Thread by @Heartofaquamari

I wasn't able to take part in the climate strike due to being ill, so instead I thought I'd make a big long series of posts here. Let's talk about one of the greatest global enviromental successes: the ozone layer hole and the Montreal and Vienna Protocols.

And just so you know, we are doing this properly, so let's start with a very simple question; what is the ozone layer? And to answer that, we need to start by looking at the structure of the atmosphere.

The first thing you need to know about the atmosphere is that it gets thinner as it goes up. A lot thinner. 90% of the atmosphere is contained in the first 10km up from the surface, and it keeps getting exponentially thinner as you go up.

The second property that really defines the layers of the atmosphere is where the source of heating in that layer is. Obviously, the ultimate source of heat is the sun, but there is a catch. See, an object will only be heated by the rays of the sun if it interacts with those rays

The sun's light peaks in the area of the spectrum we call visible light. And the reason it is invisible is that the atmosphere doesn't absorb or block photons in this range of wavelengths. It lets it pass straight through till it hits the ground or the ocean, which DO absorb them

This means that the way most of the atmosphere is warmed is not directly from the sun, but rather from the ground absorbing and re-emitting the energy from the sun. That's why it gets colder as you go higher; you are getting further away from the source of the heat!

This is also important because you probably know, hot things rise. This means that in the troposphere you (particularly during the daytime when the sun is shining) have hot air below cold air, which is an unstable situation. This is a major driver of the weather in general.

This bottom-up heating is the defining feature of the troposphere, the bottom 10-12km of the atmosphere. (Note it is also where the majority of the atmosphere is, including, crucially, the water vapour, which is rare above the troposphere. This will be important later).

So. At the top of the troposphere (the tropopause), we get a sudden temperature inversion. At this height, rather than getting colder as you rise, it suddenly gets warmer! The atmsophere becomes stable and layers of air form with colder air below warmer air.

We've reached the stratosphere (named because it is stratified into layers). But what causes this temperature inversion? Where is this warming coming from?

Now let's talk about ozone.

Ozone is formed of three oxygen atoms, rather than the usual two. It is far less stable than the two atom arrangement, which is why you rarely encounter it in the lower atmosphere; without a sustained source of ozone, it will simply break apart and form oxygen classic.

As an aside, this concept of how long chemicals last without breaking down or otherwise being removed in the atmosphere, or atmospheric lifetime, is really important in a lot of ways. It's why the very stable carbon dioxide is the greenhouse we worry about most.

Okay, so. How does ozone form in the atmosphere? Well, you need two things. First of all, you need a source of oxygen atoms. Secondly, you need something that breaks down the bonds in O2 molecules and allow the now free atoms to go and form ozone. And what does that? UV light.

Yep, we're back to the light from the sun, but a different section of the electromagnetic spectrum now. Both oxygen and ozone absorb UV light. The ozone layer is the source of heat in the stratosphere.

Both oxygen and ozone also break down in UV light. This is how the ozone layer forms; the individual ozone molecules in it are constantly breaking back down into oxygen, but at the same new ozone molecules are being formed all the time by the same process.

This is also why the ozone layer forms at the height it does (although exactly where it is varies quite a bit depending on stuff like the location on earth and the season).

You need to be high enough in the atmosphere that the UV rays haven't been absorbed, but low enough that there's enough oxygen to form the ozone. (again, the higher you go, the less gas there is).

One thing I should note is that the ozone layer isn't like, an area entirely made up of ozone. We are still talking about around 10 parts per million, but compared to the rest of the atmosphere that's about 30-100x more ozone.

So that's the ozone layer. It's a layer with more ozone than usual, which absorbs UV rays and is the major source of heat in the stratosphere. So what's the ozone hole, how did we cause it, and why was it mainly over the South Pole? We'll come back to those questions.

One thing to note is this is a fairly simplified description of the process. I haven't gone into anything about how ozone is moved around the atmosphere, the various other processes that break down ozone, or the details of the chemical processes that actually form ozone.

If you want more information, a good (free to read!) paper to look at is "Ozone: From discovery to protection", which reviews the history of ozone and how it was first detected. tandfonline.com/doi/pdf/10.313…

Anyway, I'm gonna grab a drink. It's getting pretty late so I might continue this in the morning.

