The Atmosphere, the Ocean, and Environmental Change

GG 140 - Lecture 32 - The Ozone Layer

Chapter 1: Ozone in the Atmosphere [00:00:00]

Professor Ron Smith: We're going to finish up the ozone thing by doing the stratospheric ozone. It was interesting though that just yesterday there was this big New York Times article about the politics of setting the EPA standard for ozone. How many saw that article? I know Sam did and Rachel did. But it was long, like a four page article or something. All about the interaction between the EPA and the White House trying to drop that eight hours standard from, was it 0.08 down to 0.07.

So a lot of political heartache and dispute about whether that should be dropped to 0.07. And if you're into that sort of stuff, read about the politics of that. It fits nicely with the discussion we had in class last time. Anything else? Any issues, questions?

Chapter 2: Terminology for Stratospheric Ozone [00:01:09]

OK, so we're going to do the stratospheric ozone problem today. And these are the new terminology you should be aware of. So we'll talk about the ozone layer and then the ozone hole. The Dobson unit I'll discuss, I'll define as a measure of column integrated ozone concentration.

We'll be talking about CFCs which had been used as the primary liquid in refrigerators. And you need to have a fluid that flows back and forth between the cold sink and the warm sink in a refrigerator. And CFCs was the common chemical to do that. But now they've been replaced largely by HCFCs, the hydrochlorofluorocarbons. Say that 10 times fast. And so I'll define those as well.

We'll talk at some point today about polar stratospheric clouds, catalytic reactions, which are reactions in which one particular chemical compound allows the reaction to go forward but in the end itself isn't changed. So it can be reused over and over again to make the same chemical cycle go forward. Photochemical reactions, which are chemical reactions that involve light. You have to have photons of light in order for that reaction to go forward. And we'll talk about the Montreal Protocol. So all this will come in turn today.

Chapter 3: The Ozone Layer [00:01:42]

First of all, I showed this diagram last time as we discussed this part of the ozone issue, but now we're going to talk about the ozone layer. And as it says here, contains 90% of the atmospheric ozone. So if you do a column integration from the top of the atmosphere to the bottom, most of the contribution comes from the ozone layer.

What's plotted here is the partial pressure of ozone in milliPascals. Remember that the total pressure, the air pressure, decreases strongly and exponentially all the way on up through the troposphere and the stratosphere. So this has a very different vertical structure than the total pressure.

It has a beneficial role, it acts as a shield against ultraviolet radiation that would otherwise cause skin cancer, skin damage. And the current trends are a long-term global downtrend in stratospheric ozone. And then most famously, this ozone hole which occurs in the springtime of the Antarctic part of the earth each year. And then some more recent discussion about possibly this starting up in the Northern Hemisphere as well, but this is not as well documented.

So just a couple of calculations then. So I want to compute the mixing ratio in ppmv for ozone to air using this point on this slide. So I'm going to say that at about 24 kilometers altitude you've got a partial pressure of ozone that's about 30 milliPascals. And I want to convert that to a mixing ratio because that's what we've been used to using, what we used last time.

So first of all, because it's going to be a mixing ratio you need to know how much air is up there as well, not just how much ozone, but how much air. So I'm going to use this simple formula that we've used quite often in the course. It's the approximate exponential form for pressure as a function of altitude where you use a scale height, Hs. You know the pressure at the surface of the earth, you know the altitude you're inquiring about, and this will give you the pressure at that altitude

So sea level pressure we'll take to be 1,013 millibars, which is the standard average value, convert that to Pascals. The scale height as you recall is RT over g, which is about 8,400 meters for the earth—the earth's atmosphere. So I'll put 24 kilometers up here, that scale height there, and I'll put the sea level pressure there, and that's this formula here. So I put everything where I said I would and I get a value of about 6,000 Pascals for the air pressure up here.

And then I can simply put that into a ratio of the two pressures to get the mixing ratio in the common units. So I put in that the value of 0.03/6,000, I get 20 ppmv for the ozone mixing ratio in the peak of the ozone layer. OK? Pretty straightforward calculation. Had to find out how much air was there and then from the proper ratio get it in the proper units. 

Now compare this to what we were talking about last time. The EPA one hour standard for ozone in the boundary layer, where we live, was 0.12 in those same units. So the ozone concentration in the ozone layer is about 200 times the EPA one hour limit, just to give you a relative sense for that. So if we were up there, that would be really tough for us to breathe that in.

Now remember however, there's not enough air to keep us alive anyway at those altitudes. But the ozone concentration is quite large as well. So that's an inhospitable environment up there at that altitude. Any questions on that?

But it is possible, or it was until recently, possible for us to fly at those altitudes. So the Concorde, this beautiful, supersonic, commercial airliner which stopped flying a couple years ago, it flew at an altitude typically of 17 kilometers, which isn't quite at the peak of the ozone concentration, but it's definitely up in the ozone layer, there's no doubt about that.

And of course, you have to refresh the cabin air. You can't just lock the cabin doors and expect everybody to live off that cabin air until you get to your destination. So the aircraft can continually draw in air from outside to flush the cabin and give you fresh cabin air. The question is how do you do that in this case?

Remember though that there's two problems. First of all, there's very little air outside anyway. So you're going to have to compress it with some kind of a pump before you can put it into the cabin so people can breathe it. And that's going to be a big compression.

Let's just think about this for a minute. So if you're flying at 17 kilometers and you want to get that air compressed to cabin pressure, well first of all what cabin pressure do you normally have? Airliners don't usually keep you at sea level pressure. If you're taking off from New York City, as you take off the pressure will drop--in the cabin will drop as you climb, but they'll usually only let it drop to maybe an equivalent altitude of 6,000 feet, something like that. Then they couldn't let it go any lower or you'd start to have trouble breathing, but they don't keep you at sea level.

So I'm going to assume that the, for this calculation, that the cabin altitude, what's called the cabin altitude, is going to be kept at 2 kilometers. That's about 6,000 feet above the surface. In fact, you're flying at 17 kilometers, so you've got to take air and compress it just as you would if you took a parcel of air from 17 kilometers and brought it down to two kilometers, it would compress, adiabatic compression, we've talked about this over and over again. And it would heat as you compress it. So when you put it into that pump and compress it, it's going to warm up, and how much?

Well this is a calculation you can do in your head. You know the adiabatic lapse rate is about 10 degrees per kilometer, actually it's 9.8, but I've rounded it off. That's a difference in 15 kilometers, so essentially you're going to get 150 degrees Celsius warming when you compress that air about ready to put it into the cabin. Now you’re going to--it started off pretty cold, but still that's going to be really hot air.

Two things I've got to say about that. First of all, the convenient thing is that that temperature will destroy the ozone. So just by compressing the air, getting it ready to put into the cabin, you're going to kill off that ozone automatically. So that's pretty convenient.

But then remember, you're going to have to cool it a bit before you put it into the cabin. So you're going to put it through another heat exchanger, cool it back down, and then you get the air to stick into the cabin. If you don't get it all out, you can pass the air through an activated charcoal filter and that will remove any remaining ozone that's in the air before you put it into the cabin.

So it is possible to fly at those altitudes right in the middle of the ozone layer. But that's how you have to do it. Any questions on that? Yeah?

Student: Why would you want to fly at that high of an altitude?

Professor Ron Smith: It turns out it has mostly to do with the efficiency of the engines. In fact, that's true with subsonic aircraft as well. The reason they fly quite high, 35,000 feet, 37,000 feet is because the engines are most efficient at that altitude. And for this aircraft, the more efficient—the altitude was even higher at 17 kilometers. So anything on that? OK, maybe that's a little tangential point, but--

Chapter 4: The Dobson Unit [00:11:06]

This thing called the Dobson unit is a column density. We could write it, in fact, I wish that they had chosen the unit of kilograms per square meter. You'd simply integrate top to bottom in the atmosphere and say how many kilograms of ozone are there in each square meter of footprint for that column that you've integrated over. But for historic reasons they didn't do it that way.

This is typically measured by looking at light coming from the sun and see how much ultraviolet light reaches the surface of the earth. And that will tell you how much ozone there is in the column. What they're using for the Dobson unit is this definition, it's 1 milli-atmosphere-centimeters. That's the scientific definition of a Dobson unit.

And here's how you think about that. You take all the ozone in the column, you compress it to standard temperature and pressure, that is 1 atmosphere of pressure and 0 degrees Celsius. And then you measure how thick that layer would be composed just of ozone and you measure that in thousandth of a centimeter. And that is your Dobson unit value.

And I'll be showing you this kind of data. Typical values for the earth's atmosphere run from 100 to 500 Dobson units depending on where you are around the world. But that is normally--the Dobson is what we use to describe to each other how much ozone there is in the atmosphere.

So for example, you'll see a diagram like this from NASA. It's a map of the world and some areas are blocked out because they don't have data from here for this particular time of year. But the scale is in Dobson units.

So in the tropical regions you're getting values of about 250, 260 Dobson units. In the green you're getting values maybe 325, maybe in some areas even 350 or 360. So that's the way we represent column integrated ozone in this subject.

We can also get vertical profile information either by direct in situ measurements or by looking at light passing across the earth's atmosphere horizontally. This gives you some idea where the ozone layer is as a function of latitude. So here's latitude, North Pole, South Pole, Equator, altitude. But notice the offset 0, this isn't the ground, this is starting at 16 kilometers, right?

So the ozone layer at higher latitudes, it peaks at an altitude of about 20 kilometers, 18, 19, 20 kilometers. But near the Equator, it's up around 25, 26 kilometers. I think we expected this because remember the tropopause has the same kind of a structure. It's higher in the equatorial regions and lower in the polar regions. So the ozone layer is about the same place in the stratosphere, but the stratosphere starts at a different level. So it's lifted in a sense relative to the surface of the earth in the equatorial latitudes.

The reason why--part of the reason why there is that difference is because of a broad circulation that takes place in the stratosphere calls call the Brewer-Dobson circulation, which I won't talk about, but this artist has reminded us that there is this broad slow circulation in the stratosphere.

Chapter 5: Origin and Maintenance of Ozone in the Stratosphere [00:15:05]

So why do we have ozone in the stratosphere? This is a basic chemistry of that. We know we have a lot of O2 in our atmosphere, it's the second most common molecule. And occasionally, a high energy photon will come from the sun. A high energy ultraviolet photon will dissociate that diatomic oxygen into two oxygen atoms. It doesn't happen very often because it needs a high energy photon to do that.

But then you've got some free oxygen atoms floating around. And when you have that, that'll quickly combine with an O2 molecule to form ozone. That's a fast reaction. But equally fast is that other ultraviolet photons, they don't have to be as energetic as this one, these can be in the near, or in the UVA, for example, the near ultraviolet, will dissociate that back to this again. And that's fast.

So what you've got is a rapid recycling going on between that and that that's just going out all the time in the stratosphere, but maintaining a balance here. Now occasionally an ozone will interact with one of these atoms and go back to go to O2, which is the common form. And then you've lost it. So you've lost the ozone permanently or at least quasi permanently.

So the ozone concentration is controlled by--ignoring this rapid cycling which doesn't have any long-term effect, is controlled by the balance of these two slow processes. How often are you producing oxygen atoms and how often are you removing ozone and oxygen atoms back to O2? And that would be occurring naturally because we have oxygen in the atmosphere and we have ultraviolet light coming from the sun. So that's the background to why there is ozone in the stratosphere.

The reason why it doesn't happen further down is because most of these ultraviolet photons are absorbed up there and don't get--don’t penetrate deeper into the atmosphere. So it's a layer up there because that's where these ultraviolet photons are being absorbed and doing this work causing these photochemical reactions to take place. Questions on that?

Student: I'm curious if there is some sort of feedback mechanism or if the two slow reactions are interconnected in some way. Because I guess if you have more free oxygen atoms that would allow the second process to occur more frequently?

Professor Ron Smith: Well you'd have to write down the rate equations for these to answer that question, which I'm not prepared to do. But a chemical reaction, the rate at which it goes forward depends on how much of this do you have, and how much of this do you have, and how often are they colliding to allow the reaction to go forward? So I think your instinct is right that these are going to depend on the concentrations that you have.

So just like our tank experiment, the loss rate was proportional to how much you have. And that’s what, in a system like this where the generation term is independent of how much of this you have, but the loss rate is proportional, you're going to come to an equilibrium, just like we did in the water tank. And that's what I'm implying here, that you've got that kind of balance between a source which is just there and then a sink which is dependent on how much you have. And that will give a natural equilibrium eventually.

And this is a cartoon showing that. So the sun is playing a role. Occasionally it will dissociate O2 to form a simple O, an O molecule. And then you get this rapid recycling between oxygen atoms and ozone. And then occasionally, you'll go back just to the O2 again. And the control of how much you have is primarily these processes, not this. This is just a rapid recycling of the two types of odd oxygen.

