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

High‐precision nitrous oxide (N 2 O) 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 N 2 O 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 δ 15 N (the average of both nitrogen positions) and (−0.021 ± 0.003)‰/yr for δ 18 O. The individual trends for the terminal (position 1) and central (position 2) nitrogen atoms within the N 2 O molecule are (−0.064 ± 0.016)‰/yr for 1 δ 15 N and (−0.014 ± 0.016)‰/yr for 2 δ 15 N. The average 15 N and 18 O trends compare well with recent results from measurements on air extracted from polar firn and ice, confirming earlier estimates that isotopically depleted N 2 O, mainly from soil emissions, is responsible for a large fraction of the observed N 2 O increase in the atmosphere. The position‐dependent 15 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.