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

Analysis of the complete oxygen isotopic composition (O-16, O-17, O-18) Of tropospheric N2O from various northern hemispheric locations reveals a mass independent anomaly with a O-17 excess of Delta O-17=1.0+/-0.2 parts per t housand at delta O-18=20.7+/-0.3 parts per thousand. So far, the origin of this intriguing isotope signature has remained elusive. New laboratory expe riments demonstrate that the fractionation during UV photolysis of N2O, whi ch causes N-15 and O-18 enrichments in the stratosphere, is strictly mass d ependent (Delta O-17=0). To explain the isotope anomaly in atmospheric N2O, we propose a chemical mechanism for heavy oxygen transfer from O-3 to N2O. In a first step, the NOx-O-3 photochemical interaction leads to the format ion of NO2 with significant excess O-17. 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 sou rce is of the right magnitude to explain the tropospheric observations.