Stratospheric Ozone Depletion| Global change| AP Environmental Science| Khan Academy

In this video, we're going to talk about a molecule known as ozone. Ozone you can also view as O3 or three oxygens bonded this way. These dashed 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. 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 just 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. 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. If you start messing with DNA, start mutating DNA, that can lead to things like cancer. So you could 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, it 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.

We also know that certain chemicals that we human beings have produced, in particular what's often known as chlorofluorocarbons, and 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. 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 will 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. When ozone is down there, it's actually a bad thing. 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.

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. The same thing that it's protecting us from can actually help create it—that UV light.