ACP Atmospheric Chemistry and Physics ACP 1680-7324 Copernicus GmbH Göttingen, Germany 10.5194/acp-6-493-2006 Constraints on N<sub>2</sub>O budget changes since pre-industrial time from new firn air and ice core isotope measurements Bernard S. 1 Röckmann T. 2 3 Kaiser J. 2 4 Barnola J.-M. 1 Fischer H. 5 Blunier T. 6 Chappellaz J. 1 Laboratoire de Glaciologie et Géophysique de l’Environnement (CNRS-UJF), St Martin d’Hères, France Max-Planck Institute for Nuclear Physics, Heidelberg, Germany Institute for Marine and Atmospheric Research Utrecht, Utrecht University, Utrecht, The Netherlandsed 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 16 02 2006 6 2 493 503 This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. This article is available from http://www.atmos-chem-phys.net/6/493/2006/acp-6-493-2006.html The full text article is available as a PDF file from http://www.atmos-chem-phys.net/6/493/2006/acp-6-493-2006.pdf

A historical record of changes in the N<sub>2</sub>O isotope composition is important for a better understanding of the global N<sub>2</sub>O 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 <sup>18</sup>O and position dependent <sup>15</sup>N isotopic composition of N<sub>2</sub>O. By comparing these data to simulations carried out with a firn air diffusion model, we have reconstructed the temporal evolution of the N<sub>2</sub>O 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 &delta;<sup>15</sup>N, <sup>1</sup>&delta;<sup>15</sup>N, <sup>2</sup>&delta;<sup>15</sup>N and &delta;<sup>18</sup>O, respectively, since 1700 AD. Firn air data imply a decrease of about -1.1, -1.2, -1.0 and -0.6 for &delta;<sup>15</sup>N, <sup>1</sup>&delta;<sup>15</sup>N, <sup>2</sup>&delta;<sup>15</sup>N and &delta;<sup>18</sup>O, respectively, since 1970 AD. These results imply consistent trends from firn and ice measurements for &delta;<sup>15</sup>N and &delta;<sup>18</sup>O. The trends for the intramolecular distribution of <sup>15</sup>N are less well constrained than the bulk <sup>15</sup>N trends because of the larger experimental error for the position dependent <sup>15</sup>N measurements. The decrease in the heavy isotope content of atmospheric N<sub>2</sub>O can be explained by the increasing importance of agriculture for the present atmospheric N<sub>2</sub>O budget.