Video Transcript
In this video, we will learn about
the ozone layer. We will describe it and investigate
why it is so important and how it can be damaged. What is the ozone layer?
First, the air around the Earth is
called the atmosphere. The composition of the atmosphere
changes as we move further from the Earth’s surface. The different layers have different
names. First is the troposphere, then the
stratosphere, then the mesosphere, the thermosphere, and finally, furthest from the
Earth, the exosphere.
Within the stratosphere layer is a
special layer called the ozone layer. The ozone layer exists between 20
and 40 kilometers above sea level. This layer contains high levels of
ozone molecules. Ozone molecules consist of three
oxygen atoms bonded together. And so ozone has the chemical
formula O3.
We are quite familiar with a
similar gas, oxygen gas, whose chemical formula is O2. Ozone gas has a pale-blue color and
a pungent smell like the chlorine we use in swimming pools. For many decades, scientists have
been studying this vital layer and have noticed that this layer is getting
thinner. In other words, the concentration
of ozone in this layer is being depleted. Why is this thinning of the ozone
layer a problem?
Well, ozone has the ability to
absorb harmful ultraviolet, or UV, radiation coming from the Sun. Ozone is a trace gas, which means
it exists in very small quantities. In fact, for about every 10 million
air molecules, there are only about three ozone molecules. But this is still enough to do the
job of absorbing harmful UV radiation. It is interesting to know that not
only does ozone absorb harmful UV light, but ozone is also formed when UV light
enters the atmosphere. In fact, when ozone absorbs UV
light, it breaks down. But it is constantly being reformed
in the atmosphere. So how is ozone gas formed?
First, UV radiation coming from the
Sun gets absorbed by oxygen gas molecules in the stratosphere. The energy from this UV light
breaks the double covalent bond in oxygen gas molecules. Two oxygen atoms are formed. Now, when oxygen atoms are not
bonded to other atoms, they are highly reactive. They are called free radicals in
this state. These oxygen atoms can react with
oxygen gas molecules, and in this way ozone, O3, is formed.
Let’s write the reaction equations
for this process. First, O2 gas molecules are split
apart by UV radiation to form oxygen atoms, which are in the gas state. Next, oxygen atoms react and
combine with oxygen gas molecules. Ozone gas is the final product.
Now that we know what ozone is and
how it is formed, let’s discuss its role in protecting organisms from harmful UV
radiation. Firstly, let’s talk about what UV
radiation is. The Sun emits a wide range of
light. Some is visible to us, and some is
not. This range of light is called
electromagnetic radiation. The waves in the electromagnetic
spectrum all have different wavelengths. Wavelength is the distance between
two peaks in a wave. The symbol for wavelength is a
Greek letter called 𝜆, which looks like this.
Now, UV radiation is just one part
of this spectrum of radiation. There are three categories of UV
light. UV-A has long wavelengths, about
315 to 400 nanometers. It is also known as near UV
light. UV-B has wavelengths in the range
of 280 to 315 nanometers. UV-B is also called medium UV. And UV-C has the shortest
wavelengths, from 100 to 280 nanometers. UV-C is also called far UV.
Longer wavelengths have less
energy, and shorter wavelengths have more energy. Because UV-C radiation has very
short wavelengths, it is highly energetic. UV-C radiation would be very
harmful to living organisms if it reached the Earth’s surface. However, UV-C radiation does not
reach the Earth’s surface because of the ozone layer. In fact, UV-C radiation from the
Sun is completely absorbed by ozone gas at about 35 kilometers above the Earth’s
surface.
Ozone absorbs most, about 95
percent, of UV-B radiation. Only a small percent of UV-B
radiation reaches the Earth’s surface. It is UV-B radiation which causes
sunburn if we stay out in the sun for too long. Excessive exposure to UV-B
radiation over long periods of time is one factor which contributes towards some
types of skin cancer.
The third type of UV radiation,
UV-A light, can pass easily through the ozone layer. This form of ultraviolet light
reaches the surface of the Earth. UV-B radiation is necessary for
life. For example, it causes our skin
cells to produce vitamin D.
So far, we have learnt how the
ozone layer forms and how it protects organisms from harmful UV wavelengths. Let’s now turn our attention to how
the ozone layer is damaged.
The ozone layer is about 20
kilometers wide. The ozone molecules and other air
particles in this layer are widely separated, since the atmospheric pressure is
lower here than at sea level, where the air particles are closer together. Now, in science, we tend to measure
things under standard conditions. That is, we try to measure things
at zero degrees Celsius and at the same air pressure as at sea level. The reason for using standard
conditions to measure things in science is so that we can compare information with
meaning.
A scientist called Gordon Dobson
did some complex calculations and determined that if the ozone layer was under
standard conditions, that is, zero degrees Celsius and one-atmosphere pressure, it
would be only three millimeters thick. He devised a special unit called
the Dobson unit, DU. At standard temperature and
pressure, one Dobson unit is equal to approximately 0.01-millimeter thickness of
ozone gas. So the Dobson unit is a way to
measure how much ozone is present in a portion of the ozone layer. Under standard conditions, the
amount of ozone in a given portion of the ozone layer, which would be about three
millimeters thick, is equivalent to 300 Dobson units.
