Ratios of fractionation constants

Other potentially useful diagnostics in understanding the role of physical and chemical effects on the stratospheric fractionation constant are the ratios h = e450/e540 and y = i5ebuik/e448 (Röckmann et al., 2001a; Kaiser et al., 2002a). Although mixing processes alter the absolute magnitude of the fractionation constants, they should conserve the relative magnitudes. Therefore, any process that changes either h or y within the stratosphere should be chemical and not physical in nature (Röckmann et al., 2001a; Kaiser et al., 2002a).

These ratios are calculated for the datasets in Table 14.2. Although there are differences in h and y between the lower and upper regions, in a reanalysis of the data, Park et al. (2004) showed that only the difference in h computed from the Röckmann et al. (2001a) data is statistically significant. Since temperature and actinic flux contributions are shown to have minor impacts on £strat, we can investigate what this behaviour of h means for the relative contribution of the O(1D) sink.

The ratio h has the potential to track changes in the relative contributions of the photolysis and O(1D) sinks because each process fractionates N2O differently. Specifically, e540 is greater than e450 in the O(1D) reaction but is larger than e450 in photolysis. With the relative contributions of these reactions to the total sink, h should range from ~0.5 (only O(1D) sink) to 2.5 (only photolysis) (Kaiser et al., 2002a). To explain the Röckmann et al. h value would require that 00% of the N2O sink in the lower stratosphere is due to reaction with O(1D), and 30% of the overall sink is due to O(1D). This result qualitatively agrees with standard chemistry estimates, which suggest that the O(1D) sink peaks in the lower regions, but overestimates its total strength. The standard view is that the O(1D) sink is about 10% of the total. It is unlikely then that the variation in h can be explained solely by the relative strengths of N2O sinks.

Additional analysis by Park et al. (2004) shows that h calculated from the Röckmann et al. data is larger in the mid-latitudes than the subtropics. This may suggest that the difference is due to second-order transport effects that do not cancel in the ratios.

In summary it appears that known dynamical and chemical processes can adequately explain the observed variations in the stratospheric fractionation with mixing playing the most significant role. The altitudinal dependence of the relative sink strengths likely contributes secondary effects to the variability, whereas marginal effects may be due to small shifts in the effective wavelength of the photolysing radiation.

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