Constraints on N 2O budget changes since pre-industrial time from new firn air and ice core isotope measurements
A historical record of changes in the N 2O isotope composition is important for a better understanding of the global N 2O 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 N 2O. By comparing these data to simulations carried out with a firn air diffusion model, we have reconstructed the temporal evolution of the N 2O 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 δ 15N, 1δ 15N, 2δ 15N and δ 18O, respectively, since 1700 AD. Firn air data imply a decrease of about -1.1, -1.2, -1.0 and -0.6 for δ 15N, 1δ 15N, 2δ 15N and δ 18O, respectively, since 1970 AD. These results imply consistent trends from firn and ice measurements for δ 15N and δ 18O. 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 N 2O can be explained by the increasing importance of agriculture for the present atmospheric N 2O budget.