Measurement of the isotopic fractionation of¹⁵N¹⁴N¹⁶O,¹⁴N¹⁵N ¹⁶O and¹⁴N¹⁴N¹⁸O in the UV photolysis of nitrous oxide

The isotopic analysis of atmospheric nitrous oxide (N₂O) has become a valuable tool in the investigation of its sources, sinks, and its atmospheric cycle. In particular the considerable isotopic enrichment accompanying stratospheric photolysis of N₂O, its dominant atmospheric sink process, provides a key isotope signal in the construction of a global N₂O isotope budget. Here we present the first measurements of the individual fractionation constants for ¹⁵N¹⁴NO, ¹⁵ε₁ = 10.9±1.7‰ and ¹⁴N¹⁵NO, ¹⁵ε₂ = 35.7±0.5‰ during ultraviolet photolysis at 193 nm, along with the ¹⁸O fractionation constant, ¹⁸ε = 17.3±0.5‰. Consistent results were obtained over a wide range of experimental conditions. The observed position-dependent ¹⁵N fractionation confirms theoretical predictions and provides a unique signature of N₂O that has been processed in the stratosphere, adding a new dimension to an isotope-based description of the atmospheric N₂O budget.