So I realised as I planned out what else I was gonna say in this thread that having a timeline of our understanding of ozone is going to be useful, so I'm gonna put one here now, up to the point where the ozone formation method I mentioned above is first proposed.

Ozone was first artificially created by Christian
Schönbein in 1839. It was a subject of modest interest for some researchers, but it wasn't really a great focus until the later part of the decade, where physicists were having a problem with the sun.

See, if you are on the ground looking at the various wavelengths the sun emits, then it mostly follows the prediction of a "black body" emitter with a temperature of around 6000K fairly closely.

(A quick terminology note: Kelvin is a unit of temperature that has it's zero point at absolute zero, and black bodies are objects that emit all wavelengths of light equally, so their spectrums of emitted light are only determined by temperature)

The problem is that there was this band of wavelengths of light missing in the UV range. There were two options here; either we didn't understand emission spectra correctly, or there was something in between the sun and the measuring devices blocking it.

The thing is, it is actually fairly easy to identify what was in the way. If you don't know about spectrometry, it's essentially using the unique wavelengths of em waves absorbed/emitted by substances to tell what they are, without having to actually test them chemically.

These fingerprints in the electromagnetic spectrum are super important (they are how we can tell that greenhouse gases are absorbing heat: with satellites we can see the amount of infrared in wavelengths absorbed by GHGs escaping the atmosphere decreasing over time.)

Anyway. As noted above, the optical properties of ozone had already been discovered, and it's absorption window fit the missing wavelengths nicely. With this information, the fact that there was ozone between the surface of the earth and the sun was established with further tests

Next question: Where was the ozone? It wasn't at the surface, because we could measure that easily. There was a difference in the amount of UV that reached high mountain tops and sea level measurements, so it was obviously in the upper levels of the atmosphere.

Enter one of the big names in the story of atmospheric ozone measurements; Gordon Dobson. He made a series of measurements using specially designed spectrometers around the world to measure and locate the ozone layer.

The unit for "how much ozone is in an air column" is named the Dobson after him, and while newer designs like the Brewster are now replacing them, Dobson spectrometers are still used to measure ozone concentrations.

A really cool one is an observation station in Arosa, Switzerland, which was set up with a Dobson spectrometer and is still making measurements today, giving us a really long observational record of ozone for that location. agupubs.onlinelibrary.wiley.com/doi/full/10.10…

So we knew basically where the ozone was. The next big thing was a theoretical mechanism for the formation of ozone in the layer by Chapman, 1930, which is essentially what I've been describing above. The constant breakdown and formation of ozone due to UV is the Chapman Cycle.

Around this time, we get the first studies that find ozone breaks down far more quickly in the presence of gases like chlorine. This wasn't the aim, but rather researchers were finding that small amounts of impurities could mess up their measurements. That'll be important later.

Again, this is a simplified introduction to the history. If you want a nice review of the history of ozone science, Stolarski goes further. There's a lot of additional experiments and researchers to have a look at.
acd-ext.gsfc.nasa.gov/People/Stolars…

This is one of the things that can be difficult to express about atmospheric science. There's a lot of data, but as you can see by how many concepts I've had to introduce to get this far in explaining the ozone layer, to compress it into a form that people will actually read?

It's difficult. Once I've finished this I think I'll make this into a blog post with a much more through bibliography, but for now I'll explain the basics and add links to review papers as we go.

If you want to see a more detailed and in-depth examination of this stuff, follow the citations in those papers to the rest of the research. If you want more information on any given part of this thread, let me know and I'll post them here.

Anyway. We have our explaination of the ozone layer and how it forms. Next step: what is the ozone layer hole?

First things first. The ozone layer hole isn't a literal hole. Rather, it's an area of the ozone layer with notably depleted levels of ozone. It isn't that there's no ozone, or that there's just a gap in the atmosphere or something.

The ozone layer hole (it's a catchy phrase so I'm gonna keep using it) most people refer to is a region with much lower levels of ozone than usual, centred over the South Pole. There are smaller ones throughout the world, but that's the big one.

One of the on-going issues in atmospheric science is "how do we get the observations". While there had been discussions in the scientific community for a while about the potential for human activity to damage the ozone layer, it wasn't until 1985 that proper measurements came in.

The reason for this is kind of obvious; the Antarctic is a very difficult place to get to, and satellites were also a new technology. Still, the results, first published in Nature in 1985 were shocking in how much ozone was missing, down to a third as much as expected.

