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

[1] 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 dN (the average of both nitrogen positions) and ( 0.021 ± 0.003)%/yr for dO. The individual trends for the terminal (position 1) and central (position 2) nitrogen atoms within the N2Omolecule are ( 0.064± 0.016)%/yr for dN and ( 0.014± 0.016)%/yr for dN. The average N and O trends compare well with recent results from measurements on air extracted frompolar firn and ice, confirming earlier estimates that isotopically depletedN2O, mainly from soil emissions, is responsible for a large fraction of the observedN2O increase in the atmosphere. The position-dependent N 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.