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Welcome to part three of chapter two
"Protecting the Ozone Layer" for "Chemistry in Context."
This lecture will be on how the ozone layer protects us
from the harmful rays of the sun.
Have you ever had a severe sunburn
sitting on a tropical beach like this one?
I wish I was sitting on a tropical beach like this one right now.
But I'm not.
I'm recording this lecture to help you all better yourselves
and learn about the wonderful world of science.
And actually this lecture
will have a lot of application to your daily life.
You see this picture of sunscreen over here?
We're going to learn how those
may or may not protect you from the sun and prevent skin cancer.
We're going to go into some detail
on what may have caused your sunburn.
How long did it take to get it?
And what you think life would be like
if it only took minutes to get a sunburn.
I bet you fashion would change dramatically
and perhaps not for the better.
So the ozone layer.
We know it's good.
We've heard stuff about saving it
and the banning of certain chemicals in order to protect it.
But how does it really work?
Well ozone formation occurs in the upper stratosphere.
Remember that layer of the atmosphere
that's about 10 to 50 kilometres above the earth.
Now, ultraviolet radiation can break bonds of molecules.
So we're talking about higher energy radiation than visible light.
And what happens is it will actually play a role
in breaking apart oxygen to form ozone.
So if we have a packet of UV Energy hitting an oxygen molecule,
break it apart into two oxygen atoms,
and then one of those atoms hits another oxygen molecule,
we can form a molecule of ozone.
Now this happens very quickly.
So this is an example.
This is not oxygen here, but this is just an example
of ultraviolet light breaking a bond.
We saw that in a previous chapter
or section.
So let's look at the three regions of light coming from the sun
that's of importance to us in terms of the ozone layer.
We have what's called UV-A, the A region of the UV spectrum.
It has wavelengths of light, or electromagnetic radiation,
between three hundred and twenty and four hundred nanometers.
This is the least energetic type of UV radiation.
It can cause some burns
but it doesn't really impact biological life,
like plants and animals.
It's not filtered by ozone or oxygen gas,
so it's not filtered at all.
All of the UV-A from the sun strikes the earth.
Now UV-B is a slightly higher energy region
of the UV portion of the electromagnetic spectrum.
You notice how the wavelengths are shorter.
It is harmful to biological life and it's only filtered by ozone.
Then we have the most energetic UV radiation, UV-C.
It's two hundred to two hundred and eighty nanmometers,
so we can see it's getting even shorter.
And it can cause damage to biological life,
but it is filtered by ozone and oxygen gas.
If this light hits either of those molecules
it will essentially be absorbed
and we'll look at that detail in a moment.
Here's just an image showing the sun
and you could see most of the UV-B is blocked by the ozone layer,
but a little bit gets through.
And then all the UV-A, bam, straight to the earth.
And we're not showing UV-C here because it is blocked entirely.
Now, oxygen has a stronger bond than the ozone molecule.
Why?
Let's look at the Lewis structure.
The oxygen molecule you could see here has a double bond.
Double bonds are stronger than single bonds.
And here we have the two ozone Lewis structures,
our resonance structures, representing the ozone molecule.
And remember we said before that the ozone molecule
is a combination; it's a hybrid.
The actual molecule is a hybrid of these two molecules.
So what that means
is that each bond is more like a single bond and a half,
not truly a double bond or a single bond.
That means if it's more like a one point five bond
it's going to be weaker than an oxygen molecule.
And we see that in only the most energetic UV radiation.
The UV-C radiation has the energy
to break the stronger oxygen bond.
The shortest wavelength, highest energy.
But here ozone can break down
and we've got another oxygen atom here to balance the equation.
It can break down into oxygen.
It can have one of the ozone bonds break,
but that can be done with UV-C, the high energy radiation,
but also UV-B, the slightly lower energy radiation.
And so we're saying here
any wavelength less than or equal to
three hundred and twenty nanmometers will break apart ozone.
So how do they filter this light?
What exactly is going on?
I mean, it's not like a coffee filter
where it just collects most of the light.
What happens is the molecules absorb the energy
and break apart.
So these represent oxygen.
If they're hit with the high, high energy UV radiation,
(boom) that energy is absorbed
and the bonds between the two atoms break.
And that energy is now added to the two oxygen atoms.
And then, of course, this UV-B and UV-C,
we see just a picture here of the ozone
a particle view of the ozone molecule breaking apart.
One of the bonds between the two oxygen atoms breaks apart.
Okay.
Let's now take a look at the Chapman cycle.
The Chapman cycle involves the regeneration of ozone from oxygen
and the breaking down of ozone when UV light strikes it.
So it's this cycle, a continuous cycle.
So we're starting here with an oxygen molecule.
You can see it broken apart by a UV photon.
And we need something else to collide with the system
in order for that single oxygen atom to combine and form ozone.
But wait, ozone can be hit with UV and also break apart
and those single oxygen atoms can then reform O2.
And so this is a cycle.
The Chapman cycle.
This is the chemical formula view of the Chapman cycle,
instead of the atomic, or particle, view.
You can see oxygen here, when it'*** by UV radiation,
breaks apart into single oxygen atoms.
And these are very reactive.
And if they collide with another oxygen molecule,
they'll form ozone.
But then ozone, if it is hit with UV radiation
will then break down into oxygen.
So it's a cycle that is occuring continuously
millions and millions of times every second up in the ozone layer.
