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AP®︎/College Environmental science
Course: AP®︎/College Environmental science > Unit 8
Lesson 1: OzoneStratospheric ozone depletion
Stratospheric ozone depletion is caused by anthropogenic factors, such as chlorofluorocarbons (CFC's), and natural factors, such as the melting of ice crystals in the atmosphere at the beginning of the Antarctic spring. Created by Sal Khan.
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Video transcript
- In this video we're gonna talk about a molecule known as Ozone. And Ozone you can also view as O3, or three oxygens bonded this way, these dash lines show that sometimes the double bond is on this side, sometimes it's on that side. You might recognize that as resonance in molecular structures, or resonance in bonds. But it plays some very interesting roles. It can both protect
us, and it can hurt us. And to understand this,
let's look at this diagram of the atmosphere. So there's a lot of really interesting information in this diagram. Some of it relevant to
this video, some of it not, but I encourage you to look at it. There's all sorts of cool things, how temperature changes as
we increase in altitude. We see the labels of the major
layers of the atmosphere, we can compare it to Mount
Everest and where clouds are. But we see a few things,
we have a troposphere, and then above that we
have a stratosphere. Now in the stratosphere we see this thing known as an Ozone layer, which is 10 to 20 miles above
the surface of the earth. And Ozone in this part of
the atmosphere protects us. How does it protect us? Well the sun, which most
of us consider our friend, that lives 93 million miles away, we know that life wouldn't
exist without the sun. Most of the energy we have on this planet is because of the sun. But UV frequencies from the sun, if they aren't blocked, or if they aren't mitigated a little bit, they can cause a lot of damage. They have a lot of energy,
they can affect molecules, and in particular, they
can affect our DNA. And if you start messing with DNA, start mutating DNA, that can
lead to things like cancer. And so you can imagine, we human beings, and actually most organisms can only deal with a
certain amount of UV light. Now if we had no Ozone layer, a lot of that UV light
would come in unchecked. It's going to get absorbed a little bit by the various layers of the atmosphere, but it turns out that Ozone in particular is good at protecting us from UV light. So if we didn't have this
Ozone layer above us, you would see cancer rates go up. We would just get a lot more UV energy. Now an interesting question is, how does this Ozone layer form? We're not used to seeing
oxygen in this form. Most of the oxygen that
we see in our atmosphere is molecular oxygen in
the form of two oxygens which looks like that. Well is turns out that UV light actually helps create the thing that protects us from UV light. Because O2 in the presence of UV can be broken up and
then reformed as Ozone. So it's actually the fact that you have so much UV light up here, that it reacts with oxygen that has raised to the stratosphere, and some of that gets formed into Ozone. Now Ozone is very reactive, and it can be a little bit delicate, it has a very short half life. It is way less stable than O2. And so it has to keep getting
replenished in the atmosphere. And we also know that certain chemicals that we human beings have produced, in particular what's often known as chlorofloro carbons, that's just a fancy
way of saying molecules that are hydrocarbons, so a bunch of carbons and hydrogens, that also involve chlorine and fluorine. And these molecules were generated in industrial applications, things like aerosol
sprays used to have CFCs, but it turns out that CFCs
will deplete the ozone layer. They'll come up from our
little spray paint cans and they'll interact with the
Ozone, and they'll deplete it which is not a good thing. Once again it will make
us all more susceptible to things like cancer. Now everything we've talked about so far is Ozone in the stratosphere, but it turns out that Ozone can actually also form
lower in the atmosphere, in the place where we
tend to spend our lives, in the troposphere. And when Ozone is down there,
it's actually a bad thing. And to understand why it
might form down there, we need to understand the role once again that we're playing. Nitrous oxides are formed
from automobile exhaust, and coal-fired power plants, and when those in the
presence of UV light, and what's known as
volatile organic compounds, which is just a fancy way
of saying organic compounds that evaporate very easily, and most of these are
naturally produced, the VOCs, but when you have the
UV light in conjunction with the exhaust from automobiles and nitrous oxides from
coal-fired power plants, it can produce Ozone in
the lower atmosphere. And Ozone as I mentioned
is highly reactive, it can damage living tissue, it can damage our respiratory systems, so you don't want Ozone around. So it's good in the upper atmosphere, and it is bad in the lower atmosphere. So I will leave you there, you got your primer on Ozone, and just recognize the same thing that can help us in one place, can hurt us in another place. And the same thing that
it's protecting us from can actually help create
it, that UV light.