So far, most of the things we've talked about
in this class have been pretty good black bodies.
So, we've sort of been slinging, sort of been slinging epsilon values around.
but just calling them one, in my mind, at
least, because most things are pretty good blackbodies.
Most condensed matter is pretty good at absorbing
and emitting all the different frequencies in infrared light.
But gases are different because they're so simple
They only have very simple modes of vibration.
If you take carbon dioxide and you
freeze it into dry ice, into condensed form,
it's got all these other things around it.
It's like a blop of jello.
It has all different kinds of vibrational frequencies.
And can absorb everything, in a condensed state.
But when the CO2 is floating off by itself, and not touching
anything else very much. It can only vibrate in very specific ways.
Gases are very choosy about the kinds of light they can absorb and emit.
So the way we've done this,
For black bodies, epsilon is a value close to one,
and that works for the ground and for the pane of glass in our simple model.
But for gases, in a real atmosphere,
epsilon value is not at all one, it's much smaller than that,
because gases are so picky.
So, just as an aside, before we go on, I want to explain that
most of the colors that we observe, most of the light that we see in nature
around us is got its color not because of absorption and emission by vibration
of the atoms.
But actually because the electrons exist at different energy levels.
And they can hop up to higher energy levels and pop back down again.
And when they do that they, they absorb or give off energy.
So, most things that are good dyes, like the red stuff
in this chalk, has energy levels that are fairly close together.
And so when an electron
falls down that much it gives off light that has that much energy.
Which turns out to be in the visible range and so we can see it.
But gases are so simple that the energy
levels of their electrons are so far apart.
That what they mostly do is, is make light in the ultraviolet that we can't see.
And this is why gases are transparent to the visible light that we can see.
Some exceptions to that are chlorine gas, is actually green.
Chlorine is a big atom with lots of electrons
on the outside and some of them are bound sort of loosely.
And so you can actually see it in visible light,
and then NO2 is kind of a brown color.
This is something that you find in urban smog.
When they have ozone alert days.
If you can see
far off into the distance, you see this sort of brown layer.
And that's what this is.
That's mostly what creates the colors that we see.
But what we're interested in, for the energy in the atmosphere,
is back to molecular and atomic vibrations.
For a gas to absorb light or to emit light, because it's always a two wa street,
if it can absorb, it can also emit,
two things must be true.
One is that the frequency of the light has
to be pretty close to the frequency of the vibration,
has to be kind of a match, you have to be well tuned.
But the other thing, that's very important, is that the vibration of the
molecule has to create a fluctuating electric field, an oscillating dipole.
And most gases in the atmosphere are not really capable of doing this.
So the main gases in our atmosphere
consist of molecules that have two identical atoms in them each.
O2 or N2 are mostly what the atmosphere is made out of.
Those are totally symmetric, one oxygen is
the same as another and if you vibrate it,
when they're farther apart or closer together,
it's still always symmetric,
there isn't a plus on one side and a minus on another side.
And that's why these major gases in the atmosphere are not greenhouse gases.
They don't affect the climate of the earth in a greenhouse effect sort of way.
Carbon dioxide is a symmetric molecule in its resting state.
The carbon is in the middle and then you
have these double bonds to the oxygens on either side.
It doesn't really matter to us that these are double bonds instead of
single bonds, they still act like springs and they still vibrate.
and so at first glance, you might think this is symmetric too,
and so it wouldn't be a greenhouse gas.
But there are modes of vibration of this molocule that break the symmetry.
And the one that's most important is the bend.
You are sort of bending the thing like this,
and when you do that the oxygens have sort of a minus charge to them,
and that leaves sort of a positive charge on the other side,
and then when it swings back the other way, the electric field is going to flip.
You get this oscillating dipole here.
This mode of vibration of the
CO2 molecule is the one that's most important for climate.
There are two other modes.
There's a symmetric stretch which is kind of like this, and
then there's a asymmetric stretch which is kind of like that.
So the asymmetric stretch is infrared active because it's
broken the symmetry,
but it turns out that this mode of vibration is
not as important to climate because there's just less light there, at this frequency.
So this is the frequency that is the
most important to absorbing light that's coming up from the ground.
Other greenhouse gases, the main ones, are water, water vapor.
Water has the oxygen has two lone pairs of electrons
which tend to crowd the hydrogens off to one side.
Water is an asymmetric molecule even at rest when you write it.
It's got a dipole moment even when it's just sitting there
There are lots of different ways you can vibrate this
or you can just spin it around and that will create an oscillating electric field.
Water has a very, very complicated absorption spectrum.
It can do lots of different colors of infrared light.
And then the other major greenhouse gas is methane
CH4, so there's the carbon,
and you've got four hydrogens on it.
They are symmetrically spaced around the carbon
in a tetrahedral shape.
There's a triangle in the base, and a pyramid.
And this is symmetric, too, in its resting state,
but there's lots of ways that you can vibrate this thing,
and have it break the symmetry, so this is also an important greenhouse gas.
In general, any molecule that has more than two atoms is
going to be a greenhouse gas of some sort, some strength.