It's known as a steady state condition or system.
There's a certain degree of balance and equilibrium there.
Okay, Thinker Buddy.
The Chapman cycle describes
the natural formation and destruction of ozone in the atmosphere.
Which statement is true?
Ozone is...
A) formed from an oxygen molecule and an oxygen atom,
B) destroyed by the absorption
of an appropriate wavelength of UV light,
C) formed from two oxygen atoms in the stratosphere
or D) Statements A and B are true.
And the answer is D.
Ozone is formed from an oxygen molecule and an oxygen atom.
It's also destroyed
by the absorption of an appropriate wavelength of UV light.
Another Thinker Buddy question.
How would you expect DNA damage
to __________ with _________ wavelength of UV radiation.
Would you expect it to increase with increasing wavelength?
Or decrease with increasing wavelength?
or C) increase with decreasing wavelength,
or D) decrease with decreasing wavelength.
So the correct answer is you'd expect DNA damage to decrease
with increasing wavelength of UV radiation.
Remember when the wavelength gets bigger
and longer
it's less energetic, and therefore, less dangerous.
Remember our electromagnetic spectrum.
Radio waves are very long.
They have very large wavelengths.
And ultraviolet radiation, on the other hand, is much shorter,
and therefore, higher energy.
X-rays are even higher in gamma rays, higher yet.
And visible light is here in the middle.
So as the wavelength gets longer,
it's going to be less energetic and less dangerous.
Let's take a look at short video
on what influences the degree of UV radiation in everyday life.
What causes higher versus lower radiation?
So we mentioned earlier
the majority of UV-B radiation is effectively filtered,
but UV-A is not.
What this graph is showing here
is that, okay, most of the UV-B is absorbed,
but very little of the UV-A.
And, of course, you know UV radiation causes skin cancer,
can actually damage your DNA, cause cataracts
can also damage plants.
We all are told to wear sunscreens
and protect ourselves from excess light on sunny days.
and as you learned on even partially cloudy days
that could be even more important.
But some of these sunscreens only block UV-B and not UV-A.
So you won't get burned, but it can still cause problems
if it's not blocking that UV-A.
And how do they work?
Well, zinc oxide, and if it has titanium dioxide, these are white.
These are the white substances that make suntan lotion,
always that white creamy color.
They're inorganic and they essetially reflect light.
They build up a shield on your skin to reflect light.
Then there's also organic molecules, like PABA,
that will absorb the UV before it reaches your skin.
But the problem is,
there's actually no regulation of sunscreens that block UV-A
by the food and drug administration.
So a suntan lotion can say it's got an SPF of 50,
but that is only referring to the UV-B radiation.
It tells you nothing about UV-A radiation.
Really, what you want to look for in a sunscreen
is whether or not it contains zinc oxide.
Zinc oxide is what is known as a broad spectrum sun block.
The sun screens will at times let radiation through.
Sun blocks do not.
I've even included a recipe
on how to make your own homemade sun block
in the module regarding chapter two.
And you could see here it's actually caused problems.
We've seen incidences of skin cancer go up
for light-skinned individuals.
Another problem is they think
that people aren't applying enough sunscreen frequently enough.
So if you put some on in the morning
you think you're protected all day,
when in fact you need to probably reapply sunscreen or sun block
every 2 hours.
Additionally, there has been some concern
that some of the organic compounds,
not the inorganic like titanium dioxide or zinc oxide,
some of the organic molecules, like PABA,
para-Aminobenzoic acid,
you don't need to know that,
may actually increase your likelihood of skin cancer.
So it's a bit of a mess right now.
Zinc oxide, it's all I can say.
Apply it frequently if you're going to be out in the sun.
And before you go out in the sun,
you probably want to check the EPA UV index.
And I've included that on the module links as well,
optional, if you wanna go check it out.
Of course it has, like, the air quality index.
It has a color scale in just a simple number
associated with the severity of the UV exposure.
Just one thing to think about
What would our world be like if the ozone layer did not exist?
Duhn-duhn-duh...
It would be a nightmare.
You would have to constantly cover yourself.
Plant life everywhere would be damaged by the UV radiation.
It would basically be the apocalypse.
Get your shelters ready.
No, we're actually taking some steps to protect the ozone layer.
And we'll talk about those soon.
So Thinker Buddy.
A certain brand of skin moisturizer advertises
that it contains SPF 15 sun screen
that protects from UV-A and UV-B rays.
Why isn't UV-C mentioned in its product claims?
UV-C radiation is...
A) not absorbed by ozone
so it has no connection to the story
of increased exposure to UV rays or ozone depletion,
B) not as dangerous as is UV-B radiation
so whether or not we are shielded from it does not matter
or C) extremely dangerous, but it is completely absorbed
in the upper atmosphere by oxygen as well as ozone,
or D) a natural part of the electromagnetic spectrum
so it only has the same potential for harm as visible light.
C is the correct answer.
UV-C is the highest energy UV radiation.
But oxygen will actually break apart when UV-C hits it.
It will absorb that energy and break apart.
It won't do that with the other types of radiation.
So UVC, since they're so many oxygen molecules as well as ozone,
they'll both absorb it and break apart.
It's actually completely filtered out.
Essentially, compared to UV-A and UV-B,
we can consider UV-C as not permeating our atmosphere.
Well that ends chapter two, part three.