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Lecture 3 - Snowflakes and Ozone

Listen to part of a lecture in a chemistry class.

Professor: Ok, so, today we’re going to talk about the Arctic, ozone depletion and snowflakes. And it’s all related. Let’s start with snowflakes.

Now, I find snowflakes fascinating. To even begin to understand them, you need to understand physics, chemistry, and mathematics. Even though there’s been a lot of research, there’re still actually a lot about snowflakes that we don’t understand yet.

Hard to believe, I know.

Anyway, snowflakes have a particular form, there’s a six-sided center with six branches or arms that radiate out from it. But how did they get that way? Well, you start with water vapor. You need a pretty humid atmosphere. And that water vapor condenses directly into ice, into an ice crystal. At this point it looks kind of like a thin dinner plate that rather than being circular, is hexagonal with six flat edges.

It’s at this point in the process were we begin to see why each snowflake is unique.

Imagine this dinner plate is floating around in the wind, right? And when it encounters water vapor, molecules from that vapor attached to each of the six sides. You begin this development with six arms or branches radiating out from the center plate. Each time the snowflake encounters water vapor, more molecules attached to it, leading to more and more complex structures. And of course, each snowflake takes unique route through the clouds on its way down. And so the quantity of water vapor that it goes through is going to be unique for each one.

Now one important characteristic of snowflakes is that they have something called a quasi-liquid layer, the QLL. Our snowflake is an ice crystal, right? Well, we find a quasi-liquid layer on the surface of ice is basically a thin layer of water that’s not completely frozen. And the existed temperature is well below freezing, though thickness varies at different temperatures. Now this quasi-liquid layer, it plays an important role on what we are going to talk about next.

Ah, yes, Mary?

Mary: How can liquid exist below freezing? Why doesn’t it freeze?

Professor: Well, when water becomes ice, the molecules bond together and it gets sort of…locked in the place. They can’t move around as much anymore. So each molecule is surrounded by other molecules, and they are all locked together. But what about the exterior of the ice? There is a layer of water molecules on the surface, they attached molecules only on one side. So, they are a bit freer. They can move around a bit more. Think of a… think of a brick wall. The bricks in the wall, they have other bricks above and below them, and they are all locked against each other. But that top layer, it only has a layer below it. Now this can only be taken so far because of course bricks don’t move at all. They are not liquid. But the bricks of water molecules, well, this top layer would be the quasi-liquid layer. And it wouldn’t be completely frozen. Does that make sense?

So, finally we get to the connection between snowflakes and ozone. Ozone is a gas found in the atmosphere of Earth. Now there is the ozone found in the stratosphere which is the layer of the atmosphere from 6 to 30 miles above the Earth. This is considered good ozone, which occurs naturally and helps block harmful radiation from the Sun.

But there is also ground-level ozone. It’s exactly the same gas but it’s found closer to the surface of the Earth. This ground-level ozone results from human activities, at high concentrations it can be a pollutant. Now snowflake’s quasi-liquid layer plays an important role in some complex chemical reactions. We’re going to be looking at these in detail later today. But basically, these reactions cause certain chemicals to be released. And these chemicals reduce the amount of ground-level ozone. So the more branches you have in an ice crystal, the more quasi-liquid layer there is. The more quasi-liquid layer, the more reactions and the more chemicals that reduce ground-level ozone. So you can see why this is such an important system to study and understand.