Besides physical processes, changes in temperature and actinic flux can potentially cause £strat to vary throughout the stratosphere. Photolytic experiments indicate that the magnitude of £ decreases as temperatures increase (Kaiser et al., 2002b). Since the temperature of the stratosphere increases by ~50°C from the lower to upper stratosphere, this temperature effect should produce higher fractionation constants in the lower stratosphere. This is not observed, which suggests that temperature effects are overwhelmed by other processes.
Actinic flux varies with altitude in the stratosphere in large part due to the changes in ozone concentration. Ozone-mixing ratios peak around 20-30 km and fall off with altitude. Since ozone strongly absorbs UV radiation at the red end of N2O's absorption spectrum, actinic flux will shift to longer wavelengths in the upper stratosphere. Both theoretical and experimental studies show that the enrichment of the heavy N2O species increases with longer wavelengths as differences between their absorption cross sections and 446 s increase. Rahn et al. (1998)
and Turatti et al. (2000) found that e456, e546 (15ebulk for Rahn et al.) and e448 nearly triple in absolute size from 193 to 207 nm. To account for the full increase in estrat from the lower to the upper stratosphere, the effective wavelength peak of the actinic flux would need to shift by 10 nm (Toyoda et al., 2001). This is outside the range of expected actinic flux shifts. In addition, there is little variation in measured fractionation constants when the photolysing actinic flux is changed appreciably in laboratory experiments (Rockmann et al., 2001a).
Fractionation constants may vary if the relative contribution of photolysis and O(1D) reaction to the total nitrous sink is variable throughout the stratosphere. The fraction-ation of N2O is much less for its reaction with O(1D) than for photolysis. We therefore expect estrat to reflect changes in the relative strengths of these processes. If both sink processes follow Rayleigh fractionation, for a region of the stratosphere where photolysis accounts for x fraction of the total sink, the combined sink would produce fractionation given by esink =
xephotolysis + (1 -x)eoxidation (Toyoda et ^ 2001,
2004). From Table 14.2 the average photolysis fractionations in the lower stratosphere are -22%, -9.6% and -13% for e456, e546 and e448, respectively, and in the mid-to-upper stratosphere are -35%, -15% and -22%. If we use the broadband photolysis fraction-ations of Rockmann et al. (2001a) and the O(1D) reaction fractionations of Kaiser et al. (2002a), the derived photolysis fraction x ranges from 4% to 40% in the lower stratosphere, and from 45% to 65% in the upper stratosphere. Although the trend is correct - photo-oxidation should be more important in the lower stratosphere - the fractions are too low compared with standard chemistry, which predicts that photolysis should be 90% of the total stratospheric sink. This indicates that photo-oxidation alone cannot explain the altitudinal increase in the fractionation.
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