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Stratospheric ozone depletion

Ozone, a molecule composed of three oxygen atoms, plays a dual role in our atmosphere. In the stratosphere, it forms a protective layer that absorbs harmful UV light, reducing the risk of skin cancer. However, human activities like aerosol sprays and car exhaust can deplete this layer. Conversely, in the troposphere where we live, ozone is harmful, damaging living tissue and respiratory systems. 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.