High-precision determination of the changing isotopic composition of atmospheric N2O from 1990 to 2002

Abstract

High-precision nitrous oxide (N2O) concentration and isotope ratio measurements have been carried out on archived air samples from the Antarctic station Neumayer covering the period 1990-2002. The results show that the increase in the N2O mixing ratio over this period is accompanied by a significant decrease in the heavy isotope content. The temporal isotope trends amount to (-0.040 ± 0.003)‰/yr for δ15N (the average of both nitrogen positions) and (-0.021 ± 0.003)‰/yr for δ 18O. The individual trends for the terminal (position 1) and central (position 2) nitrogen atoms within the N2O molecule are (-0.064 ± 0.016)‰/yr for 1δ 15N and (-0.014 ± 0.016)‰/yr for 2δ15N. The average 15N and 18O trends compare well with recent results from measurements on air extracted from polar fim and ice, confirming earlier estimates that isotopically depleted N2O, mainly from soil emissions, is responsible for a large fraction ofthe observed N2O increase in the atmosphere. The position-dependent 15N determinations show a strong difference between the two positions. This is in disagreement with the firn and ice core data, which imply similar fractionations at both positions. Copyright 2005 by the American Geophysical Union.