Enrichment of nitrogen-15 and oxygen-18 in stratospheric nitrous oxide: Observations, experimental results, and implications

The biogeochemical cycling of nitrous oxide plays an important role in atmospheric greenhouse forcing and in the catalytic cycling of stratospheric ozone. The abundances of the light stable isotopes of nitrogen ({dollar}sp{lcub}14{rcub}{dollar}N and {dollar}sp{lcub}15{rcub}{dollar}N) and oxygen ({dollar}sp{lcub}16{rcub}{dollar}O, {dollar}sp{lcub}17{rcub}{dollar}O, and {dollar}sp{lcub}18{rcub}{dollar}O) are useful tools in monitoring the bacterial and chemical processes which produce N{dollar}sb2{dollar}O as well as the processes in the stratosphere which are responsible for its destruction.; This work describes a technique developed for mass spectrometric analysis of directly injected N{dollar}sb2{dollar}O and applies it to a suite of samples collected from the lower stratosphere and to samples collected during photolysis experiments conducted under controlled laboratory conditions. It is shown that the isotopic signature of N{dollar}sb2{dollar}O in the lower stratosphere covaries with N{dollar}sb2{dollar}O concentration in a manner which can best be modeled as a single stage loss process also known as a Rayleigh distillation. The enrichment factors determined from these samples are {dollar}varepsilon{dollar} = {dollar}-{dollar}14.5 per mil for {dollar}sp{lcub}15{rcub}{dollar}N{dollar}sb2{dollar}O and {dollar}varepsilon{dollar} = {dollar}-{dollar}12.9 per mil for N{dollar}sb2sp{lcub}18{rcub}{dollar}O.; Laboratory experiments were conducted to determine if the ultraviolet photolytic destruction of N{dollar}sb2{dollar}O could be the source of the large enrichments observed in the stratosphere. Studies of N{dollar}sb2{dollar}O:N{dollar}sb2{dollar} mixtures irradiated at 193 and 207 nm indeed reveal a significant enrichment of the heavy nitrous oxide isotopomers in the unreacted residual gas. The isotopic signatures resulting from photolysis are also well modeled by an irreversible Rayleigh distillation process, with large enrichment factors of {dollar}varepsilonsb{lcub}15,18{rcub}{dollar}(207 nm) {dollar}-{dollar}48.7, {dollar}-{dollar}46.0 per mil and {dollar}varepsilonsb{lcub}15,18{rcub}{dollar}(193 nm) = {dollar}-{dollar}18.4, {dollar}-{dollar}14.5 per mil. The apparent incompatibility of laboratory results and stratospheric observations is accounted for by the derivation of an "effective" fractionation factor resulting from diffusive mixing processes.; Finally, a simple two box model is developed to re-examine our current understanding of the budget of nitrous oxide. It is seen that fractionation associated with ultraviolet photolysis in the stratosphere plays a key role in balancing the isotopic budget. It is also noted that increased production due to human influence should have a unique isotopic signature which should be observable in air trapped in polar ice and firn.