Funnily, this loss of ozone was actually seen in the satellite record, which was about a decade and a bit old at this point, but was assumed to be a measurement glitch and was generally ignored. When the record was reexamined, the hole went back to 1976 at least.

I couldn't fit the link to the 1985 Nature paper in the right tweet so here it is. researchgate.net/publication/24… Again, for more information a great place to start is looking at the papers this cites, and also the ones that come later and use it as a source.

So that's the ozone layer hole. But what causes it? For that, we need to go back in time, and discuss chlorofluorocarbons, or CFCs.

Like a lot of things, the invention of CFCs seemed like a really good idea at the time. They are extremely stable, non-toxic, non-flammable and don't corrode pipes, making them really good for use in a whole range of stuff, particularly cooling systems.

"What makes a good Refrigerant" is a whole thread by itself but in general CFCs are excellent choices. And for quite a while, they weren't really thought of much as a pollutant.

Anyway. In 1971, James Lovelock, on a cruise around the southern ocean, discovered something interesting; the levels of CFCs in the atmosphere indicated that basically all of them we have produced (they aren't naturally occuring chemicals, as far as I know) were still around.

As noted above, the behaviour of gases in the atmosphere is often determined by how stable they are, and how long they lasted in the atmosphere. Normally, it is difficult for stuff produced in the troposphere to reach the level of the ozone layer in the stratosphere.

For large amounts to reach those heights and overcome the stable, stratified nature of the stratosphere which surpresses upward motion, generally the substance needs to either be thrown up with great force or just needs to stick around for decades.

And CFCs were doing just that. In 1970, it was discovered that another very stable compound, nitrous oxide or NO2, was reaching the stratosphere and decaying due to the higher levels of UV light into the much more reactive nitrous oxide, NO.

This was actually the first suggestion of human activity effecting the ozone layer, since the levels of NO2 in the atmosphere were rising due to farming, and NO can react with ozone and destroy it.

In 1974, Rowland and Molina suggested that the same process might work for CFCs, which while very stable under tropospheric conditions, do in fact break down under UV light to give off chlorine. If they were to reach the stratosphere, therefore, they could do serious damage.

This hypothesis needed testing. If CFCs were causing an increase in the level of chlorine in the atmosphere, then that would leave behind observable traces, such as the remains of the CFCs that had the chlorine break off of them.

There was a range of measurements throughout the next few years that supported this idea, including James Anderson's work on measuring ClO, the compound created when chlorine reacts with ozone, such that the US National Academy of Sciences recommended banning CFCs in 1976

As an aside, the Rowland-Molina hypothesis was REALLY unpopular with companies like DuPont, which manufactored CFCs. The Chair of DuPont called it "Science Fiction", while the President of the Precision Valve Corporation wrote to Rowland's dean about his comments.

The actions of these companies are kind of fascinating as a case study in responses to the new science. For ages they denied that CFCs would have any effect at all, often while claiming that of course they would stop producing them if the science was "settled"

Note that they continued to do so even after the full extent of the damage was discovered in 1985, when it was discovered that the predictions of ozone depletion during earlier work were wrong only in that they didn't go far enough.

They also made their own groups to put pressure on governments and to delay any kind of further legislation on the issue; again, the claim that they would obviously stop as soon as the science was settled, of course, was trotted out.

There's another way they managed to still make money out of CFCs up to 2010, and related compounds up until the modern day, but that's going to come later once we reach the Montreal Protocols.

So here's where we got to so far. CFCs are stable compounds that reach the stratosphere because they last for ages in the air. Once there, they are broken down by UV light and produce chlorine atoms.

We know it's CFCs that are responsible because we can measure them and their by-products lurking in the stratosphere, and we know that human activity is responsible because, well, CFCs don't occur naturally.

Next question: why is ozone depletion focused around the poles? To answer this we need to discuss the chemistry of how chlorine breaks apart ozone, the atmospheric dynamics of the poles, and to answer another question: why is the south pole colder than the north pole?

The way chlorine destroys ozone is fairly simple. It reacts with ozone, stealing one of the three oxygen atoms, and forms Chlorine Oxide (ClO). But remember, there are also a load of free oxygen atoms roaming around this area of the atmosphere due to UV breaking down O2 and O3.