Now to remind you about the spectrum of radiation coming from the sun, I show you this, you've seen it before. The visible spectrum is here, the infrared, we're really interested in the ultraviolet over here in today's conversation, which starts at 400 nanometers wavelength and goes shorter than that.

And these standard definitions are helpful. UVA are the wavelengths closest to the visible, closest to the blue, defined as going from 400 to 315 nanometers. UVB, 315 to 280, and then UVC, 280 all the way down to 100 nanometers.

Now you may remember from physics of that the shorter wavelength photons are more energetic, they'll do more damage. So UVA would cause tanning of your skin, but probably wouldn't give you skin cancer, UVB might. But UVC is very damaging. In fact, it's used in doctor's offices to clean surgical tools. You put it into this autoclave which will put UVC on your instruments and it'll kill anything that's there very, very quickly. It's very damaging radiation.

And so here's the fate of UVA, UVB, and UVC. Some UVA comes through. Half or more of the UVB is absorbed in the ozone layer, plotted on top of this. This is altitude and Dobson units per kilometer to get where the ozone layer is. So about 1/2 or 2/3 of the UVB is absorbed by the ozone. And almost all, thankfully, of the UVC coming from the sun is absorbed by the ozone layer. So it's a very good protective shield for us.

Now what could be changing this natural ozone layer situation? Well this is the current theory for it. It involves a catalytic reaction primarily using chlorine atoms. Now bromine will do this as well and there is some bromine in the atmosphere, but I'm going to focus strictly on the chlorine.

And notice how this reaction works. So you have a chlorine atom reacting with an ozone molecule. It goes to ClO, it oxidizes the chlorine and gives you back O2. And then quite soon after that, you'll take this ClO and react it with another ozone molecule. That'll give you back the Cl that you started with and you have two O2s, two regular diatomic oxygens.

So look, the ozone is gone. In fact, two ozones are gone. And you've got your Cl back ready to do it again. So this is a catalytic reaction where the catalyst here is the chlorine atom. It can be used over and over again to destroy ozone molecules in this way. Two ozone molecules are lost and the catalyst is free, ready to go back, and do it again.

That's the idea of what's today, now that we have a measurable chlorine concentration in the atmosphere, this is why we think the ozone layer is decreasing. Questions on that?

So up here in the inset is the emission. This is an example, CFC 11, one of the chlorofluorocarbons. And here's the emission rate for it in megatons per year starting in 1960 going up to the year 2000. So you see there was a rapid rise in it as it was being used in refrigeration. And when you put it into a refrigerator, it may be contained for a few years, but eventually it leaks out or someone tosses away the refrigerator and it gets a leak and it goes to the atmosphere. So almost everything you produce eventually makes its way into the atmosphere.

The Montreal Protocol that I'll talk about was here, where you begin to see it finally decreased. But because it's a long lived molecule, here's the CFC abundance in the atmosphere, it increased through this period of time, we've stopped putting it in, but it's still there. It has a lifetime--from this data, it looks like the lifetime is about 50 years. And so it's going to take probably another 50 or 100 before we see much of a decline. Thank goodness we've stopped putting it in, but it's not going to decrease to 0 just because we’ve stopped putting it in. It's going to be stored in the atmosphere for a while. It has a long lifetime.

The circles are data from the Northern Hemisphere and the triangles are data from the Southern Hemisphere. But they're pretty much the same. So the two hemispheres, this stuff gets mixed back and forth. Probably most of it was put into the atmosphere in the Northern Hemisphere, but now it's mixed to be pretty much equal in the two hemispheres. Questions on that?

Here is the global averaged total ozone change in percent starting back in 1965. Around 1980 is when we began to see the rapid decrease, but it's not huge. Notice that it's leveled out today and it's only about 4% below where it started. So this global decrease in ozone is, I suppose it's something we should be concerned about, but it's not a huge factor. And it doesn't look like it's going to get any worse now that we’ve stopped putting CFCs into the atmosphere. So it's a concern, but not I would say a terribly difficult one.

Chapter 6: The Ozone Hole [00:26:22]

But then there's this ozone hole that I'll spend the rest of time talking about. And here's what it looked like a couple of days ago. You can go to this website, which I'm just about to do. I should have set this up before.

But here's a definition of it. So it's a brief, seasonal and local reduction in ozone. Location is Antarctica, the time of year that it happens each year is September, October. It first appeared in about 1978, apparently it wasn't happening before then. It was discovered in 1984 and then going back and looking at old data, they found that it actually had started some six years earlier than that.

The discovery of the depletion mechanism by Molina and Crutzen was a rather remarkable scientific discovery. I'll give you hints of how that went in just a minute. And they won the Nobel Prize in chemistry for that awarded in 1995 from that discovery of how the ozone depletion mechanism works. Ultimately, it's caused by these CFC emissions which then led to the Montreal Protocol, which I'll talk briefly about, a rather successful international treaty to limit the emissions of CFCs.

Here's a brief history of it. So at the top is the ozone hole area in millions of square kilometers. What you do is set a threshold for ozone concentration. If it's lower than that you say you're in the hole, if it's larger than that you're out of the hole. And then you keep track of that area.

And the ozone hole was nonexistent before '79. But now it has reached about 25 million square kilometers, you saw a picture of it. And it's kind of leveled out, it's not getting it better not getting any worse. And the minimum ozone in the middle of that hole is shown here in Dobson units, of course, getting worse but now leveling out to a value of about 100 Dobson units in the middle of the hole each year when it forms.

So give me just a moment here to bring up this website. I forgot to do this before. Let’s see here. I'll just Google and find it. Yeah, that's probably it. That's not the one I want. That's a good one, but it's not the one I'm looking for. I'll try to find this other one. Yeah, let's do an animation for this season. I’ll try this one.

Yeah, so this is this year's hole. It hasn't formed yet, we're back in July. This is Antarctica sitting there waiting. Now wait until we get into September. It's already beginning to form in the late summer, but now we're getting the ozone hole forming. October, peaks in October, and then it'll start to fill in up to the present date. It's starting to weaken now that we're into November.

So that's the deal. It happens every year in that region. You get this large area that forms with about half the ozone it would otherwise have. Ozone depletion is about 50 percent in that region compared to remember only 4% globally. So this is a localized anomaly.

You can go to that website yourself and do some other animations. So in this year then this tracks it, the ozone hole area climbed up to the 12th of September, now it's decreased. The minimum ozone dropped to a minimum now it's increasing now that we're into November.

And I'm going to be talking about the stratospheric temperature. Stratospheric temperature in that part of the world now at that altitude is beginning to climb because we're getting out of their winter and towards of their Southern Hemisphere summer. And I'll come back to talk about the role of temperature in just a moment.

So I think the most curious thing about the ozone hole is why only in September and October? Why just over the South Pole? And I guess what is the link to human activity? So there's a lot of natural processes and factors controlling this. And then there's the added impact of humans and somehow they have all come together to form this curious phenomena of the ozone hole.

In the few minutes I have left I want to just try to explain some of that. Now I hope you can see this, I'll read it through for you. This is the seasonal cycle of temperature in the stratosphere. And this is for 90 North to 65 North, so this is the north polar cap. This is 25 North to 25 South, so this is the equatorial region. And this is 65 South to 90 South, so this is the south polar region.

Now the temperatures in the Northern Hemisphere get down to minus 70 on occasion. These are at different altitudes, I believe, having trouble reading those. They get down to minus 70. In the equatorial regions, those temperatures get down to about minus 75. But here in the South Pole, they get down to minus 80 and even minus 85. So of all the places in the world's stratosphere, the Southern Hemisphere polar stratosphere, that is over Antarctica, has a winter time temperature colder than any other place on the planet.

That's one of the things that makes it special in regards to why the ozone hole forms there. Now why would that be? It has to do with the phenomena of polar stratospheric clouds, abbreviation PSCs. Polar stratospheric clouds. I haven't spoken about these in the course up to this present time, so this is new material.

These are ice clouds that form up in the stratosphere. Normally we think clouds don't form in the stratosphere, it's pretty dry up there, but these clouds do. They are not entirely made of water. They're  a mixture of water and nitric acid in these clouds. And they require an extraordinarily cold temperature in order to form, temperatures colder than minus 70 degrees Celsius, some would say even colder than that.

This is probably why the Antarctic stratosphere is so special because it's temperature gets cold enough in winter to allow these polar stratospheric clouds to occur, whereas other places around the globe in the stratosphere you don't get those clouds. What do they do then? How do they influence ozone?

Well these ice clouds, these ice particles, freeze up diatomic chlorine by holding the nitric acid in the ice itself. Notice when it forms it's storing HNO3, nitric acid. And here's the reaction. You start with HCl, and ClONO2, reaction goes to Cl2 and the nitric acid, and then this is locked up in the ice. So what you've done, you had your chlorine locked up in HCl and in this compound, there's chlorines in both. Now you've put the nitric acid in the ice and you've freed up the chlorine diatomic molecule. There’s only one step to go.

When the light returns, remember, that minimum of temperature occurred in perpetual darkness. At that season of the year, it's dark over Antarctica, there's no light. So you can get the ice forming, you can get the Cl all ready to go, but it's in the diatomic form. Then the light comes back, Southern Hemisphere spring, the light comes back, you can dissociate the Cl2 to form two Cl molecules. And then go back a few slides to that catalytic reaction and it starts to destroy ozone.

So you get that? So the ice frees up the chlorine, it then dissociates when light returns, and the catalytic destruction of ozone begins to take place, and the ozone hole forms. That's why it occurs where it does and when it does because of that necessary condition for cold. And then the ozone hole forms just after the light returns to that region. Any questions on that? Yes?.

Student: Where does the hydrochloric acid come from, the HCl?

Professor Ron Smith: I don't know. That's the way the chlorine is stored most of the time in the stratosphere, is in these two molecules. After the CFCs break down, the chlorine is transferred into these two forms and just hangs around. Then the PSCs form and that sucks off the nitric acid and the Cl is released. So this is the storage form of the chlorine during most of the year in the stratosphere.

Student: Is there normally chlorine in the atmosphere besides the HCl?

Professor Ron Smith: Not naturally. Remember this was CFCs and not chlorine. And that's a critical difference. But let me just go back to that. There were no CFCs in the atmosphere before that. There probably was a little bit of chlorine, but only a very small amount. But now we're putting it in through this CFC source. Sorry, that's not a complete answer, but that's the best I can do.

Chapter 7: The Montreal Protocol [00:37:51]

So I think this–yeah, this is the last one. So the Montreal Protocol, the full name of it is Montreal Protocol on Substances that Deplete the Ozone layer. It's an international treaty. And of course it was first signed in Montreal, it's got the name. Since then it has been modified several times. Certain forms of substances have been added to the banned list, others have been allowed. So it's been modified a few times.

Generally what it did is by the time we came to 1994, it had banned pretty much all emissions of CFCs into the atmosphere. It largely replaced them with the HCFC, which can also be used as a refrigerant, but has a much shorter lifetime and will not cause the chlorine to build up in the atmosphere. It may have some other problems, it may be a greenhouse gas. But it doesn't have the problem of putting permanently chlorine into the atmosphere the way that the CFCs do.

Now one of the reasons why this treaty was signed, because as you'd expect the refrigerant industry vigorously opposed this, was at about the time it was signed, Dupont which was this large chemical company and the world's largest producer of CFCs, discovered a replacement for it. And they realized that they would not lose their business because they had a replacement ready to go and they might even be able to make more money by doing this changeover to a different type of refrigerant. So in the end, they dropped their opposition and many countries of the world got together and signed this international treaty.

And it's viewed in environmental circles as being one of the best examples of a successful situation where the scientists discovered the problem, suggested how to solve it, and then you have the countries of the world actually getting together to pass a treaty and acting on it. As you saw, the CFC emissions have dropped nearly to 0 now. So it's looked at as, well in some cases, in wonderment because how could we do something like this today, it's not clear. But at least we have this example of how environmental science can prevent these problems.

Any questions on this? Great we're done a little bit early again today. So enjoy your Thanksgiving break and I'll see you a week from Monday.

Student: Professor, I'm still a little unclear as to why that same phenomenon with the ozone hole doesn't occur in the Northern Hemisphere.

Professor Ron Smith: Yeah, I should have said more about that. As you saw, it doesn't get as cold. And that is because it's believed there's more mixing between the high and low latitudes. It does want to get cold over the pole in winter, there's no sunlight getting there, it's radiating to space. But in the Northern Hemisphere, there's more north south mixing in the stratosphere to keep it warm. And that they think is because there's more continents, more of a continent ocean contrast in the Northern Hemisphere. So more disturbances in general that are causing a north south mixing, keeping it warmer up there.

