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 (0øC) 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 0ø 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 45ø. 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.
Click here to return
Last updated Friday July 11 1997