In the absence of any other factors, you could expect that total ozone levels would be highest over the tropics and correspondingly lower in the polar regions because of the greater intensity of solar ultraviolet radiation in equatorial regions. Recall the ozone life-cycle reactions from section 3.0 above. UV radiation is part of the cycle. We expect that ozone levels in the stratosphere should decrease as you move toward the poles, since UV intensity decreases toward the poles. However, this is not the case. Indeed, it is actually the reverse of this that is true.

The actual distribution of ozone is not simply a balance between production and loss. Winds can transport ozone away from the production region, altering the basic distribution of ozone, and the impact of ozone on the surface UV. Note that the erythemal exposure shown in Fig. 1.03 has very high levels in the tropics. These high levels result from the direct overhead sun and the naturally low levels of ozone found in the tropics. Satellite measurements of ozone reveal these low levels in the tropics. Figure 1.05 displays this Total Ozone Mapping Spectrometer (TOMS) satellite measurements of the total amount of ozone between the surface and the top of the atmosphere (column ozone) over the course of the year (TOMS cannot measure ozone during the continuous darkness of polar night, as explained in the measurements section in Chapter 7).

Before exploring the causes of this observed ozone distribution, let's take a closer look at what the TOMS instrument is actually measuring. TOMS measures the total number of ozone molecules between the surface of Earth and the top of the atmosphere.The amount of ozone in this column is numerically expressed in Dobson Units. It is essentially a measure of the "thickness" of the ozone layer.

4.1 The Dobson Unit

The Dobson Unit (DU) is a measure of the "thickness" of the ozone layer. The column measurement can be conceptualized by imagining that all of the overhead ozone molecules (spread over the depth of the stratosphere) could be brought down to the surface (at standard temperature and pressure). This "layer" of ozone would only be about 3 millimeters (mm) thick, equivalent to the height of two stacked pennies. This amount of ozone has a Dobson Unit value of 300 DU (approximately the global average of total ozone). Chapter 3 explores the concept of the Dobson Unit and its relationship to total column ozone in more depth.

4.2 Distribution of Ozone by Season and Location

As we saw in Figure 1.05, total column ozone amounts near the equator are rather low (less than 300 DU) over the course of the year. Throughout the year, low total ozone amounts in the tropics combines with the direct overhead sun to create the very high amounts of UV erythemal exposure, as shown in Figure 1.03. The total column amount of ozone generally increases as we move from the tropics to higher latitudes in both hemispheres. However, as we shall see, the overall column amounts are greater in the northern hemisphere high latitudes than in the southern hemisphere high latitudes. In addition, while the highest amounts of column ozone over the Arctic occur in the northern spring (March-April), the opposite is true over the Antarctic, where the lowest amounts of column ozone occur in the southern spring (September-October). Indeed, the highest amounts of column ozone anywhere in the world are found over the Arctic region during the northern spring period of March and April. The amounts then decrease over the course of the northern summer. Meanwhile, the lowest amounts of column ozone anywhere in the world are found over the Antarctic in the southern spring period of September and October, owing to the ozone hole phenomenon explained in section 6.0 of this Chapter and in great detail in Chapter 11.

Ozone amounts over the continental United States (25°N to 49°N) are highest in the northern spring (April and May). These ozone amounts fall over the course of the summer to their lowest amounts in October, and then rise again over the course of the winter. Wind transport of ozone is principally responsible for the seasonal evolution of these higher latitude ozone patterns displayed in Figure 1.05.

These patterns of column ozone, however, only tell part of the story. Ozone also varies with height. The resulting picture or profile of ozone variations with height gives us information on how ozone is distributed vertically in the atmosphere at different locations. While the Dobson Unit gives us the total column amount of ozone from the surface to the top of the atmosphere, it is number density that is useful for exploring the vertical distribution. Number density is discussed in the next section, while other types of profile measurements are examined in Chapter 3.

4.3 Ozone Number Density

Density refers to the amount of material per unit of volume. The density of a mixture of molecules is typically measured in kilograms per cubic meter. The density can also be measured as a number density, or the number of molecules per cubic meter. We can also total up all of the overhead molecules to determine the number of molecules over a unit area of surface, such as a square meter. Total column ozone is just such a measure of the number of ozone molecules per square meter.

As shown in Figure 1.05, total column ozone has an average value of 300 Dobson Units (a convenient unit of measurement). This 300 DU corresponds to 8.07x1022 molecules/m2 or 6.42x10-3 kg/m2. Because we use the Dobson Unit so commonly, we also express the density versus altitude in units of DU per kilometer, where 10 DU/km is equal to 2.69x1018 molecules/m3 (number density) or 2.14x10-7 kg/m3 (density). Figure 1.06 displays satellite measured ozone number density in DU per kilometer averaged over a 10 year period. The data is based on the measurements of the Nimbus-7 SBUV instrument from 1980-1989.

The difference between Figures 1.01 and 1.06 is that the first figure shows the ozone layer near about 22 km for a "typical" midlatitude profile, while the second figure shows the variation of these profiles from the tropics to high latitudes, that is, for all latitudes.

4.4 Circulation Patterns in the Stratosphere

As discussed in section 3.0, ozone is primarily produced by solar UV radiation. This production principally occurs in the tropical upper stratosphere. We should expect, therefore, to find the greatest concentrations of stratospheric ozone in the tropics However, as we see in Figure 1.05, most of the ozone is found in the mid-to-high latitudes of the northern and southern hemispheres. This puzzle is explained by the stratospheric circulation, known as the Brewer-Dobson circulation, which transports high ozone from the tropics poleward and downward to the lower stratosphere of the high latitudes.

While the Brewer-Dobson circulation is explored in depth in Chapter 5, Figure 1.06 contains a schematic diagram of the flow pattern that makes up this circulation. The black arrows in the figure represent the annual average of the air circulation pattern in the stratosphere.

Figure 1.06 also reveals that the ozone layer is higher in altitude in the tropics, and lower in altitude in the extratropics, especially in the polar regions. This altitude variation of ozone results from the slow circulation that lifts the ozone-poor air out of the troposphere into the stratosphere. As this air slowly rises in the tropics, ozone is produced by the overhead sun which photolyzes oxygen molecules (as shown in the previous section). As this slow circulation bends towards the mid-latitudes, it carries the ozone-rich air from the tropical middle stratosphere to the mid-and-high latitudes lower stratosphere. The high ozone concentrations at high latitudes are due to the accumulation of ozone at lower altitudes.

The circulation pictured in Figure 1.06 moves at a literal snail's pace. The time needed to lift an air parcel from the tropical tropopause near 16 km (50,000 feet) to 20 km is about 4-5 months (about 30 feet per day). Even though ozone in the lower tropical stratosphere is produced at a very slow rate, the lifting circulation is so slow that ozone can build up to relatively high levels by the time it reaches 26 km (85,000 feet). As we will see in the next section, this slow circulation has very important implications for the times necessary to cleanse the stratosphere of human produced atmospheric pollutants.