Fractionation in the laboratory

Photolysis fractionation has been well studied in the laboratory and we now have a clear picture of the process (Table 14.2). The degree of fractionation depends on a number of factors, of which the most important is the isotopologue of interest. In the wavelength range 190-215 nm, where most N2O is photo-lysed, the fractionation constants follow the relation I e4561 > I e4481 > ie540!, indicating that 456 is most highly enriched. At 207 nm, for example, e450 = -00%, e448 = -49% and e540 = -27% (Turatti et al., 2000). Similar results are found at other wavelengths.

The enrichment is wavelength-dependent. While e450 = -00% at 207 nm, it falls to -27% at 193 nm (Turatti et al., 2000). A similar decrease is observed for the other isotopo-logues. These results, which are consistent throughout all published reports, indicate that the enrichment of the heavy species decreases at longer wavelengths. Because the make-up of the photolysing radiation shifts to longer wavelengths towards the top of the stratosphere, the enrichment of the heavy isotopologues should increase upwards through the stratosphere.

The above relational order of the frac-tionation factors is maintained at wavelengths down to 185 nm. At this wavelength, which is near the peak of the absorption spectrum, Kaiser et al. (2003) find that e450 = -18.5%, e540 = 3.7% and e448 = 4.5%. Not only is the relational order between 540 and 448 reversed, but these species are also depleted relative to 440, not enriched.

The variation of the fractionation constant with isotopologue and wavelength is caused by small differences between the absorption spectra of the different N2O spe cies. High-resolution spectroscopy reveals that the absorption spectra of the heavy species are blue-shifted relative to 440 by ~0.1-0.2 nm (Selwyn and Johnston, 1981). The spectrum of 450 is most highly shifted, followed by 448 and 540, respectively. Because of the shifts, the cross sections of the heavy species are reduced relative to 440 at wavelengths longer than the peak of the absorption spectrum. This reduces the rate of photolysis for the heavy species and leads to their enrichment.

At wavelengths near the absorption peak, the spectrum is highly structured and banded with many peaks. This small-scale structure produces complex variations in the ratios between cross sections as one spectrum is shifted with respect to the other. At some wavelengths the cross section of the heavy species will be larger than 440 and the heavy species will be depleted due to its faster photolysis. This explains why 540 and 448 are depleted during photolysis at 185 nm.

Laboratory experiments show that the enrichment of heavy N2O increases with decreasing temperature (Kaiser et al., 2002b). During broadband photolysis e450 changed from -75% at 190 K to -57% at 290 K, e448 changed from -45% to -37% and e540 changed from -30% to -25%. Because the temperature of the stratosphere increases with altitude, this dependence will reduce the enrichment in the upper stratosphere.

In addition to photolysis, reaction with O(1D) also fractionates N2O. Since this process is only 10% of the total N2O sink, its overall contribution to the stratospheric enrichment is small, but may be important in the lower stratosphere where the reaction peaks. In the laboratory Kaiser et al. (2002a) found that this reaction fractionates according to e448 = -12.4%, e540 = -8.9% and e450 = -2.2%. Not only are these constants lower than their respective photolysis counterparts, but the relational order is also changed: I e448 I > I e540 I > I e450 I. Thus, the sink processes have their own unique signatures and these can be used to assess the relative strengths of the sink processes in the stratosphere.

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