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Chapter 1: Ozone in the Atmosphere [00:00:00]

Professor Ron Smith:--stuff, so show off a little bit. We're going to finish up the ozone thing by doing the stratospheric ozone. It was interesting though that just yesterday there was this big New York Times article about the politics of setting the EPA standard for ozone. How many saw that article? I know Sam did and Rachel did. But it was long, like a four page article or something. All about the interaction between the EPA and the White House trying to drop that eight hours standard from, was it 0.08 down to 0.07.

So a lot of political heartache and dispute about whether that should be dropped to 0.07. And if you're into that sort of stuff, read about the politics of that. It fits nicely with the discussion we had in class last time. Anything else? Any issues, questions?

Chapter 2: Terminology for Stratospheric Ozone [00:01:13]

OK, so we're going to do the stratospheric ozone problem today. And these are the new terminology you should be aware of. So we'll talk about the ozone layer and then the ozone hole. The Dobson unit I'll discuss, I'll define as a measure of column integrated ozone concentration.

We'll be talking about CFCs which had been used as the primary liquid in refrigerators. And you need to have a fluid that flows back and forth between the cold sink and the warm sink in a refrigerator. And CFCs was the common chemical to do that. But now they've been replaced largely by HCFCs, the hydrochlorofluorocarbons. Say that 10 times fast. And so I'll define those as well.

We'll talk at some point today about polar stratospheric clouds, catalytic reactions, which are reactions in which one particular chemical compound allows the reaction to go forward but in the end itself isn't changed. So it can be reused over and over again to make the same chemical cycle go forward. Photochemical reactions, which are chemical reactions that involve light. You have to have photons of light in order for that reaction to go forward. And we'll talk about the Montreal Protocol. So all this will come in turn today.

Chapter 3: The Ozone Layer [00:02:55]

First of all, I showed this diagram last time as we discussed this part of the ozone issue, but now we're going to talk about the ozone layer. And as it says here, contains 90% of the atmospheric ozone. So if you do a column integration from the top of the atmosphere to the bottom, most of the contribution comes from the ozone layer.

What's plotted here is the partial pressure of ozone in milliPascals. Remember that the total pressure, the air pressure, decreases strongly and exponentially all the way on up through the troposphere and the stratosphere. So this has a very different vertical structure than the total pressure.

It has a beneficial role, it acts as a shield against ultraviolet radiation that would otherwise cause skin cancer, skin damage. And the current trends are a long-term global downtrend in stratospheric ozone. And then most famously, this ozone hole which occurs in the springtime of the Antarctic part of the earth each year. And then some more recent discussion about possibly this starting up in the Northern Hemisphere as well, but this is not as well documented.

So just a couple of calculations then. So I want to compute the mixing ratio in ppmv for ozone to air using this point on this slide. So I'm going to say that at about 24 kilometers altitude you've got a partial pressure of ozone that's about 30 milliPascals. And I want to convert that to a mixing ratio because that's what we've been used to using, what we used last time.

So first of all, because it's going to be a mixing ratio you need to know how much air is up there as well, not just how much ozone, but how much air. So I'm going to use this simple formula that we've used quite often in the course. It's the approximate exponential form for pressure as a function of altitude where you use a scale height, Hs. You know the pressure at the surface of the earth, you know the altitude you're inquiring about, and this will give you the pressure at that altitude

So sea level pressure we'll take to be 1,013 millibars, which is the standard average value, convert that to Pascals. The scale height as you recall is RT over g, which is about 8,400 meters for the earth—the earth's atmosphere. So I'll put 24 kilometers up here, that scale height there, and I'll put the sea level pressure there, and that's this formula here. So I put everything where I said I would and I get a value of about 6,000 Pascals for the air pressure up here.

And then I can simply put that into a ratio of the two pressures to get the mixing ratio in the common units. So I put in that the value of 0.03/6,000, I get 20 ppmv for the ozone mixing ratio in the peak of the ozone layer. OK? Pretty straightforward calculation. Had to find out how much air was there and then from the proper ratio get it in the proper units. 

Now compare this to what we were talking about last time. The EPA one hour standard for ozone in the boundary layer, where we live, was 0.12 in those same units. So the ozone concentration in the ozone layer is about 200 times the EPA one hour limit, just to give you a relative sense for that. So if we were up there, that would be really tough for us to breathe that in.

Now remember however, there's not enough air to keep us alive anyway at those altitudes. But the ozone concentration is quite large as well. So that's an inhospitable environment up there at that altitude. Any questions on that?

But it is possible, or it was until recently, possible for us to fly at those altitudes. So the Concorde, this beautiful, supersonic, commercial airliner which stopped flying a couple years ago, it flew at an altitude typically of 17 kilometers, which isn't quite at the peak of the ozone concentration, but it's definitely up in the ozone layer, there's no doubt about that.

And of course, you have to refresh the cabin air. You can't just lock the cabin doors and expect everybody to live off that cabin air until you get to your destination. So the aircraft can continually draw in air from outside to flush the cabin and give you fresh cabin air. The question is how do you do that in this case?

Remember though that there's two problems. First of all, there's very little air outside anyway. So you're going to have to compress it with some kind of a pump before you can put it into the cabin so people can breathe it. And that's going to be a big compression.

Let's just think about this for a minute. So if you're flying at 17 kilometers and you want to get that air compressed to cabin pressure, well first of all what cabin pressure do you normally have? Airliners don't usually keep you at sea level pressure. If you're taking off from New York City, as you take off the pressure will drop--in the cabin will drop as you climb, but they'll usually only let it drop to maybe an equivalent altitude of 6,000 feet, something like that. Then they couldn't let it go any lower or you'd start to have trouble breathing, but they don't keep you at sea level.

So I'm going to assume that the, for this calculation, that the cabin altitude, what's called the cabin altitude, is going to be kept at 2 kilometers. That's about 6,000 feet above the surface. In fact, you're flying at 17 kilometers, so you've got to take air and compress it just as you would if you took a parcel of air from 17 kilometers and brought it down to two kilometers, it would compress, adiabatic compression, we've talked about this over and over again. And it would heat as you compress it. So when you put it into that pump and compress it, it's going to warm up, and how much?

Well this is a calculation you can do in your head. You know the adiabatic lapse rate is about 10 degrees per kilometer, actually it's 9.8, but I've rounded it off. That's a difference in 15 kilometers, so essentially you're going to get 150 degrees Celsius warming when you compress that air about ready to put it into the cabin. Now you’re going to--it started off pretty cold, but still that's going to be really hot air.

Two things I've got to say about that. First of all, the convenient thing is that that temperature will destroy the ozone. So just by compressing the air, getting it ready to put into the cabin, you're going to kill off that ozone automatically. So that's pretty convenient.

But then remember, you're going to have to cool it a bit before you put it into the cabin. So you're going to put it through another heat exchanger, cool it back down, and then you get the air to stick into the cabin. If you don't get it all out, you can pass the air through an activated charcoal filter and that will remove any remaining ozone that's in the air before you put it into the cabin.

So it is possible to fly at those altitudes right in the middle of the ozone layer. But that's how you have to do it. Any questions on that? Yeah?

Student: Why would you want to fly at that high of an altitude?

Professor Ron Smith: It turns out it has mostly to do with the efficiency of the engines. In fact, that's true with subsonic aircraft as well. The reason they fly quite high, 35,000 feet, 37,000 feet is because the engines are most efficient at that altitude. And for this aircraft, the more efficient—the altitude was even higher at 17 kilometers. So anything on that? OK, maybe that's a little tangential point, but--

Chapter 4: The Dobson Unit [00:12:17]

This thing called the Dobson unit is a column density. We could write it, in fact, I wish that they had chosen the unit of kilograms per square meter. You'd simply integrate top to bottom in the atmosphere and say how many kilograms of ozone are there in each square meter of footprint for that column that you've integrated over. But for historic reasons they didn't do it that way.

This is typically measured by looking at light coming from the sun and see how much ultraviolet light reaches the surface of the earth. And that will tell you how much ozone there is in the column. What they're using for the Dobson unit is this definition, it's 1 milli-atmosphere-centimeters. That's the scientific definition of a Dobson unit.

And here's how you think about that. You take all the ozone in the column, you compress it to standard temperature and pressure, that is 1 atmosphere of pressure and 0 degrees Celsius. And then you measure how thick that layer would be composed just of ozone and you measure that in thousandth of a centimeter. And that is your Dobson unit value.

And I'll be showing you this kind of data. Typical values for the earth's atmosphere run from 100 to 500 Dobson units depending on where you are around the world. But that is normally--the Dobson is what we use to describe to each other how much ozone there is in the atmosphere.

So for example, you'll see a diagram like this from NASA. It's a map of the world and some areas are blocked out because they don't have data from here for this particular time of year. But the scale is in Dobson units.

So in the tropical regions you're getting values of about 250, 260 Dobson units. In the green you're getting values maybe 325, maybe in some areas even 350 or 360. So that's the way we represent column integrated ozone in this subject.

We can also get vertical profile information either by direct in situ measurements or by looking at light passing across the earth's atmosphere horizontally. This gives you some idea where the ozone layer is as a function of latitude. So here's latitude, North Pole, South Pole, Equator, altitude. But notice the offset 0, this isn't the ground, this is starting at 16 kilometers, right?

So the ozone layer at higher latitudes, it peaks at an altitude of about 20 kilometers, 18, 19, 20 kilometers. But near the Equator, it's up around 25, 26 kilometers. I think we expected this because remember the tropopause has the same kind of a structure. It's higher in the equatorial regions and lower in the polar regions. So the ozone layer is about the same place in the stratosphere, but the stratosphere starts at a different level. So it's lifted in a sense relative to the surface of the earth in the equatorial latitudes.

The reason why--part of the reason why there is that difference is because of a broad circulation that takes place in the stratosphere calls call the Brewer-Dobson circulation, which I won't talk about, but this artist has reminded us that there is this broad slow circulation in the stratosphere.

Chapter 5: Origin and Maintenance of Ozone in the Stratosphere [00:16:18]

So why do we have ozone in the stratosphere? This is a basic chemistry of that. We know we have a lot of O2 in our atmosphere, it's the second most common molecule. And occasionally, a high energy photon will come from the sun. A high energy ultraviolet photon will dissociate that diatomic oxygen into two oxygen atoms. It doesn't happen very often because it needs a high energy photon to do that.

But then you've got some free oxygen atoms floating around. And when you have that, that'll quickly combine with an O2 molecule to form ozone. That's a fast reaction. But equally fast is that other ultraviolet photons, they don't have to be as energetic as this one, these can be in the near, or in the UVA, for example, the near ultraviolet, will dissociate that back to this again. And that's fast.

So what you've got is a rapid recycling going on between that and that that's just going out all the time in the stratosphere, but maintaining a balance here. Now occasionally an ozone will interact with one of these atoms and go back to go to O2, which is the common form. And then you've lost it. So you've lost the ozone permanently or at least quasi permanently.

So the ozone concentration is controlled by--ignoring this rapid cycling which doesn't have any long-term effect, is controlled by the balance of these two slow processes. How often are you producing oxygen atoms and how often are you removing ozone and oxygen atoms back to O2? And that would be occurring naturally because we have oxygen in the atmosphere and we have ultraviolet light coming from the sun. So that's the background to why there is ozone in the stratosphere.

The reason why it doesn't happen further down is because most of these ultraviolet photons are absorbed up there and don't get--don’t penetrate deeper into the atmosphere. So it's a layer up there because that's where these ultraviolet photons are being absorbed and doing this work causing these photochemical reactions to take place. Questions on that?

Student: I'm curious if there is some sort of feedback mechanism or if the two slow reactions are interconnected in some way. Because I guess if you have more free oxygen atoms that would allow the second process to occur more frequently?

Professor Ron Smith: Well you'd have to write down the rate equations for these to answer that question, which I'm not prepared to do. But a chemical reaction, the rate at which it goes forward depends on how much of this do you have, and how much of this do you have, and how often are they colliding to allow the reaction to go forward? So I think your instinct is right that these are going to depend on the concentrations that you have.

So just like our tank experiment, the loss rate was proportional to how much you have. And that’s what, in a system like this where the generation term is independent of how much of this you have, but the loss rate is proportional, you're going to come to an equilibrium, just like we did in the water tank. And that's what I'm implying here, that you've got that kind of balance between a source which is just there and then a sink which is dependent on how much you have. And that will give a natural equilibrium eventually.

And this is a cartoon showing that. So the sun is playing a role. Occasionally it will dissociate O2 to form a simple O, an O molecule. And then you get this rapid recycling between oxygen atoms and ozone. And then occasionally, you'll go back just to the O2 again. And the control of how much you have is primarily these processes, not this. This is just a rapid recycling of the two types of odd oxygen.

