Constraints on N2O budget changes since pre-industrial time from new firn air and ice core isotope measurements

[摘要] A historical record of changes in the N₂O isotope composition isimportant for a better understanding of the global N₂O atmosphericbudget. Here we have combined measurements of trapped gases in the firn andin ice cores of one Arctic site (North GReenland Ice core Project - NGRIP)and one Antarctic site (Berkner Island). We have performed measurements ofthe ¹⁸O and position dependent ¹⁵N isotopic composition ofN₂O. By comparing these data to simulations carried out with a firn airdiffusion model, we have reconstructed the temporal evolution of theN₂O isotope signatures since pre-industrial times. The decreaseobserved for all signatures is consistent from one pole to the other.Results obtained from the air occluded in the ice suggest a decrease ofabout -2.8, -2.4, -3.2 and -1.6 for δ¹⁵N,¹δ¹⁵N, ²δ¹⁵N and δ¹⁸O, respectively, since1700 AD. Firn air data imply a decrease of about -1.1, -1.2, -1.0 and-0.6 for δ¹⁵N, ¹δ¹⁵N, ²δ¹⁵Nand δ¹⁸O, respectively, since 1970 AD. These resultsimply consistent trends from firn and ice measurements for δ¹⁵Nand δ¹⁸O. The trends for the intramolecular distribution of ¹⁵N are less wellconstrained than the bulk ¹⁵N trends because of the larger experimentalerror for the position dependent ¹⁵N measurements. The decrease in theheavy isotope content of atmospheric N₂O can be explained by theincreasing importance of agriculture for the present atmospheric N₂O budget.

[学科分类] 大气科学