Isotopic Fractionation of Nitrous Oxide: Constraints on Its Biogenic Sources

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Nitrous oxide (N2O) plays a significant role in terms of controlling stratospheric ozone and is an important greenhouse gas. N2O is increasing in the atmosphere and has been targeted for regulation under the Montreal Protocol. Previous stable isotopic measurements of stratospheric N2O have demonstrated highly enriched 15/14N and 18/16O ratios. It has been proposed that this fractionation is caused by variations in the photolysis rate of various isotopomers. This project involves the laboratory investigation of the isotopic fractionation of N2O by ultraviolet photolysis in the wavelength range 198-220 nm. The ultraviolet absorption cross sections of various nitrous oxide isotopomers will be measured at high spectral resolution. Photolysis experiments will also be carried out with mass spectrometric and near-infrared Fourier transform spectroscopy to examine the isotopic fractionation of residual N2O. The results of these experiments will be used to refine the interpretation of atmospheric nitrous oxide isotopic ratio measurements and improve our understanding of the contribution of various sources and sinks to the global atmospheric nitrous oxide budget.