Variation of Fractionation Constants Throughout the Stratosphere

Vertical profiles of N2O isotopes in the stratosphere reveal that not only does the enrichment of the heavy species increase with altitude, but the fractionation constants of these species increase as well (Griffith et al., 2000; Rockmann et al., 2000; Toyoda et al., 2001; Park et al., 2004; Toyoda et al., 2004). The former observation is understood by the simple recognition that N2O undergoes continuous fractionation through photolysis and reaction with O(1D) as it moves from the lower to the upper layers of the stratosphere. As a result, air in the upper stratosphere is isotopically older and is more enriched in the heavy species relative to fresh air entering from the troposphere.

The latter observation that fractionation constants increase with altitude (i.e. they become more negative) is difficult to explain. That they do is clearly seen in the isotopic measurements in Fig. 14.8 and is evident from the summary of stratospheric measurements in Table 14.2. In general, fractionation constants are on an average approximately twice as large in the upper stratosphere as in the lower stratosphere. The altitude at which this transition occurs is not clearly delimited but rather falls in a band between 18 and 26 km (Toyoda et al., 2004). In addition, fractionation constants measured in the laboratory are consistently larger than those in the lower stratosphere and agree approximately with those measured in the middle-to-upper stratosphere. In the following sections we explore a number of both physical and chemical factors that account for the observed behaviour of e.

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