High-precision determination of the changing isotopic composition of atmospheric N2O from 1990 to 2002 -: art. no. D21304

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)parts per thousand/yr for delta N-15 (the average of both nitrogen positions) and (-0.021 +/- 0.003)parts per thousand/yr for delta O-18. The individual trends for the terminal (position 1) and central (position 2) nitrogen atoms within the N2O molecule are (-0.064 +/- 0.016)parts per thousand/yr for (1)delta N-15 and (-0.014 +/- 0.016)%/yr for (2)delta N-15. The average N-15 and O-18 trends compare well with recent results from measurements on air extracted from polar firn and ice, confirming earlier estimates that isotopically depleted N2O, mainly from soil emissions, is responsible for a large fraction of the observed N2O increase in the atmosphere. The position-dependent N-15 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.