Now to remind you about the spectrum of radiation coming from the sun, I show you this, you've seen it before. The visible spectrum is here, the infrared, we're really interested in the ultraviolet over here in today's conversation, which starts at 400 nanometers wavelength and goes shorter than that.

And these standard definitions are helpful. UVA are the wavelengths closest to the visible, closest to the blue, defined as going from 400 to 315 nanometers. UVB, 315 to 280, and then UVC, 280 all the way down to 100 nanometers.

Now you may remember from physics of that the shorter wavelength photons are more energetic, they'll do more damage. So UVA would cause tanning of your skin, but probably wouldn't give you skin cancer, UVB might. But UVC is very damaging. In fact, it's used in doctor's offices to clean surgical tools. You put it into this autoclave which will put UVC on your instruments and it'll kill anything that's there very, very quickly. It's very damaging radiation.

And so here's the fate of UVA, UVB, and UVC. Some UVA comes through. Half or more of the UVB is absorbed in the ozone layer, plotted on top of this. This is altitude and Dobson units per kilometer to get where the ozone layer is. So about 1/2 or 2/3 of the UVB is absorbed by the ozone. And almost all, thankfully, of the UVC coming from the sun is absorbed by the ozone layer. So it's a very good protective shield for us.

Now what could be changing this natural ozone layer situation? Well this is the current theory for it. It involves a catalytic reaction primarily using chlorine atoms. Now bromine will do this as well and there is some bromine in the atmosphere, but I'm going to focus strictly on the chlorine.

And notice how this reaction works. So you have a chlorine atom reacting with an ozone molecule. It goes to ClO, it oxidizes the chlorine and gives you back O2. And then quite soon after that, you'll take this ClO and react it with another ozone molecule. That'll give you back the Cl that you started with and you have two O2s, two regular diatomic oxygens.

So look, the ozone is gone. In fact, two ozones are gone. And you've got your Cl back ready to do it again. So this is a catalytic reaction where the catalyst here is the chlorine atom. It can be used over and over again to destroy ozone molecules in this way. Two ozone molecules are lost and the catalyst is free, ready to go back, and do it again.

That's the idea of what's today, now that we have a measurable chlorine concentration in the atmosphere, this is why we think the ozone layer is decreasing. Questions on that?

So up here in the inset is the emission. This is an example, CFC 11, one of the chlorofluorocarbons. And here's the emission rate for it in megatons per year starting in 1960 going up to the year 2000. So you see there was a rapid rise in it as it was being used in refrigeration. And when you put it into a refrigerator, it may be contained for a few years, but eventually it leaks out or someone tosses away the refrigerator and it gets a leak and it goes to the atmosphere. So almost everything you produce eventually makes its way into the atmosphere.

The Montreal Protocol that I'll talk about was here, where you begin to see it finally decreased. But because it's a long lived molecule, here's the CFC abundance in the atmosphere, it increased through this period of time, we've stopped putting it in, but it's still there. It has a lifetime--from this data, it looks like the lifetime is about 50 years. And so it's going to take probably another 50 or 100 before we see much of a decline. Thank goodness we've stopped putting it in, but it's not going to decrease to 0 just because we’ve stopped putting it in. It's going to be stored in the atmosphere for a while. It has a long lifetime.

The circles are data from the Northern Hemisphere and the triangles are data from the Southern Hemisphere. But they're pretty much the same. So the two hemispheres, this stuff gets mixed back and forth. Probably most of it was put into the atmosphere in the Northern Hemisphere, but now it's mixed to be pretty much equal in the two hemispheres. Questions on that?

Here is the global averaged total ozone change in percent starting back in 1965. Around 1980 is when we began to see the rapid decrease, but it's not huge. Notice that it's leveled out today and it's only about 4% below where it started. So this global decrease in ozone is, I suppose it's something we should be concerned about, but it's not a huge factor. And it doesn't look like it's going to get any worse now that we’ve stopped putting CFCs into the atmosphere. So it's a concern, but not I would say a terribly difficult one.

Chapter 6: The Ozone Hole [00:27:35]

But then there's this ozone hole that I'll spend the rest of time talking about. And here's what it looked like a couple of days ago. You can go to this website, which I'm just about to do. I should have set this up before.

But here's a definition of it. So it's a brief, seasonal and local reduction in ozone. Location is Antarctica, the time of year that it happens each year is September, October. It first appeared in about 1978, apparently it wasn't happening before then. It was discovered in 1984 and then going back and looking at old data, they found that it actually had started some six years earlier than that.

The discovery of the depletion mechanism by Molina and Crutzen was a rather remarkable scientific discovery. I'll give you hints of how that went in just a minute. And they won the Nobel Prize in chemistry for that awarded in 1995 from that discovery of how the ozone depletion mechanism works. Ultimately, it's caused by these CFC emissions which then led to the Montreal Protocol, which I'll talk briefly about, a rather successful international treaty to limit the emissions of CFCs.

Here's a brief history of it. So at the top is the ozone hole area in millions of square kilometers. What you do is set a threshold for ozone concentration. If it's lower than that you say you're in the hole, if it's larger than that you're out of the hole. And then you keep track of that area.

And the ozone hole was nonexistent before '79. But now it has reached about 25 million square kilometers, you saw a picture of it. And it's kind of leveled out, it's not getting it better not getting any worse. And the minimum ozone in the middle of that hole is shown here in Dobson units, of course, getting worse but now leveling out to a value of about 100 Dobson units in the middle of the hole each year when it forms.

So give me just a moment here to bring up this website. I forgot to do this before. Let’s see here. I'll just Google and find it. Yeah, that's probably it. That's not the one I want. That's a good one, but it's not the one I'm looking for. I'll try to find this other one. Yeah, let's do an animation for this season. I’ll try this one.

Yeah, so this is this year's hole. It hasn't formed yet, we're back in July. This is Antarctica sitting there waiting. Now wait until we get into September. It's already beginning to form in the late summer, but now we're getting the ozone hole forming. October, peaks in October, and then it'll start to fill in up to the present date. It's starting to weaken now that we're into November.

So that's the deal. It happens every year in that region. You get this large area that forms with about half the ozone it would otherwise have. Ozone depletion is about 50 percent in that region compared to remember only 4% globally. So this is a localized anomaly.

You can go to that website yourself and do some other animations. So in this year then this tracks it, the ozone hole area climbed up to the 12th of September, now it's decreased. The minimum ozone dropped to a minimum now it's increasing now that we're into November.

And I'm going to be talking about the stratospheric temperature. Stratospheric temperature in that part of the world now at that altitude is beginning to climb because we're getting out of their winter and towards of their Southern Hemisphere summer. And I'll come back to talk about the role of temperature in just a moment.

So I think the most curious thing about the ozone hole is why only in September and October? Why just over the South Pole? And I guess what is the link to human activity? So there's a lot of natural processes and factors controlling this. And then there's the added impact of humans and somehow they have all come together to form this curious phenomena of the ozone hole.

In the few minutes I have left I want to just try to explain some of that. Now I hope you can see this, I'll read it through for you. This is the seasonal cycle of temperature in the stratosphere. And this is for 90 North to 65 North, so this is the north polar cap. This is 25 North to 25 South, so this is the equatorial region. And this is 65 South to 90 South, so this is the south polar region.

Now the temperatures in the Northern Hemisphere get down to minus 70 on occasion. These are at different altitudes, I believe, having trouble reading those. They get down to minus 70. In the equatorial regions, those temperatures get down to about minus 75. But here in the South Pole, they get down to minus 80 and even minus 85. So of all the places in the world's stratosphere, the Southern Hemisphere polar stratosphere, that is over Antarctica, has a winter time temperature colder than any other place on the planet.

That's one of the things that makes it special in regards to why the ozone hole forms there. Now why would that be? It has to do with the phenomena of polar stratospheric clouds, abbreviation PSCs. Polar stratospheric clouds. I haven't spoken about these in the course up to this present time, so this is new material.

These are ice clouds that form up in the stratosphere. Normally we think clouds don't form in the stratosphere, it's pretty dry up there, but these clouds do. They are not entirely made of water. They're  a mixture of water and nitric acid in these clouds. And they require an extraordinarily cold temperature in order to form, temperatures colder than minus 70 degrees Celsius, some would say even colder than that.

This is probably why the Antarctic stratosphere is so special because it's temperature gets cold enough in winter to allow these polar stratospheric clouds to occur, whereas other places around the globe in the stratosphere you don't get those clouds. What do they do then? How do they influence ozone?

Well these ice clouds, these ice particles, freeze up diatomic chlorine by holding the nitric acid in the ice itself. Notice when it forms it's storing HNO3, nitric acid. And here's the reaction. You start with HCl, and ClONO2, reaction goes to Cl2 and the nitric acid, and then this is locked up in the ice. So what you've done, you had your chlorine locked up in HCl and in this compound, there's chlorines in both. Now you've put the nitric acid in the ice and you've freed up the chlorine diatomic molecule. There’s only one step to go.

When the light returns, remember, that minimum of temperature occurred in perpetual darkness. At that season of the year, it's dark over Antarctica, there's no light. So you can get the ice forming, you can get the Cl all ready to go, but it's in the diatomic form. Then the light comes back, Southern Hemisphere spring, the light comes back, you can dissociate the Cl2 to form two Cl molecules. And then go back a few slides to that catalytic reaction and it starts to destroy ozone.

So you get that? So the ice frees up the chlorine, it then dissociates when light returns, and the catalytic destruction of ozone begins to take place, and the ozone hole forms. That's why it occurs where it does and when it does because of that necessary condition for cold. And then the ozone hole forms just after the light returns to that region. Any questions on that? Yes?.

Student: Where does the hydrochloric acid come from, the HCl?

Professor Ron Smith: I don't know. That's the way the chlorine is stored most of the time in the stratosphere, is in these two molecules. After the CFCs break down, the chlorine is transferred into these two forms and just hangs around. Then the PSCs form and that sucks off the nitric acid and the Cl is released. So this is the storage form of the chlorine during most of the year in the stratosphere.

Student: Is there normally chlorine in the atmosphere besides the HCl?

Professor Ron Smith: Not naturally. Remember this was CFCs and not chlorine. And that's a critical difference. But let me just go back to that. There were no CFCs in the atmosphere before that. There probably was a little bit of chlorine, but only a very small amount. But now we're putting it in through this CFC source. Sorry, that's not a complete answer, but that's the best I can do.

Chapter 7: The Montreal Protocol [00:39:04]

So I think this–yeah, this is the last one. So the Montreal Protocol, the full name of it is Montreal Protocol on Substances that Deplete the Ozone layer. It's an international treaty. And of course it was first signed in Montreal, it's got the name. Since then it has been modified several times. Certain forms of substances have been added to the banned list, others have been allowed. So it's been modified a few times.

Generally what it did is by the time we came to 1994, it had banned pretty much all emissions of CFCs into the atmosphere. It largely replaced them with the HCFC, which can also be used as a refrigerant, but has a much shorter lifetime and will not cause the chlorine to build up in the atmosphere. It may have some other problems, it may be a greenhouse gas. But it doesn't have the problem of putting permanently chlorine into the atmosphere the way that the CFCs do.

Now one of the reasons why this treaty was signed, because as you'd expect the refrigerant industry vigorously opposed this, was at about the time it was signed, Dupont which was this large chemical company and the world's largest producer of CFCs, discovered a replacement for it. And they realized that they would not lose their business because they had a replacement ready to go and they might even be able to make more money by doing this changeover to a different type of refrigerant. So in the end, they dropped their opposition and many countries of the world got together and signed this international treaty.

And it's viewed in environmental circles as being one of the best examples of a successful situation where the scientists discovered the problem, suggested how to solve it, and then you have the countries of the world actually getting together to pass a treaty and acting on it. As you saw, the CFC emissions have dropped nearly to 0 now. So it's looked at as, well in some cases, in wonderment because how could we do something like this today, it's not clear. But at least we have this example of how environmental science can prevent these problems.

Any questions on this? Great we're done a little bit early again today. So enjoy your Thanksgiving break and I'll see you a week from Monday.

Student: Professor, I'm still a little unclear as to why that same phenomenon with the ozone hole doesn't occur in the Northern Hemisphere.

Professor Ron Smith: Yeah, I should have said more about that. As you saw, it doesn't get as cold. And that is because it's believed there's more mixing between the high and low latitudes. It does want to get cold over the pole in winter, there's no sunlight getting there, it's radiating to space. But in the Northern Hemisphere, there's more north south mixing in the stratosphere to keep it warm. And that they think is because there's more continents, more of a continent ocean contrast in the Northern Hemisphere. So more disturbances in general that are causing a north south mixing, keeping it warmer up there.

