Using Isotopic Fingerprints to Trace Nitrous Oxide in the Atmosphere.

submitted for GGMT-2017 Using Isotopic Fingerprints to Trace Nitrous Oxide in the Atmosphere Joachim Mohn, Béla Tuzson, Eliza Harris, Stephan Henne, Benjamin Wolf, Erkan Ibraim, Longfei Yu, Christoph Zellweger, Lukas Emmenegger 1 Empa, Laboratory for Air Pollution / Environmental Technology, CH-8600 Dübendorf, Switzerland; [email protected] 2 University of Innsbruck, Institute of Ecology, Plant, Soil and Ecosystem Processes Research Group, A-6020 Innsbruck, Austria 3 Karlsruhe Institute of Technology, Institute for Meteorology and Climate Research (IMK-IFU), D-82467 Garmisch-Partenkirchen, Germany Nitrous oxide (N2O) is a potent greenhouse gas and the strongest ozone-destroying substance emitted this century. Reliable predictions of future emissions, requires knowledge of the responsible N2O source processes. Isotopic composition of N2O is a tracer to distinguish between different emission pathways, as well as constraining the stratospheric N2O sink. The four most abundant N2O isotopic species are: NNO (99%), NNO (α, 0.4%), NNO (β, 0.4%) and NNO (0.2%). Due to its asymmetric molecular structure, N2O (α) and N2O (β) differ only in the position of the N atom, and the difference in their abundance – known as site preference (SP) – is a particularly powerful indicator for different N2O production mechanisms. Here we illustrate the potential of laser spectroscopy for real-time, high-precision analysis of isotopic composition in ambient N2O. Furthermore, we present applications, in agricultural as well as suburban environments, illustrating the advantage and necessity of real-time data of trace gas isotope ratios. In an extensive campaign above a managed grassland, nitrifierdenitrification and denitrification were identified as prevalent sources of N2O and variations in isotopic composition were attributed to alterations in the extent to which N2O was reduced to N2 [1]. In an ongoing project, we validate the real-time N2O isotope data against a process-based biogeochemical soil model (DNDC) with an isotope sub-module (SIMONE), which is based on published isotope effects [2]. At a suburban site, the isotopic composition of atmospheric N2O was monitored over 18 months to determine the source isotopic composition, which varied significantly compared to chemical and meteorological parameters. FLEXPART-COSMO transport modelling in combination with modified EDGAR inventory emissions was able to capture variability in N2O mole fraction well, but simulations of isotopic composition showed little agreement with observations, indicating that the range of literature values of isotopic source signatures significantly underestimates the true variability [3]. In summary, we are convinced that real-time analysis of N2O isotopic composition is an efficient approach to disentangle N2O source / sink processes in agricultural as well as suburban / industrial environments. Combination of point measurements with modelling approaches provides spatial resolution and enables validation of emission inventories.