Source fluxes remain the greatest uncertainty in the global budget of N2O. Emissions from N2O's primary sources - soil, ocean and agricultural practices - are variable over small spatial and temporal scales, and depend on many factors such as temperature, soil moisture and soil type (Prather and Ehhalt, 2001). Since these sources have unique isotopic signatures their fluxes can be constrained if the other contributions to the isotopic budget are known. One element of this budget is the isotopic flux from the stratosphere to the troposphere. The net isotopic flux reflects differences in the iso-topic composition of the troposphere and stratosphere. Specifically it is the isotopic composition of the lower stratosphere that is relevant since this is the source region of the stratosphere-troposphere exchange.
Through a series of reasonable approximations and assumptions (Park et al., 2004), the isotopic flux FST is related to the net loss of N2O in the stratosphere L and the frac-tionation constant for the isotopologue of interest e (McLinden et al., 2003; Park et al., 2004) by:
where ¿T and 5S are representative 5 values for the troposphere and lower stratosphere. Estimates of the isotopic flux from both McLinden et al. (2003) and Park et al. (2004) are shown in Table 14.3. Both analyses agree in spite of some error, and the bulk of any difference is due to the use of different fractionation constants. Those used by McLinden et al. are mass flux-weighted model averages, whereas Park et al. used values based on measurements in the lower stratosphere at high latitudes. As Kim and Craig (1993) first suggested, exchange between the stratosphere and the troposphere is a significant contributor to the isotopic composition of the troposphere. Calculations show that tropospheric ¿15Nbulk and 518O increase from 5%o to 25%o and from 1%> to 17% respectively, if the contribution from the stratospheric isotopic flux is considered (Park et al., 2004).
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