Recommended rate constants for Eqs 14.3a and 14.3b are 6.7 x 10-11 and 4.9 x 10-11 cm3/molecule/s respectively, and produce a branching ratio (R = k3a/k3) of 0.57 (Sander et al., 2003). This ratio is slightly lower than the average branching ratio of 0.61 ± 0.06 calculated from relevant measurements since 1957 (Cantrell et al., 1994).
As Eq. 14.3a is a major source of odd NOX species to the stratosphere, there is concern that increased concentrations of N2O will contribute to the decline of stratospheric ozone (Crutzen, 1970). The production of nitric oxide (NO) from Eq. 14.3a peaks near the equator at an altitude of ~26 km (Johnston et al., 1979) where it reaches a maximum value of 320 molecules/cm3/s (Cantrell et al., 1994). Production is symmetric around the equator in both hemispheres during spring and fall but is biased towards the summer hemisphere during winter and summer. The global production of NO is estimated to be 1.25 Tg N/year (Cantrell et al., 1994). As ozone photolysis is the main source of O(1D), the loss of N2O by O(1D) diminishes with increasing altitude in the stratosphere.
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