[end of transcript]

Chapter 1: Ozone in the Atmosphere [00:00:00]

Professor Ron Smith:--stuff, so show off a little bit. We're going to finish up the ozone thing by doing the stratospheric ozone. It was interesting though that just yesterday there was this big New York Times article about the politics of setting the EPA standard for ozone. How many saw that article? I know Sam did and Rachel did. But it was long, like a four page article or something. All about the interaction between the EPA and the White House trying to drop that eight hours standard from, was it 0.08 down to 0.07.

So a lot of political heartache and dispute about whether that should be dropped to 0.07. And if you're into that sort of stuff, read about the politics of that. It fits nicely with the discussion we had in class last time. Anything else? Any issues, questions?

Chapter 2: Terminology for Stratospheric Ozone [00:01:13]

OK, so we're going to do the stratospheric ozone problem today. And these are the new terminology you should be aware of. So we'll talk about the ozone layer and then the ozone hole. The Dobson unit I'll discuss, I'll define as a measure of column integrated ozone concentration.

We'll be talking about CFCs which had been used as the primary liquid in refrigerators. And you need to have a fluid that flows back and forth between the cold sink and the warm sink in a refrigerator. And CFCs was the common chemical to do that. But now they've been replaced largely by HCFCs, the hydrochlorofluorocarbons. Say that 10 times fast. And so I'll define those as well.

We'll talk at some point today about polar stratospheric clouds, catalytic reactions, which are reactions in which one particular chemical compound allows the reaction to go forward but in the end itself isn't changed. So it can be reused over and over again to make the same chemical cycle go forward. Photochemical reactions, which are chemical reactions that involve light. You have to have photons of light in order for that reaction to go forward. And we'll talk about the Montreal Protocol. So all this will come in turn today.

Chapter 3: The Ozone Layer [00:02:55]

First of all, I showed this diagram last time as we discussed this part of the ozone issue, but now we're going to talk about the ozone layer. And as it says here, contains 90% of the atmospheric ozone. So if you do a column integration from the top of the atmosphere to the bottom, most of the contribution comes from the ozone layer.

What's plotted here is the partial pressure of ozone in milliPascals. Remember that the total pressure, the air pressure, decreases strongly and exponentially all the way on up through the troposphere and the stratosphere. So this has a very different vertical structure than the total pressure.

It has a beneficial role, it acts as a shield against ultraviolet radiation that would otherwise cause skin cancer, skin damage. And the current trends are a long-term global downtrend in stratospheric ozone. And then most famously, this ozone hole which occurs in the springtime of the Antarctic part of the earth each year. And then some more recent discussion about possibly this starting up in the Northern Hemisphere as well, but this is not as well documented.

So just a couple of calculations then. So I want to compute the mixing ratio in ppmv for ozone to air using this point on this slide. So I'm going to say that at about 24 kilometers altitude you've got a partial pressure of ozone that's about 30 milliPascals. And I want to convert that to a mixing ratio because that's what we've been used to using, what we used last time.

So first of all, because it's going to be a mixing ratio you need to know how much air is up there as well, not just how much ozone, but how much air. So I'm going to use this simple formula that we've used quite often in the course. It's the approximate exponential form for pressure as a function of altitude where you use a scale height, Hs. You know the pressure at the surface of the earth, you know the altitude you're inquiring about, and this will give you the pressure at that altitude

So sea level pressure we'll take to be 1,013 millibars, which is the standard average value, convert that to Pascals. The scale height as you recall is RT over g, which is about 8,400 meters for the earth—the earth's atmosphere. So I'll put 24 kilometers up here, that scale height there, and I'll put the sea level pressure there, and that's this formula here. So I put everything where I said I would and I get a value of about 6,000 Pascals for the air pressure up here.

And then I can simply put that into a ratio of the two pressures to get the mixing ratio in the common units. So I put in that the value of 0.03/6,000, I get 20 ppmv for the ozone mixing ratio in the peak of the ozone layer. OK? Pretty straightforward calculation. Had to find out how much air was there and then from the proper ratio get it in the proper units. 

Now compare this to what we were talking about last time. The EPA one hour standard for ozone in the boundary layer, where we live, was 0.12 in those same units. So the ozone concentration in the ozone layer is about 200 times the EPA one hour limit, just to give you a relative sense for that. So if we were up there, that would be really tough for us to breathe that in.

Now remember however, there's not enough air to keep us alive anyway at those altitudes. But the ozone concentration is quite large as well. So that's an inhospitable environment up there at that altitude. Any questions on that?

But it is possible, or it was until recently, possible for us to fly at those altitudes. So the Concorde, this beautiful, supersonic, commercial airliner which stopped flying a couple years ago, it flew at an altitude typically of 17 kilometers, which isn't quite at the peak of the ozone concentration, but it's definitely up in the ozone layer, there's no doubt about that.

And of course, you have to refresh the cabin air. You can't just lock the cabin doors and expect everybody to live off that cabin air until you get to your destination. So the aircraft can continually draw in air from outside to flush the cabin and give you fresh cabin air. The question is how do you do that in this case?

Remember though that there's two problems. First of all, there's very little air outside anyway. So you're going to have to compress it with some kind of a pump before you can put it into the cabin so people can breathe it. And that's going to be a big compression.

Let's just think about this for a minute. So if you're flying at 17 kilometers and you want to get that air compressed to cabin pressure, well first of all what cabin pressure do you normally have? Airliners don't usually keep you at sea level pressure. If you're taking off from New York City, as you take off the pressure will drop--in the cabin will drop as you climb, but they'll usually only let it drop to maybe an equivalent altitude of 6,000 feet, something like that. Then they couldn't let it go any lower or you'd start to have trouble breathing, but they don't keep you at sea level.

So I'm going to assume that the, for this calculation, that the cabin altitude, what's called the cabin altitude, is going to be kept at 2 kilometers. That's about 6,000 feet above the surface. In fact, you're flying at 17 kilometers, so you've got to take air and compress it just as you would if you took a parcel of air from 17 kilometers and brought it down to two kilometers, it would compress, adiabatic compression, we've talked about this over and over again. And it would heat as you compress it. So when you put it into that pump and compress it, it's going to warm up, and how much?

Well this is a calculation you can do in your head. You know the adiabatic lapse rate is about 10 degrees per kilometer, actually it's 9.8, but I've rounded it off. That's a difference in 15 kilometers, so essentially you're going to get 150 degrees Celsius warming when you compress that air about ready to put it into the cabin. Now you’re going to--it started off pretty cold, but still that's going to be really hot air.

Two things I've got to say about that. First of all, the convenient thing is that that temperature will destroy the ozone. So just by compressing the air, getting it ready to put into the cabin, you're going to kill off that ozone automatically. So that's pretty convenient.

But then remember, you're going to have to cool it a bit before you put it into the cabin. So you're going to put it through another heat exchanger, cool it back down, and then you get the air to stick into the cabin. If you don't get it all out, you can pass the air through an activated charcoal filter and that will remove any remaining ozone that's in the air before you put it into the cabin.

So it is possible to fly at those altitudes right in the middle of the ozone layer. But that's how you have to do it. Any questions on that? Yeah?

Student: Why would you want to fly at that high of an altitude?

Professor Ron Smith: It turns out it has mostly to do with the efficiency of the engines. In fact, that's true with subsonic aircraft as well. The reason they fly quite high, 35,000 feet, 37,000 feet is because the engines are most efficient at that altitude. And for this aircraft, the more efficient—the altitude was even higher at 17 kilometers. So anything on that? OK, maybe that's a little tangential point, but--

Chapter 4: The Dobson Unit [00:12:17]

This thing called the Dobson unit is a column density. We could write it, in fact, I wish that they had chosen the unit of kilograms per square meter. You'd simply integrate top to bottom in the atmosphere and say how many kilograms of ozone are there in each square meter of footprint for that column that you've integrated over. But for historic reasons they didn't do it that way.

This is typically measured by looking at light coming from the sun and see how much ultraviolet light reaches the surface of the earth. And that will tell you how much ozone there is in the column. What they're using for the Dobson unit is this definition, it's 1 milli-atmosphere-centimeters. That's the scientific definition of a Dobson unit.

And here's how you think about that. You take all the ozone in the column, you compress it to standard temperature and pressure, that is 1 atmosphere of pressure and 0 degrees Celsius. And then you measure how thick that layer would be composed just of ozone and you measure that in thousandth of a centimeter. And that is your Dobson unit value.

And I'll be showing you this kind of data. Typical values for the earth's atmosphere run from 100 to 500 Dobson units depending on where you are around the world. But that is normally--the Dobson is what we use to describe to each other how much ozone there is in the atmosphere.

So for example, you'll see a diagram like this from NASA. It's a map of the world and some areas are blocked out because they don't have data from here for this particular time of year. But the scale is in Dobson units.

So in the tropical regions you're getting values of about 250, 260 Dobson units. In the green you're getting values maybe 325, maybe in some areas even 350 or 360. So that's the way we represent column integrated ozone in this subject.

We can also get vertical profile information either by direct in situ measurements or by looking at light passing across the earth's atmosphere horizontally. This gives you some idea where the ozone layer is as a function of latitude. So here's latitude, North Pole, South Pole, Equator, altitude. But notice the offset 0, this isn't the ground, this is starting at 16 kilometers, right?

So the ozone layer at higher latitudes, it peaks at an altitude of about 20 kilometers, 18, 19, 20 kilometers. But near the Equator, it's up around 25, 26 kilometers. I think we expected this because remember the tropopause has the same kind of a structure. It's higher in the equatorial regions and lower in the polar regions. So the ozone layer is about the same place in the stratosphere, but the stratosphere starts at a different level. So it's lifted in a sense relative to the surface of the earth in the equatorial latitudes.

The reason why--part of the reason why there is that difference is because of a broad circulation that takes place in the stratosphere calls call the Brewer-Dobson circulation, which I won't talk about, but this artist has reminded us that there is this broad slow circulation in the stratosphere.

Chapter 5: Origin and Maintenance of Ozone in the Stratosphere [00:16:18]

So why do we have ozone in the stratosphere? This is a basic chemistry of that. We know we have a lot of O2 in our atmosphere, it's the second most common molecule. And occasionally, a high energy photon will come from the sun. A high energy ultraviolet photon will dissociate that diatomic oxygen into two oxygen atoms. It doesn't happen very often because it needs a high energy photon to do that.

But then you've got some free oxygen atoms floating around. And when you have that, that'll quickly combine with an O2 molecule to form ozone. That's a fast reaction. But equally fast is that other ultraviolet photons, they don't have to be as energetic as this one, these can be in the near, or in the UVA, for example, the near ultraviolet, will dissociate that back to this again. And that's fast.

So what you've got is a rapid recycling going on between that and that that's just going out all the time in the stratosphere, but maintaining a balance here. Now occasionally an ozone will interact with one of these atoms and go back to go to O2, which is the common form. And then you've lost it. So you've lost the ozone permanently or at least quasi permanently.

So the ozone concentration is controlled by--ignoring this rapid cycling which doesn't have any long-term effect, is controlled by the balance of these two slow processes. How often are you producing oxygen atoms and how often are you removing ozone and oxygen atoms back to O2? And that would be occurring naturally because we have oxygen in the atmosphere and we have ultraviolet light coming from the sun. So that's the background to why there is ozone in the stratosphere.

The reason why it doesn't happen further down is because most of these ultraviolet photons are absorbed up there and don't get--don’t penetrate deeper into the atmosphere. So it's a layer up there because that's where these ultraviolet photons are being absorbed and doing this work causing these photochemical reactions to take place. Questions on that?

Student: I'm curious if there is some sort of feedback mechanism or if the two slow reactions are interconnected in some way. Because I guess if you have more free oxygen atoms that would allow the second process to occur more frequently?

Professor Ron Smith: Well you'd have to write down the rate equations for these to answer that question, which I'm not prepared to do. But a chemical reaction, the rate at which it goes forward depends on how much of this do you have, and how much of this do you have, and how often are they colliding to allow the reaction to go forward? So I think your instinct is right that these are going to depend on the concentrations that you have.

So just like our tank experiment, the loss rate was proportional to how much you have. And that’s what, in a system like this where the generation term is independent of how much of this you have, but the loss rate is proportional, you're going to come to an equilibrium, just like we did in the water tank. And that's what I'm implying here, that you've got that kind of balance between a source which is just there and then a sink which is dependent on how much you have. And that will give a natural equilibrium eventually.

And this is a cartoon showing that. So the sun is playing a role. Occasionally it will dissociate O2 to form a simple O, an O molecule. And then you get this rapid recycling between oxygen atoms and ozone. And then occasionally, you'll go back just to the O2 again. And the control of how much you have is primarily these processes, not this. This is just a rapid recycling of the two types of odd oxygen.

