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Figure 5.12 Cross-section of Cl I, from Figure 5.7. Solid arrows indicate the effects of the Brewer-Dobson circulation: lifting the isopleths up in the tropics and pushing the isopleths down in the extratropics. Dashed arrows indicate the isopleth-flattening effect of quasi-horizontal eddy transport.

abundance of CH.t decreases. Eddy processes, on the other hand (dashed arrows in Figure 5.12), transport constituents quasi-horizontally and attempt to remove any horizontal gradients. In other words, eddy processes tend to flatten the isopleths, opposing the isopleth-steepening effects of the Brewer-Dobson circulation. The actual slope of the isopleth observed in the atmosphere is a result of the competition between these two processes.

As we have already said, air enters the stratosphere primarily through the tropical tropopause. By a combination of the Brewer-Dobson circulation and quasihorizontal eddy transport, the air then moves poleward and eventually exits the stratosphere by descending back across the tropopause at mid- and high latitudes. As the air travels through the stratosphere, the abundances of Clv, Brv, and NO,, build up as the source molecules for these species (CFCs, halons, and N,0) are photolyzed. The abundance of CI,., Br , and NO, in an air parcel is therefore determined by the total exposure of the air parcel to radiation and oxidizing species [ 146].

The mean age of air in the stratosphere is shown in Figure 5.13, where age is defined as the number of years since the air crossed the tropopause. In general, the air in the stratosphere is a few years old, with the average age increasing with both altitude and latitude in a way that is consistent with the general 2D circulation. "Age of air" is actually a fairly complex subject; it is discussed more thoroughly elsewhere [82,189],

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