These react with the ClO, forming a normal oxygen molecule O2 and re-releasing the chlorine atom by itself back out to react with more ozone. This is why chlorine and other similar atoms like bromine can do so much damage to the ozone layer; they keep being released back.

Now, this can occur just naturally, when all of the components are gases. This is why there was a worldwide decrease of ozone levels by about 4% throughout the 70s. However, there is a way to make it far more efficient; if there is a solid surface available.

See, most chlorine in the stratosphere isn't just floating around as free atoms. There are stable compounds it forms, such as when it reacts with hydrogen in HCl. A really important one is Chlorine Nitrate (ClONO2), which breaks down more easily when there is a solid catalyst

I will be honest, this particular bit of atmospheric chemistry is outside my direct wheelhouse, given I'm more on the physics side, but essentially a lot of reactions are sped up if they have a solid surface, such as ice crystals in clouds, to act on.

Another way clouds encourage the production of Chlorine atoms is that they react with and remove nitrogen dioxide, the NO2 part of ClONO2, to form nitric acid which then drops out through another process. This means that ClO molecules can't react with the NO2 to reform ClONO2

So essentially, the more clouds there are in the stratosphere, the more chlorine is released from stable molecules, and thus more damage is done to the ozone layer. But you might be asking "well, there are clouds everywhere. Why are the poles special?"

Once again, we return to the idea of how long things stay in the atmosphere. The major source of water is of course evaporation from the oceans. However (and this is worth an entire discussion on its own), water is a weird chemical.

Essentially, water is one of the only chemicals that exists in solid, liquid and gaseous forms in the temperature ranges that exist on Earth. This is really important, because it introduces state changes as a way to remove water from the atmosphere, not just chemical reactions.

You've seen how excess water is removed from the atmosphere; it's called rain or snow or other forms of precipitation. Essentially, water does not last long enough in the atmosphere for any large amounts to reach the stratosphere, and that is why the stratosphere is so dry.

For clouds to form, you need a source of water vapour, and it generally needs to be cold enough for that water to condense. Again, cloud formation is a super complicated topic and I want to keep on track here, so we're not going to go into this today.

But essentially you can form clouds out of very little water vapour if it is cold enough to do so. The only places that it is cold enough on Earth to do so reliably are at the poles during the season long nights they go through at winter.

They do form at both poles, but the South Pole is colder than the North Pole so more depletion is found there. Why?

So at the tropospheric level, this temperature difference is primarily due to elevation. Antarctica has the highest average height above sea level of any continent due to the ice sheets, and is one big land mass without the oceans to provide heat.

Meanwhile the Arctic is mainly sea ice, which is far low down, and is utterly surrounded by the ocean, which absorbs, transports and releases heat very well.

The explaination as to why the stratospheric temperatures of the two poles are different is a bit more complicated, and is linked to a term you've almost certainly heard recently; the polar vortex.

The polar vortexs are large low pressure areas that sit on top of the poles during the winter months, with extremely strong boundary winds going eastwards around the poles starting in the mid troposphere and extending into the stratosphere.

When the polar vortex is strong, it prevents warmer air reaching the pole and also stops the cold air around the poles escaping towards lower latitudes.

A good way to weaken polar vortexes is to dump heat into the upper troposphere, which disrupts their formation and allows air to move. This is what happens when the polar vortex drops over areas of the US and Europe, causing really harsh winters there but mild winters at the pole

The Northern Hemisphere has a lot of bits of land poking into the arctic circle, and a good way to force warmer air into the upper atmosphere to weaken the vortex is to have a mountain in the way. The South pole, meanwhile, is surrounded by ocean and is just one continent

This is why the Southern polar vortex is so much stronger than the Northern one, and why the southern stratosphere is so much colder than the northern polar area. (I'm simplifying; again, the dynamics here are a big subject)

(as an aside, there is some evidence that global warming will result in the polar vortex weakening more often, driving harsher winters in lower latitudes but meaning that the north pole will be far warmer than it should be as the cold air escapes so...that's fun)

So. South Pole stratosphere is colder -> more clouds form -> more chlorine is released from otherwise stable compounds -> far more damage done to the ozone layer.

And with that I think we've reached the end of the scientific explanations. Now we are moving into the realm of politics as we look at how the world responded to this discovery.

@threadreaderapp unroll please