The isotopic fingerprint of the pre-industrial and the anthropogenic N₂O source

Abstract. We have performed high-precision measurements of the ¹⁸O and position dependent ¹⁵N isotopic composition of N₂O 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 N₂O isotope signatures since pre-industrial times. The heavy isotope content of atmospheric N₂O is presently decreasing for all signatures at rates of about -0.038 %o yr ^-1 for ¹d¹⁵N, -0.046 %o yr ^-1 for ²d ¹⁵N and -0.025 %o yr ^-1 for d¹⁸O. The total decrease since pre-industrial times is estimated to be about -1.8%o for ¹d¹⁵N at both positions and -2.2%o for ²d¹⁵N. Isotope budget calculations using these trends and recent stratospheric measurements allow to isotopically characterize the present and the pre-industrial global average N₂O source, as well as the additional N₂O emissions that have caused the global N₂O 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 N₂O source signature is calculated to be significantly depleted today relative to the pre-industrial value, in agreement with recent indications from soil emission measurements.