Part F: A Closer Look at....

Stratospheric Ozone

Why the Ozone Layer is in the Stratosphere
The ozone and the stratosphere are inseparable. The ozone that is formed there differentiates the stratosphere from the lower part of the atmosphere (troposphere).

Formation of Ozone by Ultraviolet
Radiation in the stratosphere

(1) O2 + UV Energy ------- 2 O

(2) O + O2 ------- O3

The Equilibrium concentration of O3 depends on the altitude

A chain of events explains this interdependence:
1. High energy UV photons at the top of the atmosphere convert some of the O2 into O3 (ozone).
2. Only a small amount of ozone accumulates because other UV photons break down O3 molecules, turning them back into O2. When O3 is being broken down as fast as it is formed, its concentration remains constant
3. Ozone absorbs many of the highest-energy UV photons, converting their energy into heat. This energy heats the air, the higher temperature differentiates the stratosphere from the troposphere.
4. Because the ozone absorbs the higher-energy UV photons, no significant amount of ozone forms in the lower part of the atmosphere.

The Units of Ozone Concentration

Since the sunlight has to shine down through the entire thickness of the atmosphere, we need to study the total amount of ozone between us and the sun to understand its effect on UV radiation. This amount, called the column abundance of ozone is the total amount of ozone in a column that the sunlight has to pass through to reach the surface. Think about the column of atmosphere that has an area of 1 sq.m and extends from the surface to the top of the atmosphere. Of course the top of the atmosphere isn't well defined; it just sort of peters out.
We commonly measure the column abundance of ozone in the Dobson Unit (DU) (Figure 1). In Dobson Units the average value for the entire globe is about 300 DU, which equals 8 10^22 molecules/sq.m. If this amount of ozone were reduced to standard temperature (0C) and pressure (1 atm), then it would have a thickness of only 3 mm. The ozone layer is indeed somewhat delicate.

Figure 1: The units of ozone

The Ozone Layer Isn't Uniform around the World

The global distribution of ozone in the stratosphere (see Fig. 2) follows this general pattern:
1. In the tropics it is below average and more or less the same the year around.
2. In the middle latitudes the levels are generally higher, reaching a maximum over the latitudes of the northern United States and Canada, being highest in the spring and lowest in the fall.
3. The poles are extremely variable. In the polar spring a collar of high ozone concentrations forms surrounding a region of very low concentration. This "ozone hole" has become increasingly severe in the antarctic during the last decade.

How the Ozone Gets Where It Is

The distribution of ozone in the stratosphere over different parts of the world is complicated and surprising. Because the sunlight (including UV) is most intense in the tropics, you might expect the most ozone over that region. In fact, however, the average ozone concentrations throughout the year are lowest in the tropics, because the ozone in the stratosphere doesn't stay where it is formed. In the fall and spring, when the sun is above the equator, the general pattern of global circulation carries the ozone and other gases in the tropical stratosphere toward the poles. Because UV is not as intense at these higher latitudes, the ozone accumulates there. In the summer and winter, when the sun is more directly over one hemisphere than the other, the global circulation moves from the "summer" hemisphere to the "winter" hemisphere. Therefore, in the temperate latitudes, where most of the United States is, the ozone concentrations vary with the seasons. Finally, in recent years the ozone levels in the polar regions have been declining sharply in the spring to as low as half the global average. Occurrences of low ozone concentrations are called "ozone holes," and are believed by many to be caused by release of chloro-fluorocarbon compounds (CFC) into the atmosphere.

The Angle of the Sun

The amount of UV that is removed from sunlight as it passes through the atmosphere depends on how many molecules of ozone obstruct its path. Therefore, both the concentration of ozone in the atmosphere, and the distance the light must travel are important. If the sun is directly overhead, which only happens in the tropics, the sunlight travels the shortest possible distance through the atmosphere. As the sun gets lower in the sky, the distance its light must travel increases. This means that the amount of UV in the sunlight that reaches the surface depends on time of day (greatest near noon), the season of the year (greatest in the summer), and the latitude (greatest in the tropics).
The zenith angle of the sun is 0 when the sun is directly overhead. The greater this angle becomes, the more atmosphere the sun's radiation must penetrate to reach the surface. Consequently, when the angle is greater, the protection by the atmosphere is greater. A simple way to estimate the angle of the sun is to notice the length of your shadow (Figure 3). If your shadow is as long as you are tall, the zenith angle is 45. At this angle and beyond the risk of overexposure becomes relatively small.

Figure 2 : Global and seasonal distribution of ozone.

Figure 3: The effect of the angle of the sun on the length of your shadow.

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Last updated Friday July 11 1997