Now to remind you about the spectrum of radiation coming from the sun, I show you this, you've seen it before. The visible spectrum is here, the infrared, we're really interested in the ultraviolet over here in today's conversation, which starts at 400 nanometers wavelength and goes shorter than that.

And these standard definitions are helpful. UVA are the wavelengths closest to the visible, closest to the blue, defined as going from 400 to 315 nanometers. UVB, 315 to 280, and then UVC, 280 all the way down to 100 nanometers.

Now you may remember from physics of that the shorter wavelength photons are more energetic, they'll do more damage. So UVA would cause tanning of your skin, but probably wouldn't give you skin cancer, UVB might. But UVC is very damaging. In fact, it's used in doctor's offices to clean surgical tools. You put it into this autoclave which will put UVC on your instruments and it'll kill anything that's there very, very quickly. It's very damaging radiation.

And so here's the fate of UVA, UVB, and UVC. Some UVA comes through. Half or more of the UVB is absorbed in the ozone layer, plotted on top of this. This is altitude and Dobson units per kilometer to get where the ozone layer is. So about 1/2 or 2/3 of the UVB is absorbed by the ozone. And almost all, thankfully, of the UVC coming from the sun is absorbed by the ozone layer. So it's a very good protective shield for us.

Now what could be changing this natural ozone layer situation? Well this is the current theory for it. It involves a catalytic reaction primarily using chlorine atoms. Now bromine will do this as well and there is some bromine in the atmosphere, but I'm going to focus strictly on the chlorine.

And notice how this reaction works. So you have a chlorine atom reacting with an ozone molecule. It goes to ClO, it oxidizes the chlorine and gives you back O2. And then quite soon after that, you'll take this ClO and react it with another ozone molecule. That'll give you back the Cl that you started with and you have two O2s, two regular diatomic oxygens.

So look, the ozone is gone. In fact, two ozones are gone. And you've got your Cl back ready to do it again. So this is a catalytic reaction where the catalyst here is the chlorine atom. It can be used over and over again to destroy ozone molecules in this way. Two ozone molecules are lost and the catalyst is free, ready to go back, and do it again.

That's the idea of what's today, now that we have a measurable chlorine concentration in the atmosphere, this is why we think the ozone layer is decreasing. Questions on that?

So up here in the inset is the emission. This is an example, CFC 11, one of the chlorofluorocarbons. And here's the emission rate for it in megatons per year starting in 1960 going up to the year 2000. So you see there was a rapid rise in it as it was being used in refrigeration. And when you put it into a refrigerator, it may be contained for a few years, but eventually it leaks out or someone tosses away the refrigerator and it gets a leak and it goes to the atmosphere. So almost everything you produce eventually makes its way into the atmosphere.

The Montreal Protocol that I'll talk about was here, where you begin to see it finally decreased. But because it's a long lived molecule, here's the CFC abundance in the atmosphere, it increased through this period of time, we've stopped putting it in, but it's still there. It has a lifetime--from this data, it looks like the lifetime is about 50 years. And so it's going to take probably another 50 or 100 before we see much of a decline. Thank goodness we've stopped putting it in, but it's not going to decrease to 0 just because we’ve stopped putting it in. It's going to be stored in the atmosphere for a while. It has a long lifetime.

The circles are data from the Northern Hemisphere and the triangles are data from the Southern Hemisphere. But they're pretty much the same. So the two hemispheres, this stuff gets mixed back and forth. Probably most of it was put into the atmosphere in the Northern Hemisphere, but now it's mixed to be pretty much equal in the two hemispheres. Questions on that?

Here is the global averaged total ozone change in percent starting back in 1965. Around 1980 is when we began to see the rapid decrease, but it's not huge. Notice that it's leveled out today and it's only about 4% below where it started. So this global decrease in ozone is, I suppose it's something we should be concerned about, but it's not a huge factor. And it doesn't look like it's going to get any worse now that we’ve stopped putting CFCs into the atmosphere. So it's a concern, but not I would say a terribly difficult one.

Chapter 6: The Ozone Hole [00:27:35]

But then there's this ozone hole that I'll spend the rest of time talking about. And here's what it looked like a couple of days ago. You can go to this website, which I'm just about to do. I should have set this up before.

But here's a definition of it. So it's a brief, seasonal and local reduction in ozone. Location is Antarctica, the time of year that it happens each year is September, October. It first appeared in about 1978, apparently it wasn't happening before then. It was discovered in 1984 and then going back and looking at old data, they found that it actually had started some six years earlier than that.

The discovery of the depletion mechanism by Molina and Crutzen was a rather remarkable scientific discovery. I'll give you hints of how that went in just a minute. And they won the Nobel Prize in chemistry for that awarded in 1995 from that discovery of how the ozone depletion mechanism works. Ultimately, it's caused by these CFC emissions which then led to the Montreal Protocol, which I'll talk briefly about, a rather successful international treaty to limit the emissions of CFCs.

Here's a brief history of it. So at the top is the ozone hole area in millions of square kilometers. What you do is set a threshold for ozone concentration. If it's lower than that you say you're in the hole, if it's larger than that you're out of the hole. And then you keep track of that area.

And the ozone hole was nonexistent before '79. But now it has reached about 25 million square kilometers, you saw a picture of it. And it's kind of leveled out, it's not getting it better not getting any worse. And the minimum ozone in the middle of that hole is shown here in Dobson units, of course, getting worse but now leveling out to a value of about 100 Dobson units in the middle of the hole each year when it forms.

So give me just a moment here to bring up this website. I forgot to do this before. Let’s see here. I'll just Google and find it. Yeah, that's probably it. That's not the one I want. That's a good one, but it's not the one I'm looking for. I'll try to find this other one. Yeah, let's do an animation for this season. I’ll try this one.

Yeah, so this is this year's hole. It hasn't formed yet, we're back in July. This is Antarctica sitting there waiting. Now wait until we get into September. It's already beginning to form in the late summer, but now we're getting the ozone hole forming. October, peaks in October, and then it'll start to fill in up to the present date. It's starting to weaken now that we're into November.

So that's the deal. It happens every year in that region. You get this large area that forms with about half the ozone it would otherwise have. Ozone depletion is about 50 percent in that region compared to remember only 4% globally. So this is a localized anomaly.

You can go to that website yourself and do some other animations. So in this year then this tracks it, the ozone hole area climbed up to the 12th of September, now it's decreased. The minimum ozone dropped to a minimum now it's increasing now that we're into November.

And I'm going to be talking about the stratospheric temperature. Stratospheric temperature in that part of the world now at that altitude is beginning to climb because we're getting out of their winter and towards of their Southern Hemisphere summer. And I'll come back to talk about the role of temperature in just a moment.

So I think the most curious thing about the ozone hole is why only in September and October? Why just over the South Pole? And I guess what is the link to human activity? So there's a lot of natural processes and factors controlling this. And then there's the added impact of humans and somehow they have all come together to form this curious phenomena of the ozone hole.

In the few minutes I have left I want to just try to explain some of that. Now I hope you can see this, I'll read it through for you. This is the seasonal cycle of temperature in the stratosphere. And this is for 90 North to 65 North, so this is the north polar cap. This is 25 North to 25 South, so this is the equatorial region. And this is 65 South to 90 South, so this is the south polar region.

Now the temperatures in the Northern Hemisphere get down to minus 70 on occasion. These are at different altitudes, I believe, having trouble reading those. They get down to minus 70. In the equatorial regions, those temperatures get down to about minus 75. But here in the South Pole, they get down to minus 80 and even minus 85. So of all the places in the world's stratosphere, the Southern Hemisphere polar stratosphere, that is over Antarctica, has a winter time temperature colder than any other place on the planet.

That's one of the things that makes it special in regards to why the ozone hole forms there. Now why would that be? It has to do with the phenomena of polar stratospheric clouds, abbreviation PSCs. Polar stratospheric clouds. I haven't spoken about these in the course up to this present time, so this is new material.

These are ice clouds that form up in the stratosphere. Normally we think clouds don't form in the stratosphere, it's pretty dry up there, but these clouds do. They are not entirely made of water. They're  a mixture of water and nitric acid in these clouds. And they require an extraordinarily cold temperature in order to form, temperatures colder than minus 70 degrees Celsius, some would say even colder than that.

This is probably why the Antarctic stratosphere is so special because it's temperature gets cold enough in winter to allow these polar stratospheric clouds to occur, whereas other places around the globe in the stratosphere you don't get those clouds. What do they do then? How do they influence ozone?

Well these ice clouds, these ice particles, freeze up diatomic chlorine by holding the nitric acid in the ice itself. Notice when it forms it's storing HNO3, nitric acid. And here's the reaction. You start with HCl, and ClONO2, reaction goes to Cl2 and the nitric acid, and then this is locked up in the ice. So what you've done, you had your chlorine locked up in HCl and in this compound, there's chlorines in both. Now you've put the nitric acid in the ice and you've freed up the chlorine diatomic molecule. There’s only one step to go.

When the light returns, remember, that minimum of temperature occurred in perpetual darkness. At that season of the year, it's dark over Antarctica, there's no light. So you can get the ice forming, you can get the Cl all ready to go, but it's in the diatomic form. Then the light comes back, Southern Hemisphere spring, the light comes back, you can dissociate the Cl2 to form two Cl molecules. And then go back a few slides to that catalytic reaction and it starts to destroy ozone.

So you get that? So the ice frees up the chlorine, it then dissociates when light returns, and the catalytic destruction of ozone begins to take place, and the ozone hole forms. That's why it occurs where it does and when it does because of that necessary condition for cold. And then the ozone hole forms just after the light returns to that region. Any questions on that? Yes?.

Student: Where does the hydrochloric acid come from, the HCl?

Professor Ron Smith: I don't know. That's the way the chlorine is stored most of the time in the stratosphere, is in these two molecules. After the CFCs break down, the chlorine is transferred into these two forms and just hangs around. Then the PSCs form and that sucks off the nitric acid and the Cl is released. So this is the storage form of the chlorine during most of the year in the stratosphere.

Student: Is there normally chlorine in the atmosphere besides the HCl?

Professor Ron Smith: Not naturally. Remember this was CFCs and not chlorine. And that's a critical difference. But let me just go back to that. There were no CFCs in the atmosphere before that. There probably was a little bit of chlorine, but only a very small amount. But now we're putting it in through this CFC source. Sorry, that's not a complete answer, but that's the best I can do.

Chapter 7: The Montreal Protocol [00:39:04]

So I think this–yeah, this is the last one. So the Montreal Protocol, the full name of it is Montreal Protocol on Substances that Deplete the Ozone layer. It's an international treaty. And of course it was first signed in Montreal, it's got the name. Since then it has been modified several times. Certain forms of substances have been added to the banned list, others have been allowed. So it's been modified a few times.

Generally what it did is by the time we came to 1994, it had banned pretty much all emissions of CFCs into the atmosphere. It largely replaced them with the HCFC, which can also be used as a refrigerant, but has a much shorter lifetime and will not cause the chlorine to build up in the atmosphere. It may have some other problems, it may be a greenhouse gas. But it doesn't have the problem of putting permanently chlorine into the atmosphere the way that the CFCs do.

Now one of the reasons why this treaty was signed, because as you'd expect the refrigerant industry vigorously opposed this, was at about the time it was signed, Dupont which was this large chemical company and the world's largest producer of CFCs, discovered a replacement for it. And they realized that they would not lose their business because they had a replacement ready to go and they might even be able to make more money by doing this changeover to a different type of refrigerant. So in the end, they dropped their opposition and many countries of the world got together and signed this international treaty.

And it's viewed in environmental circles as being one of the best examples of a successful situation where the scientists discovered the problem, suggested how to solve it, and then you have the countries of the world actually getting together to pass a treaty and acting on it. As you saw, the CFC emissions have dropped nearly to 0 now. So it's looked at as, well in some cases, in wonderment because how could we do something like this today, it's not clear. But at least we have this example of how environmental science can prevent these problems.

Any questions on this? Great we're done a little bit early again today. So enjoy your Thanksgiving break and I'll see you a week from Monday.

Student: Professor, I'm still a little unclear as to why that same phenomenon with the ozone hole doesn't occur in the Northern Hemisphere.

Professor Ron Smith: Yeah, I should have said more about that. As you saw, it doesn't get as cold. And that is because it's believed there's more mixing between the high and low latitudes. It does want to get cold over the pole in winter, there's no sunlight getting there, it's radiating to space. But in the Northern Hemisphere, there's more north south mixing in the stratosphere to keep it warm. And that they think is because there's more continents, more of a continent ocean contrast in the Northern Hemisphere. So more disturbances in general that are causing a north south mixing, keeping it warmer up there.

