| Summary: | We have performed high-precision measurements of the 18 O and position dependent 15 N isotopic composition of N 2 O from Antarctic firn air samples. By comparing these data to simulations carried out with a firn air diffusion model, we have reconstructed the temporal evolution of the N 2 O isotope signatures since pre-industrial times. The heavy isotope content of atmospheric N 2 O is presently decreasing for all signatures at rates of about -0.038 %o yr -1 for 1 d 15 N, -0.046 %o yr -1 for 2 d 15 N and -0.025 %o yr -1 for d 18 O. The total decrease since pre-industrial times is estimated to be about -1.8%o for 1 d 15 N at both positions and -2.2%o for 2 d 15 N. Isotope budget calculations using these trends and recent stratospheric measurements allow to isotopically characterize the present and the pre-industrial global average N 2 O source, as well as the additional N 2 O emissions that have caused the global N 2 O increase since pre-industrial times. The increased fluxes from the depleted surface sources alone are insufficient to explain the inferred temporal isotope changes. In addition, the global average N 2 O source signature is calculated to be significantly depleted today relative to the pre-industrial value, in agreement with recent indications from soil emission measurements.
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