Kaiser, J. (2002). “Stable isotope investigations of atmospheric nitrous oxide”. Ph. D. Thesis, Max Planck Institute for Chemistry/University of Mainz. (Download)

The analysis of isotope ratios is of increasing importance to study the sources and sinks of atmospheric trace gases and to investigate their chemical reaction pathways. Nitrous oxide (N2O) has four mono-substituted rare isotopic species: 14N15N16O, 15N14N16O, 14N217O and 14N218O. Mass-spectrometric measurement techniques have been developed during the work described here which enable a complete characterisation of abundance variations of these species. The hitherto most comprehensive account of these variations in the troposphere and stratosphere is given and interpreted in detail with reference to a suite of laboratory experiments.

The laboratory experiments represent a major part of this thesis and focus on the isotopic fractionation of N2O in its stratospheric sink reactions, i.e. ultraviolet photolysis and reaction with electronically excited oxygen atoms, O(1D). These processes are of dominant influence for the isotopic composition of atmospheric N2O. Parameters of potential importance such as temperature and pressure variations as well as wavelength changes in case of UV photolysis were considered. Photolysis at stratospherically relevant wavelengths > 190 nm invariably showed enrichments in 15N at both nitrogen atoms of the residual N2O as well as in 17O and 18O. Enrichments were significantly larger for the central N atom than for the terminal N (with intermediate values for 18O) and increased towards longer wavelengths and colder temperatures. For the first time, isotopic depletions were noted for 18O and 15N at the terminal nitrogen site in N2O photolysis at 185 nm. In contrast, the second important N2O sink, reaction with O(1D), causes comparatively smaller isotopic enrichments in stratospheric N2O. However, its position-dependent fractionation pattern is directly opposite to the one in photolysis corresponding to larger enrichments at the terminal N atom. Hence, both sink processes leave distinct isotope signatures in stratospheric N2O. Further N2O photolysis experiments showed that 15N216O is most likely to be present in the atmosphere at its statistically dictated abundance.

Small stratospheric samples required adaptation of mass-spectrometric methods to continuous-flow techniques which were also used for measurements on firn air samples from two Antarctic locations. The firn air "archive" allowed to determine present trend and pre-industrial values of the tropospheric N2O isotope signature. A global N2O isotope budget constructed therefrom is in line with current best estimates of total N2O emissions from soils and oceans.

17O measurements confirmed the presence of an oxygen isotope anomaly in atmospheric N2O, but also showed that N2O photolysis enriches oxygen isotopes according to a mass-dependent fractionation law. A tropospheric origin for part of the 17O excess was proposed due to reaction of NH2 with NO2 which transfers the oxygen isotope anomaly of O3 via NO2 to N2O.