Within the range of the estimated source fluxes of N2O the global budget can be considered closed (Kroeze et al., 1999; Prather and Ehhalt, 2001). In addition, the observed atmospheric vertical profile of N2O agrees well with modelled predictions based upon known sources and sinks. With our current knowledge, there is no need for additional sources or sinks, though such processes could exist if their fluxes were small (Rahn and Wahlen, 2000). A number of new reactions have been proposed in recent years to close the N2O budget (Prasad, 1994, 1997; Prasad et al., 1997).
Most relevant to this chapter are the proposed reactions of N2O with vibra-tionally excited molecular oxygen:
and with electronically excited oxygen:
O2(b1X) + N2O ^ O3 + N2 (14.11a) ^ NO + NO2 (14.11b)
In the absence of kinetic rate coefficients, it is difficult to assess the significance of these reactions although McElroy and Jones (1990) rule out Eqs 14.10b and 14.11a on quantum mechanical grounds, and feel that Eq. 14.10a is not likely since quenching of the excited oxygen molecule by N2 may dominate. Prasad et al. (1997) show that if the above reactions are considered along with other new source reactions proposed by Prasad (1997), they retain the vertical structure of N2O as predicted by standard chemistry alone. Therefore, these reactions are consistent with the observations.
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