[end of transcript]

Chapter 1: Ozone in the Atmosphere [00:00:00]

Professor Ron Smith:--stuff, so show off a little bit. We're going to finish up the ozone thing by doing the stratospheric ozone. It was interesting though that just yesterday there was this big New York Times article about the politics of setting the EPA standard for ozone. How many saw that article? I know Sam did and Rachel did. But it was long, like a four page article or something. All about the interaction between the EPA and the White House trying to drop that eight hours standard from, was it 0.08 down to 0.07.

So a lot of political heartache and dispute about whether that should be dropped to 0.07. And if you're into that sort of stuff, read about the politics of that. It fits nicely with the discussion we had in class last time. Anything else? Any issues, questions?

Chapter 2: Terminology for Stratospheric Ozone [00:01:13]

OK, so we're going to do the stratospheric ozone problem today. And these are the new terminology you should be aware of. So we'll talk about the ozone layer and then the ozone hole. The Dobson unit I'll discuss, I'll define as a measure of column integrated ozone concentration.

We'll be talking about CFCs which had been used as the primary liquid in refrigerators. And you need to have a fluid that flows back and forth between the cold sink and the warm sink in a refrigerator. And CFCs was the common chemical to do that. But now they've been replaced largely by HCFCs, the hydrochlorofluorocarbons. Say that 10 times fast. And so I'll define those as well.

We'll talk at some point today about polar stratospheric clouds, catalytic reactions, which are reactions in which one particular chemical compound allows the reaction to go forward but in the end itself isn't changed. So it can be reused over and over again to make the same chemical cycle go forward. Photochemical reactions, which are chemical reactions that involve light. You have to have photons of light in order for that reaction to go forward. And we'll talk about the Montreal Protocol. So all this will come in turn today.

Chapter 3: The Ozone Layer [00:02:55]

First of all, I showed this diagram last time as we discussed this part of the ozone issue, but now we're going to talk about the ozone layer. And as it says here, contains 90% of the atmospheric ozone. So if you do a column integration from the top of the atmosphere to the bottom, most of the contribution comes from the ozone layer.

What's plotted here is the partial pressure of ozone in milliPascals. Remember that the total pressure, the air pressure, decreases strongly and exponentially all the way on up through the troposphere and the stratosphere. So this has a very different vertical structure than the total pressure.

It has a beneficial role, it acts as a shield against ultraviolet radiation that would otherwise cause skin cancer, skin damage. And the current trends are a long-term global downtrend in stratospheric ozone. And then most famously, this ozone hole which occurs in the springtime of the Antarctic part of the earth each year. And then some more recent discussion about possibly this starting up in the Northern Hemisphere as well, but this is not as well documented.

So just a couple of calculations then. So I want to compute the mixing ratio in ppmv for ozone to air using this point on this slide. So I'm going to say that at about 24 kilometers altitude you've got a partial pressure of ozone that's about 30 milliPascals. And I want to convert that to a mixing ratio because that's what we've been used to using, what we used last time.

So first of all, because it's going to be a mixing ratio you need to know how much air is up there as well, not just how much ozone, but how much air. So I'm going to use this simple formula that we've used quite often in the course. It's the approximate exponential form for pressure as a function of altitude where you use a scale height, Hs. You know the pressure at the surface of the earth, you know the altitude you're inquiring about, and this will give you the pressure at that altitude

So sea level pressure we'll take to be 1,013 millibars, which is the standard average value, convert that to Pascals. The scale height as you recall is RT over g, which is about 8,400 meters for the earth—the earth's atmosphere. So I'll put 24 kilometers up here, that scale height there, and I'll put the sea level pressure there, and that's this formula here. So I put everything where I said I would and I get a value of about 6,000 Pascals for the air pressure up here.

And then I can simply put that into a ratio of the two pressures to get the mixing ratio in the common units. So I put in that the value of 0.03/6,000, I get 20 ppmv for the ozone mixing ratio in the peak of the ozone layer. OK? Pretty straightforward calculation. Had to find out how much air was there and then from the proper ratio get it in the proper units. 

Now compare this to what we were talking about last time. The EPA one hour standard for ozone in the boundary layer, where we live, was 0.12 in those same units. So the ozone concentration in the ozone layer is about 200 times the EPA one hour limit, just to give you a relative sense for that. So if we were up there, that would be really tough for us to breathe that in.

Now remember however, there's not enough air to keep us alive anyway at those altitudes. But the ozone concentration is quite large as well. So that's an inhospitable environment up there at that altitude. Any questions on that?

But it is possible, or it was until recently, possible for us to fly at those altitudes. So the Concorde, this beautiful, supersonic, commercial airliner which stopped flying a couple years ago, it flew at an altitude typically of 17 kilometers, which isn't quite at the peak of the ozone concentration, but it's definitely up in the ozone layer, there's no doubt about that.

And of course, you have to refresh the cabin air. You can't just lock the cabin doors and expect everybody to live off that cabin air until you get to your destination. So the aircraft can continually draw in air from outside to flush the cabin and give you fresh cabin air. The question is how do you do that in this case?

Remember though that there's two problems. First of all, there's very little air outside anyway. So you're going to have to compress it with some kind of a pump before you can put it into the cabin so people can breathe it. And that's going to be a big compression.

Let's just think about this for a minute. So if you're flying at 17 kilometers and you want to get that air compressed to cabin pressure, well first of all what cabin pressure do you normally have? Airliners don't usually keep you at sea level pressure. If you're taking off from New York City, as you take off the pressure will drop--in the cabin will drop as you climb, but they'll usually only let it drop to maybe an equivalent altitude of 6,000 feet, something like that. Then they couldn't let it go any lower or you'd start to have trouble breathing, but they don't keep you at sea level.

So I'm going to assume that the, for this calculation, that the cabin altitude, what's called the cabin altitude, is going to be kept at 2 kilometers. That's about 6,000 feet above the surface. In fact, you're flying at 17 kilometers, so you've got to take air and compress it just as you would if you took a parcel of air from 17 kilometers and brought it down to two kilometers, it would compress, adiabatic compression, we've talked about this over and over again. And it would heat as you compress it. So when you put it into that pump and compress it, it's going to warm up, and how much?

Well this is a calculation you can do in your head. You know the adiabatic lapse rate is about 10 degrees per kilometer, actually it's 9.8, but I've rounded it off. That's a difference in 15 kilometers, so essentially you're going to get 150 degrees Celsius warming when you compress that air about ready to put it into the cabin. Now you’re going to--it started off pretty cold, but still that's going to be really hot air.

Two things I've got to say about that. First of all, the convenient thing is that that temperature will destroy the ozone. So just by compressing the air, getting it ready to put into the cabin, you're going to kill off that ozone automatically. So that's pretty convenient.

But then remember, you're going to have to cool it a bit before you put it into the cabin. So you're going to put it through another heat exchanger, cool it back down, and then you get the air to stick into the cabin. If you don't get it all out, you can pass the air through an activated charcoal filter and that will remove any remaining ozone that's in the air before you put it into the cabin.

So it is possible to fly at those altitudes right in the middle of the ozone layer. But that's how you have to do it. Any questions on that? Yeah?

Student: Why would you want to fly at that high of an altitude?

Professor Ron Smith: It turns out it has mostly to do with the efficiency of the engines. In fact, that's true with subsonic aircraft as well. The reason they fly quite high, 35,000 feet, 37,000 feet is because the engines are most efficient at that altitude. And for this aircraft, the more efficient—the altitude was even higher at 17 kilometers. So anything on that? OK, maybe that's a little tangential point, but--

Chapter 4: The Dobson Unit [00:12:17]

This thing called the Dobson unit is a column density. We could write it, in fact, I wish that they had chosen the unit of kilograms per square meter. You'd simply integrate top to bottom in the atmosphere and say how many kilograms of ozone are there in each square meter of footprint for that column that you've integrated over. But for historic reasons they didn't do it that way.

This is typically measured by looking at light coming from the sun and see how much ultraviolet light reaches the surface of the earth. And that will tell you how much ozone there is in the column. What they're using for the Dobson unit is this definition, it's 1 milli-atmosphere-centimeters. That's the scientific definition of a Dobson unit.

And here's how you think about that. You take all the ozone in the column, you compress it to standard temperature and pressure, that is 1 atmosphere of pressure and 0 degrees Celsius. And then you measure how thick that layer would be composed just of ozone and you measure that in thousandth of a centimeter. And that is your Dobson unit value.

And I'll be showing you this kind of data. Typical values for the earth's atmosphere run from 100 to 500 Dobson units depending on where you are around the world. But that is normally--the Dobson is what we use to describe to each other how much ozone there is in the atmosphere.

So for example, you'll see a diagram like this from NASA. It's a map of the world and some areas are blocked out because they don't have data from here for this particular time of year. But the scale is in Dobson units.

So in the tropical regions you're getting values of about 250, 260 Dobson units. In the green you're getting values maybe 325, maybe in some areas even 350 or 360. So that's the way we represent column integrated ozone in this subject.

We can also get vertical profile information either by direct in situ measurements or by looking at light passing across the earth's atmosphere horizontally. This gives you some idea where the ozone layer is as a function of latitude. So here's latitude, North Pole, South Pole, Equator, altitude. But notice the offset 0, this isn't the ground, this is starting at 16 kilometers, right?

So the ozone layer at higher latitudes, it peaks at an altitude of about 20 kilometers, 18, 19, 20 kilometers. But near the Equator, it's up around 25, 26 kilometers. I think we expected this because remember the tropopause has the same kind of a structure. It's higher in the equatorial regions and lower in the polar regions. So the ozone layer is about the same place in the stratosphere, but the stratosphere starts at a different level. So it's lifted in a sense relative to the surface of the earth in the equatorial latitudes.

The reason why--part of the reason why there is that difference is because of a broad circulation that takes place in the stratosphere calls call the Brewer-Dobson circulation, which I won't talk about, but this artist has reminded us that there is this broad slow circulation in the stratosphere.

Chapter 5: Origin and Maintenance of Ozone in the Stratosphere [00:16:18]

So why do we have ozone in the stratosphere? This is a basic chemistry of that. We know we have a lot of O2 in our atmosphere, it's the second most common molecule. And occasionally, a high energy photon will come from the sun. A high energy ultraviolet photon will dissociate that diatomic oxygen into two oxygen atoms. It doesn't happen very often because it needs a high energy photon to do that.

But then you've got some free oxygen atoms floating around. And when you have that, that'll quickly combine with an O2 molecule to form ozone. That's a fast reaction. But equally fast is that other ultraviolet photons, they don't have to be as energetic as this one, these can be in the near, or in the UVA, for example, the near ultraviolet, will dissociate that back to this again. And that's fast.

So what you've got is a rapid recycling going on between that and that that's just going out all the time in the stratosphere, but maintaining a balance here. Now occasionally an ozone will interact with one of these atoms and go back to go to O2, which is the common form. And then you've lost it. So you've lost the ozone permanently or at least quasi permanently.

So the ozone concentration is controlled by--ignoring this rapid cycling which doesn't have any long-term effect, is controlled by the balance of these two slow processes. How often are you producing oxygen atoms and how often are you removing ozone and oxygen atoms back to O2? And that would be occurring naturally because we have oxygen in the atmosphere and we have ultraviolet light coming from the sun. So that's the background to why there is ozone in the stratosphere.

The reason why it doesn't happen further down is because most of these ultraviolet photons are absorbed up there and don't get--don’t penetrate deeper into the atmosphere. So it's a layer up there because that's where these ultraviolet photons are being absorbed and doing this work causing these photochemical reactions to take place. Questions on that?

Student: I'm curious if there is some sort of feedback mechanism or if the two slow reactions are interconnected in some way. Because I guess if you have more free oxygen atoms that would allow the second process to occur more frequently?

Professor Ron Smith: Well you'd have to write down the rate equations for these to answer that question, which I'm not prepared to do. But a chemical reaction, the rate at which it goes forward depends on how much of this do you have, and how much of this do you have, and how often are they colliding to allow the reaction to go forward? So I think your instinct is right that these are going to depend on the concentrations that you have.

So just like our tank experiment, the loss rate was proportional to how much you have. And that’s what, in a system like this where the generation term is independent of how much of this you have, but the loss rate is proportional, you're going to come to an equilibrium, just like we did in the water tank. And that's what I'm implying here, that you've got that kind of balance between a source which is just there and then a sink which is dependent on how much you have. And that will give a natural equilibrium eventually.

And this is a cartoon showing that. So the sun is playing a role. Occasionally it will dissociate O2 to form a simple O, an O molecule. And then you get this rapid recycling between oxygen atoms and ozone. And then occasionally, you'll go back just to the O2 again. And the control of how much you have is primarily these processes, not this. This is just a rapid recycling of the two types of odd oxygen.

