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FIGURE 12.1 Typical vertical ozone profiles at Edmonton, Alberta, Canada, in the January-April period for 1980-1982 (solid line) and 1993 (dashed line) (adapted from Kerr et al., 1993).

FIGURE 12.2 UV absorption of 03 at room temperature in the Hartley (200-300 nm) and Huggins (300-360 nm) bands (adapted from Daumont et al, 1992).

FIGURE 12.2 UV absorption of 03 at room temperature in the Hartley (200-300 nm) and Huggins (300-360 nm) bands (adapted from Daumont et al, 1992).

of hours to days, whereas the time scale for similar mixing in the stratosphere is months to years.

Figure 12.3 schematically depicts large, global-scale motions that transport tropospheric air into the stratosphere in the tropics. This has been described recently in terms of a "global-scale fluid dynamical suction pump" (Holton et al., 1995), which slowly moves air from the troposphere into the stratosphere in the tropics. This transport is what largely controls the tropospheric lifetimes of species such as the CFCs and N20 that have no significant tropospheric chemical sinks. As a result of this motion, the mixing ratios of such species in the stratosphere peak in the tropical regions and are approximately the same as their tropospheric mixing ratios. This large-scale upward motion in the tropics and downward motion toward the poles is what is known as Hadley circulation.

As air is transported rapidly upward, for example in a convective system, cooling occurs (see Chapter 2), leading to the condensation of water as ice crystals. Because of this removal of water as moist tropospheric air rises, air in the stratosphere is very dry, of the order of a few ppm. Some water is also produced directly in the stratosphere from the oxidation of CH4 and H2. The so-called extratropical pump then moves the air poleward and downward at higher latitudes (Path I), warming the air as it descends.

Using the terminology of Hoskins (1991), Holton and co-workers (1995) distinguish between the "over-

Path I

Extratropical

--Overworld

Pump

i^i , Large-scale ascent

Large-scale i i subsidence ri

Lowe'most_ __ ""__1 "" ' 11 Cumulonimbus

Stratosphe-e clouds

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