The origin of anomalous or "mass-independent" oxygen in isotope fractionation in tropospheric N₂O

Analysis of the complete oxygen isotopic composition (¹⁶O, ¹⁷O, ¹⁸O) of tropospheric N₂O from various northern hemispheric locations reveals a mass independent anomaly with a ¹⁷O excess of Δ¹⁷O=1.0 ± 0.2‰. at δ¹⁸O=20.7 ± 0.3‰. So far, the origin of this intriguing isotope signature has remained elusive. New laboratory experiments demonstrate that the fractionation during UV photolysis of N₂O, which causes ¹⁵N and ¹⁸O enrichments in the stratosphere, is strictly mass dependent (Δ¹⁷O=0). To explain the isotope anomaly in atmospheric N₂O, we propose a chemical mechanism for heavy oxygen transfer from O₃ to N₂O. In a first step, the NO_x-O₃ photochemical interaction leads to the formation of NO₂ with significant excess ¹⁷O. In a second step, the heavy oxygen anomaly is transferred to N₂O via the reaction NO₂+NH₂→N₂O+H₂O, as part of the gas phase degradation of ammonia. This small but significant N₂O source is of the right magnitude to explain the tropospheric observations.