumed in the definition of the oxygen isotope anomaly. Up to 44% of the observed anomaly might be explained by this “numerical source” that was not recognized previously. As a prerequisite to understand this possibly surprising result, the existing definitions of isotope anomalies and their practical consequences are analyzed. An accurate terminology will also benefit future generations of researchers in the rapidly growing fields of atmospheric isotope chemistry and physics.