Now to remind you about the spectrum of radiation coming from the sun, I show you this, you've seen it before. The visible spectrum is here, the infrared, we're really interested in the ultraviolet over here in today's conversation, which starts at 400 nanometers wavelength and goes shorter than that.

And these standard definitions are helpful. UVA are the wavelengths closest to the visible, closest to the blue, defined as going from 400 to 315 nanometers. UVB, 315 to 280, and then UVC, 280 all the way down to 100 nanometers.

Now you may remember from physics of that the shorter wavelength photons are more energetic, they'll do more damage. So UVA would cause tanning of your skin, but probably wouldn't give you skin cancer, UVB might. But UVC is very damaging. In fact, it's used in doctor's offices to clean surgical tools. You put it into this autoclave which will put UVC on your instruments and it'll kill anything that's there very, very quickly. It's very damaging radiation.

And so here's the fate of UVA, UVB, and UVC. Some UVA comes through. Half or more of the UVB is absorbed in the ozone layer, plotted on top of this. This is altitude and Dobson units per kilometer to get where the ozone layer is. So about 1/2 or 2/3 of the UVB is absorbed by the ozone. And almost all, thankfully, of the UVC coming from the sun is absorbed by the ozone layer. So it's a very good protective shield for us.

Now what could be changing this natural ozone layer situation? Well this is the current theory for it. It involves a catalytic reaction primarily using chlorine atoms. Now bromine will do this as well and there is some bromine in the atmosphere, but I'm going to focus strictly on the chlorine.

And notice how this reaction works. So you have a chlorine atom reacting with an ozone molecule. It goes to ClO, it oxidizes the chlorine and gives you back O2. And then quite soon after that, you'll take this ClO and react it with another ozone molecule. That'll give you back the Cl that you started with and you have two O2s, two regular diatomic oxygens.

So look, the ozone is gone. In fact, two ozones are gone. And you've got your Cl back ready to do it again. So this is a catalytic reaction where the catalyst here is the chlorine atom. It can be used over and over again to destroy ozone molecules in this way. Two ozone molecules are lost and the catalyst is free, ready to go back, and do it again.

That's the idea of what's today, now that we have a measurable chlorine concentration in the atmosphere, this is why we think the ozone layer is decreasing. Questions on that?

So up here in the inset is the emission. This is an example, CFC 11, one of the chlorofluorocarbons. And here's the emission rate for it in megatons per year starting in 1960 going up to the year 2000. So you see there was a rapid rise in it as it was being used in refrigeration. And when you put it into a refrigerator, it may be contained for a few years, but eventually it leaks out or someone tosses away the refrigerator and it gets a leak and it goes to the atmosphere. So almost everything you produce eventually makes its way into the atmosphere.

The Montreal Protocol that I'll talk about was here, where you begin to see it finally decreased. But because it's a long lived molecule, here's the CFC abundance in the atmosphere, it increased through this period of time, we've stopped putting it in, but it's still there. It has a lifetime--from this data, it looks like the lifetime is about 50 years. And so it's going to take probably another 50 or 100 before we see much of a decline. Thank goodness we've stopped putting it in, but it's not going to decrease to 0 just because we’ve stopped putting it in. It's going to be stored in the atmosphere for a while. It has a long lifetime.

The circles are data from the Northern Hemisphere and the triangles are data from the Southern Hemisphere. But they're pretty much the same. So the two hemispheres, this stuff gets mixed back and forth. Probably most of it was put into the atmosphere in the Northern Hemisphere, but now it's mixed to be pretty much equal in the two hemispheres. Questions on that?

Here is the global averaged total ozone change in percent starting back in 1965. Around 1980 is when we began to see the rapid decrease, but it's not huge. Notice that it's leveled out today and it's only about 4% below where it started. So this global decrease in ozone is, I suppose it's something we should be concerned about, but it's not a huge factor. And it doesn't look like it's going to get any worse now that we’ve stopped putting CFCs into the atmosphere. So it's a concern, but not I would say a terribly difficult one.

Chapter 6: The Ozone Hole [00:27:35]

But then there's this ozone hole that I'll spend the rest of time talking about. And here's what it looked like a couple of days ago. You can go to this website, which I'm just about to do. I should have set this up before.

But here's a definition of it. So it's a brief, seasonal and local reduction in ozone. Location is Antarctica, the time of year that it happens each year is September, October. It first appeared in about 1978, apparently it wasn't happening before then. It was discovered in 1984 and then going back and looking at old data, they found that it actually had started some six years earlier than that.

The discovery of the depletion mechanism by Molina and Crutzen was a rather remarkable scientific discovery. I'll give you hints of how that went in just a minute. And they won the Nobel Prize in chemistry for that awarded in 1995 from that discovery of how the ozone depletion mechanism works. Ultimately, it's caused by these CFC emissions which then led to the Montreal Protocol, which I'll talk briefly about, a rather successful international treaty to limit the emissions of CFCs.

Here's a brief history of it. So at the top is the ozone hole area in millions of square kilometers. What you do is set a threshold for ozone concentration. If it's lower than that you say you're in the hole, if it's larger than that you're out of the hole. And then you keep track of that area.

And the ozone hole was nonexistent before '79. But now it has reached about 25 million square kilometers, you saw a picture of it. And it's kind of leveled out, it's not getting it better not getting any worse. And the minimum ozone in the middle of that hole is shown here in Dobson units, of course, getting worse but now leveling out to a value of about 100 Dobson units in the middle of the hole each year when it forms.

So give me just a moment here to bring up this website. I forgot to do this before. Let’s see here. I'll just Google and find it. Yeah, that's probably it. That's not the one I want. That's a good one, but it's not the one I'm looking for. I'll try to find this other one. Yeah, let's do an animation for this season. I’ll try this one.

Yeah, so this is this year's hole. It hasn't formed yet, we're back in July. This is Antarctica sitting there waiting. Now wait until we get into September. It's already beginning to form in the late summer, but now we're getting the ozone hole forming. October, peaks in October, and then it'll start to fill in up to the present date. It's starting to weaken now that we're into November.

So that's the deal. It happens every year in that region. You get this large area that forms with about half the ozone it would otherwise have. Ozone depletion is about 50 percent in that region compared to remember only 4% globally. So this is a localized anomaly.

You can go to that website yourself and do some other animations. So in this year then this tracks it, the ozone hole area climbed up to the 12th of September, now it's decreased. The minimum ozone dropped to a minimum now it's increasing now that we're into November.

And I'm going to be talking about the stratospheric temperature. Stratospheric temperature in that part of the world now at that altitude is beginning to climb because we're getting out of their winter and towards of their Southern Hemisphere summer. And I'll come back to talk about the role of temperature in just a moment.

So I think the most curious thing about the ozone hole is why only in September and October? Why just over the South Pole? And I guess what is the link to human activity? So there's a lot of natural processes and factors controlling this. And then there's the added impact of humans and somehow they have all come together to form this curious phenomena of the ozone hole.

In the few minutes I have left I want to just try to explain some of that. Now I hope you can see this, I'll read it through for you. This is the seasonal cycle of temperature in the stratosphere. And this is for 90 North to 65 North, so this is the north polar cap. This is 25 North to 25 South, so this is the equatorial region. And this is 65 South to 90 South, so this is the south polar region.

Now the temperatures in the Northern Hemisphere get down to minus 70 on occasion. These are at different altitudes, I believe, having trouble reading those. They get down to minus 70. In the equatorial regions, those temperatures get down to about minus 75. But here in the South Pole, they get down to minus 80 and even minus 85. So of all the places in the world's stratosphere, the Southern Hemisphere polar stratosphere, that is over Antarctica, has a winter time temperature colder than any other place on the planet.

That's one of the things that makes it special in regards to why the ozone hole forms there. Now why would that be? It has to do with the phenomena of polar stratospheric clouds, abbreviation PSCs. Polar stratospheric clouds. I haven't spoken about these in the course up to this present time, so this is new material.

These are ice clouds that form up in the stratosphere. Normally we think clouds don't form in the stratosphere, it's pretty dry up there, but these clouds do. They are not entirely made of water. They're  a mixture of water and nitric acid in these clouds. And they require an extraordinarily cold temperature in order to form, temperatures colder than minus 70 degrees Celsius, some would say even colder than that.

This is probably why the Antarctic stratosphere is so special because it's temperature gets cold enough in winter to allow these polar stratospheric clouds to occur, whereas other places around the globe in the stratosphere you don't get those clouds. What do they do then? How do they influence ozone?

Well these ice clouds, these ice particles, freeze up diatomic chlorine by holding the nitric acid in the ice itself. Notice when it forms it's storing HNO3, nitric acid. And here's the reaction. You start with HCl, and ClONO2, reaction goes to Cl2 and the nitric acid, and then this is locked up in the ice. So what you've done, you had your chlorine locked up in HCl and in this compound, there's chlorines in both. Now you've put the nitric acid in the ice and you've freed up the chlorine diatomic molecule. There’s only one step to go.

When the light returns, remember, that minimum of temperature occurred in perpetual darkness. At that season of the year, it's dark over Antarctica, there's no light. So you can get the ice forming, you can get the Cl all ready to go, but it's in the diatomic form. Then the light comes back, Southern Hemisphere spring, the light comes back, you can dissociate the Cl2 to form two Cl molecules. And then go back a few slides to that catalytic reaction and it starts to destroy ozone.

So you get that? So the ice frees up the chlorine, it then dissociates when light returns, and the catalytic destruction of ozone begins to take place, and the ozone hole forms. That's why it occurs where it does and when it does because of that necessary condition for cold. And then the ozone hole forms just after the light returns to that region. Any questions on that? Yes?.

Student: Where does the hydrochloric acid come from, the HCl?

Professor Ron Smith: I don't know. That's the way the chlorine is stored most of the time in the stratosphere, is in these two molecules. After the CFCs break down, the chlorine is transferred into these two forms and just hangs around. Then the PSCs form and that sucks off the nitric acid and the Cl is released. So this is the storage form of the chlorine during most of the year in the stratosphere.

Student: Is there normally chlorine in the atmosphere besides the HCl?

Professor Ron Smith: Not naturally. Remember this was CFCs and not chlorine. And that's a critical difference. But let me just go back to that. There were no CFCs in the atmosphere before that. There probably was a little bit of chlorine, but only a very small amount. But now we're putting it in through this CFC source. Sorry, that's not a complete answer, but that's the best I can do.

Chapter 7: The Montreal Protocol [00:39:04]

So I think this–yeah, this is the last one. So the Montreal Protocol, the full name of it is Montreal Protocol on Substances that Deplete the Ozone layer. It's an international treaty. And of course it was first signed in Montreal, it's got the name. Since then it has been modified several times. Certain forms of substances have been added to the banned list, others have been allowed. So it's been modified a few times.

Generally what it did is by the time we came to 1994, it had banned pretty much all emissions of CFCs into the atmosphere. It largely replaced them with the HCFC, which can also be used as a refrigerant, but has a much shorter lifetime and will not cause the chlorine to build up in the atmosphere. It may have some other problems, it may be a greenhouse gas. But it doesn't have the problem of putting permanently chlorine into the atmosphere the way that the CFCs do.

Now one of the reasons why this treaty was signed, because as you'd expect the refrigerant industry vigorously opposed this, was at about the time it was signed, Dupont which was this large chemical company and the world's largest producer of CFCs, discovered a replacement for it. And they realized that they would not lose their business because they had a replacement ready to go and they might even be able to make more money by doing this changeover to a different type of refrigerant. So in the end, they dropped their opposition and many countries of the world got together and signed this international treaty.

And it's viewed in environmental circles as being one of the best examples of a successful situation where the scientists discovered the problem, suggested how to solve it, and then you have the countries of the world actually getting together to pass a treaty and acting on it. As you saw, the CFC emissions have dropped nearly to 0 now. So it's looked at as, well in some cases, in wonderment because how could we do something like this today, it's not clear. But at least we have this example of how environmental science can prevent these problems.

Any questions on this? Great we're done a little bit early again today. So enjoy your Thanksgiving break and I'll see you a week from Monday.

Student: Professor, I'm still a little unclear as to why that same phenomenon with the ozone hole doesn't occur in the Northern Hemisphere.

Professor Ron Smith: Yeah, I should have said more about that. As you saw, it doesn't get as cold. And that is because it's believed there's more mixing between the high and low latitudes. It does want to get cold over the pole in winter, there's no sunlight getting there, it's radiating to space. But in the Northern Hemisphere, there's more north south mixing in the stratosphere to keep it warm. And that they think is because there's more continents, more of a continent ocean contrast in the Northern Hemisphere. So more disturbances in general that are causing a north south mixing, keeping it warmer up there.

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