Intramolecular 15 N and 18 O fractionation in the reaction of N 2 O with O( 1 D ) and its implications for the stratospheric N 2 O isotope signature

Atmospheric nitrous oxide (N 2 O) is enriched in heavy oxygen and nitrogen isotopes relative to its tropospheric sources. This enrichment is traced back to kinetic isotope effects in the two stratospheric N 2 O sink mechanisms, i.e., photolysis and reaction with O( 1 D ). Most of the previous studies on the cause of isotopic enrichment in N 2 O have focused on photolysis. Here we present results on the 18 O and the position‐resolved 15 N kinetic isotope effects in the reaction of nitrous oxide with O( 1 D ) obtained by recently developed mass spectrometric techniques. Just as in the photolysis sink, a heavy isotope enrichment in the residual N 2 O was found, but of smaller magnitude. However, the fractionation pattern of nitrogen isotopes at the two nonequivalent positions in the molecule is clearly distinct from that in photolytic N 2 O destruction. The fractionation constant for the terminal nitrogen atom, 15 ε 1 = k ( 14 N 2 O)/ k ( 15 N 14 NO) − 1 = (8.87 ± 0.15) %, is larger than for the central nitrogen atom, 15 ε 2 = k ( 14 N 2 O)/ k ( 14 N 15 NO) − 1 = (2.22 ± 0.12) % (all errors are 2σ). The fractionation constant for oxygen, 18 ε = k (N 2 16 O)/ k (N 2 18 O) − 1 = (12.38 ± 0.14) %, was found to be larger than for nitrogen and amounts to about twice the value from the single previous determination. The larger influence of the O( 1 D ) sink at lower stratospheric altitudes could probably explain the lower ratio of 15 ε 2 / 15 ε 1 (≡η) observed there, which is shown to be only marginally influenced by transport. The published data on stratospheric η values suggest that, if there are no other chemical reactions involved, up to 60% of the overall N 2 O loss at lower altitudes could be from the reaction with O( 1 D ).