Chemistry Of The Unperturbed Stratosphere

Figure 12.1 shows typical average vertical ozone profiles measured at Edmonton, Alberta, Canada, in the January-April period in 1980-1982 and in 1993, respectively. The high concentrations of ozone in the stratosphere are evident, demonstrating why this region is often referred to colloquially as the "ozone layer." (The significant decrease in stratospheric ozone from the period 1980-f982 to 1993 will be discussed below. While a small decrease in tropospheric ozone is seen in these particular data, ozone levels have generally increased in the troposphere; see Chapter 14.)

The total ozone integrated through a column in the atmosphere from the earth's surface is often used as a measure of stratospheric ozone, since as seen in Fig. 12.1, about 85-90% of the total ozone is found in this region. Dobson units are used to express the amount of total column ozone. One Dobson unit (DU) is the height of the layer of pure gaseous ozone in units of 10"5 m that one would have if one separated all of the atmospheric 03 and compressed it into a layer at 1 atm and 273 K. That is, 100 DU is equivalent to a layer of pure ozone of thickness of 1 mm.

The importance of this high concentration of ozone in the stratosphere resides in its effects on radiation and associated feedbacks. Figure 12.2 shows the ozone absorption spectrum, which was discussed in detail

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