Atmospheric Chemistry and Physics Discussions www.atmos-chem-phys-discuss.net 1680-7367 1680-7375 5 4 2005 10.5194/acpd-5-7547-2005 http://www.atmos-chem-phys-discuss.net/5/7547/2005/ http://www.atmos-chem-phys-discuss.net/5/7547/2005/acpd-5-7547-2005.html http://www.atmos-chem-phys-discuss.net/5/7547/2005/acpd-5-7547-2005.pdf 7547 7575 2005-08-29 Constraints on N₂O budget changes since pre-industrial time from new firn air and ice core isotope measurements S. Bernard T. Röckmann J. Kaiser J.-M. Barnola H. Fischer T. Blunier J. Chappellaz Laboratoire de Glaciologie et Géophysique de l’Environnement (CNRS-UJF), St Martin d’Hères, France Max-Planck Institute for Nuclear Physics, Heidelberg, Germany Department of Geosciences, Princeton University, Princeton, New Jersey, USA Alfred-Wegener Institute for Polar and Marine Research, Bremerhaven, Germany Climate and Environmental Physics Institute, University of Bern, Bern, Switzerland now at: Institute for Marine and Atmospheric Research Utrecht, Utrecht University, Utrecht, The Netherlands A historical record of changes in the N₂O isotope composition is important for a better understanding of the global N₂O atmospheric budget. Here we have combined measurements of trapped gases in the firn and in ice cores of one Arctic site (North GReenland Ice core Project – NGRIP) and one Antarctic site (Berkner Island). We have performed measurements of the ¹⁸O and position dependent ¹⁵N isotopic composition of N₂O. 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 decrease observed for all signatures is consistent from one pole to the other. Results obtained from the air occluded in the ice suggest a decrease of about −2.8‰, −2.4‰, −3.2‰ and −1.6‰ for δ¹⁵N, ¹δ¹⁵N, ²δ¹⁵N and δ¹⁸O, respectively, since 1700 AD. Firn air data imply a decrease of about −1.1‰, −1.2‰, −1.0‰ and −0.6‰ for δ¹⁵N, ¹δ¹⁵N, ²δ¹⁵N and δ¹⁸O, respectively, since 1970 AD. These results imply consistent trends from firn and ice measurements for δ¹⁵N and δ¹⁸O. The trends for the intramolecular distribution of ¹⁵N are less well constrained than the bulk ¹⁵N trends because of the larger experimental error for the position dependent ¹⁵N measurements. The decrease in the heavy isotope content of atmospheric N₂O can be explained by the increasing importance of agriculture for the present atmospheric N₂O budget.