Intramolecular ¹⁵N and¹⁸O fractionation in the reaction of N₂O with O(¹D) and its implications for the stratospheric N₂O isotope signature

Atmospheric nitrous oxide (N₂O) 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 N₂O sink mechanisms, i.e., photolysis and reaction with O(¹D). Most of the previous studies on the cause of isotopic enrichment in N₂O have focused on photolysis. Here we present results on the ¹⁸O and the positionresolved ¹⁵N kinetic isotope effects in the reaction of nitrous oxide with O(¹D) obtained by recently developed mass spectrometric techniques. Just as in the photolysis sink, a heavy isotope enrichment in the residual N_2O 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 N2O destruction. The fractionation constant for the terminal nitrogen atom, ¹⁵ε₁, = k(¹⁴N₂O)/k( ¹⁵N¹⁴NO) - 1 = (8.87 ± 0.15) %o, is larger than for the central nitrogen atom, ¹⁵ε₂ = k( ¹⁴N₂O)/k(¹⁴N¹⁵NO) - 1 = (2.22 ± 0.12) %o (all errors are 2σ). The fractionation constant for oxygen, 18ε = k(N₂¹⁶O)/k(N₂¹⁸O) - 1 = (12.38 ± 0.14) %o, 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(¹D) sink at lower stratospheric altitudes could probably explain the lower ratio of ^l5ε₂/ ¹⁵ε₁ (≡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 N₂O loss at lower altitudes could be from the reaction with 0(D).