The stratosphere

The stratosphere is the layer of our atmosphere that is between ~10 and 50 km above the Earth's surface. Butenhoff and Khalil (Chapter 14, this volume) describe how most N2O is removed from the atmosphere by direct photolysis by sunlight in the stratosphere, with ~10% being removed by chemical reaction with the excited oxygen atom O(1D). This latter reaction is also of significance since it is a major source of N2O to the stratosphere that plays a significant role in the catalytic removal of stratospheric ozone. The photolysis of N2O peaks at wavelengths between 195 and 205 nm and at altitudes between 25 and 35 km. Together, photolysis and the O(1D) reaction remove ~12-13 Tg N2O/year from the atmosphere.

The isotopic composition of N2O in the stratosphere can be used to constrain estimates of N2O source fluxes. Photolysis preferentially removes the light molecule 14N14N16O from the stratosphere, leaving the remaining N2O enriched in the heavy isotopically substituted species 14N15N16O,

15N14N16O, 14N14N17O and 14N14N18O. The enrichment of these species increases through the stratosphere as the air ages. Back-flux of mass from the lower stratosphere to the troposphere enriches the isotopic composition of tropospheric N2O, which balances the flux of light N2O from surface sources. Successful efforts at modelling the fractionation processes of N2O reveal that the dynamical mixing of tropo-spheric air into the lower stratosphere decreases the fractionation constants in the lower stratosphere, though dependencies of the fractionation on temperature and wavelength also contribute to a certain extent.

Although some outstanding questions remain about proposed exotic sinks of N2O, as well as the origin of the oxygen isotope anomaly, our understanding of the stratospheric sinks of N2O is essentially complete. Detailed examination of the rates of these removal processes is important since, coupled with observations of the rates of change in atmospheric concentration of the gas, it allows the rates of emission of N2O from the Earth's surface to be constrained, and hence future concentrations and the resultant radiative forcing to be estimated.

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