Constraints on N2O budget changes since pre-industrial time from new firn air and ice core isotope measurements S. Bernard1, T. Röckmann2,3, J. Kaiser2,4, J.-M. Barnola1, H. Fischer5, T. Blunier6, and J. Chappellaz1 1Laboratoire de Glaciologie et Géophysique de l’Environnement (CNRS-UJF), St Martin d’Hères, France 2Max-Planck Institute for Nuclear Physics, Heidelberg, Germany 3Institute for Marine and Atmospheric Research Utrecht, Utrecht University, Utrecht, The Netherlandsed 4Department of Geosciences, Princeton University, Princeton, New Jersey, USA 5Alfred-Wegener Institute for Polar and Marine Research, Bremerhaven, Germany 6Climate and Environmental Physics Institute, University of Bern, Bern, Switzerland
Abstract. 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.
Citation: Bernard, S., Röckmann, T., Kaiser, J., Barnola, J.-M., Fischer, H., Blunier, T., and Chappellaz, J.: Constraints on N2O budget changes since pre-industrial time from new firn air and ice core isotope measurements, Atmos. Chem. Phys., 6, 493-503, doi:10.5194/acp-6-493-2006, 2006.