Scientists use the Dobson unit when
monitoring concentration of ozone in the ozone layer. They have noticed that ozone
concentrations change slightly at different times of the year, especially above
Antarctica. The Dobson unit value tends to drop
below 300 Dobson units over Antarctica in its springtime. This diagram shows Antarctica in
winter. When Antarctica is in winter, the
Sun does not rise there for a few days up to several months, depending on how close
to the South Pole we are talking. This is because of the tilt of the
Earth and its position in its orbit around the Sun. So different parts of Antarctica
get days to months of darkness during winter.
Even though the Earth is spinning,
it spins on a tilted axis. So Antarctica points away from the
Sun and doesn’t receive sunshine. But when September comes, the Earth
has moved in its orbit around the Sun a bit further. And now some of Antarctica gets
some sunshine. Spring has arrived in
Antarctica. The sun shines there again. UV radiation now reaching the
atmosphere above the Antarctic causes chemical reactions to occur in the atmosphere
there. Ozone molecules are broken down
because of the energy from the incoming UV light and artificial pollutant gas
molecules in the air there. An ozone hole begins to form.
Now, don’t get confused. Pollutant gas molecules are found
distributed throughout the Earth’s atmosphere. So why is the ozone layer in
Antarctica specifically so badly affected? The unique conditions in Antarctica
also have a part to play in ozone being destroyed. The cloud cover there and its low
temperatures also affect this process.
Now let’s look at a special image
showing the Dobson unit amount or thickness of ozone in different places around the
Antarctic. This image shows the South Pole of
the Earth. The colors show the concentrations
of ozone around Antarctica in Dobson units when the ozone hole forms. We can see that dark blue
represents low ozone concentrations. This is the ozone hole. It is not really a hole, but very
low concentrations of ozone. Green represents near 300 Dobson
units or a normal ozone thickness, and red represents a high Dobson unit value. In other words, small patches of
very concentrated ozone occur because of special conditions.
What are the pollutant chemicals
which cause the ozone hole to form? The table shows some of these
substances. Chlorofluorocarbon compounds, or
CFCs, were widely used in aerosol cans as propellants and as coolants in
air-conditioning. These substances are major causes
of ozone depletion. Thankfully, the use of CFCs is
significantly less nowadays. In 1987, many countries agreed to
phase out the use of CFCs. And a treaty called the Montreal
Protocol was signed. Other pollutant chemicals that end
up in the atmosphere and damage the ozone layer are methyl bromide, which is used in
insecticides, halons from fire extinguishers, and nitrogen oxides produced by the
combustion of fuels in airplanes.
It’s now time to practice what
we’ve learnt.
The diagram shows the stages of
ozone formation. What is the correct order of the
illustrations? (A) 2, 1, 3, 4; (B) 3, 4, 1, 2; (C)
3, 4, 2, 1; (D) 1, 4, 2, 3; (E) 2, 1, 4, 3.
The question mentions ozone. Ozone is a chemical made from three
oxygen atoms bonded together. In the stratosphere layer of the
atmosphere is a special layer of air called the ozone layer. This 20-kilometer-thick layer has a
high concentration of ozone gas molecules. Although the concentration of ozone
here is higher than in other parts of the atmosphere, ozone is still considered a
trace gas. Nevertheless, ozone molecules in
the ozone layer are important to life on Earth, as they absorb harmful wavelengths
of ultraviolet, UV, radiation. UV light also causes new ozone
molecules to form in the atmosphere.
The diagram shows the steps in
ozone formation. But the steps are not necessarily
in the correct order. We need to figure out the correct
order. We can immediately assign image (3)
to the last step in the process, as image (3) is a picture of ozone. We can see that answers (D) and (E)
have number (3) as the last step in the process. So one of these must be the correct
answer. At this point, we can eliminate
answers (A), (B), and (C), which do not end with (3), the ozone molecule.
So how is ozone in the atmosphere
formed? First, UV light from the Sun is
absorbed by oxygen molecules. This description matches image
(2). The energy from the absorbed UV
radiation causes the double covalent bond of the oxygen molecule to break. Two separate oxygen atoms called
free radicals are produced. This looks like image (1). These particles are very
reactive. The reactive oxygen free radicals
each then collide with other oxygen molecules. We can see this process happening
in image (4). Finally, the oxygen free radicals
and oxygen molecules, which have collided, react and bond together, forming ozone
molecules. This is image (3). So the correct order of the images
for the formation of ozone is (E) 2, 1, 4, 3.
It’s time to summarize what we have
learnt in this video. We learnt that ozone molecules are
made from three oxygen atoms bonded together. The ozone layer has a high
concentration of ozone molecules. The ozone layer is critical for
life since it protects organisms from harmful UV wavelengths. We saw that UV-A wavelengths pass
through the ozone layer and a small percent of UV-B wavelengths too. And we learnt that the ozone layer
absorbs 100 percent of UV-C radiation, which is harmful to organisms. And finally we learnt that certain
man-made air pollutants, such as CFCs, halons, methyl bromide, and nitrogen oxides,
cause damage to the ozone layer.
