Ozone; The hole that always was

I’ve explained the Ozone layer so many times on other people’s blogs that I finally decided to write it up on my own blog so I could just point at it.  There’s an assumption that people make that the famous Ozone holes are man made.  While some pollutants can in fact destroy Ozone, the holes themselves are in fact natural.

Ozone exists as a layer of gas in the earth’s atmosphere.   The layer right underneath it is Oxygen.  Both are made of the same atoms (O) but Oxygen is two of them stuck together (O2) and Ozone is a threesome (O3).  So to illustrate, here’s a picture of earth surrounded by a layer of oxygen which is in turn surrounded by a layer of Ozone.  This is not to scale, it is not in the right proportions, there are many other layers, much mixing of gases in each layer, but this is close enough to explain the issues:

 

The next thing to understand is what happens when Ultraviolet light (UV) strikes the atmosphere.  There are various wavelengths of UV (most commonly grouped as UV-A, UV-B and UV-C).  Because of the big scare concerning Ozone, most people are aware that Ozone absorbs UV which can be harmful to people, so running out of it sounded awful bad.  But some frequencies of UV destroy Ozone, breaking the molecules apart.  Here’s a little movie that NASA made to show UV rays breaking apart Ozone http://svs.gsfc.nasa.gov/vis/a000000/a000800/a000824/a000824.mpg  As the various O and O2 float around, they will meet up and form Ozone again as the video shows.  What the video doesn’t show is that in the real Ozone layer there are all sorts of other molecules hanging around.  Everything from Hydrogen to Methane to sulfur to yes, CFC’s.  Since Oxygen is highly reactive, a free O or even O2 is more likely to combine with them to create H2O, CO2 and other compounds than to go right back to Ozone.

 Other frequencies of UV pass through the Ozone layer, some getting absorbed and some getting through and hitting Oxygen, which causes Ozone to be formed.  NASA made a movie to show that too http://svs.gsfc.nasa.gov/vis/a000000/a000800/a000823/a000823.mpg 

So what we have is a balanced system where Ozone is constantly being destroyed at the top of the Ozone layer, and constantly being created at the top of the Oxygen layer:

 

So what if something happens to destroy a lot of Ozone?  Well the Ozone layer would thin, and as a result less UV gets absorbed by the Ozone.  This means the amount of UV that gets through the Ozone layer to the Oxygen layer goes up, and so the rate of Ozone creation increases:

 

The same is true in reverse if the Ozone layer gets thicker .  Less UV reaches the Oxygen layer and so less Ozone is produced until it shrinks to normal once more:

 

So what about the Ozone holes over the poles?  They’re supposed to be there.  Consider what happens to sunlight as it goes through the atmosphere in earth’s higher latitudes.  Since it has to pass through the Ozone layer at an angle, a lot more gets absorbed than at lower altitudes.  But when we get to the very top of the planet, we reach a height at which the UV is destroying Ozone, but it never gets low enough to strike Oxygen, so the rate at which Ozone is being created falls off:

 

 As a consequence, most of the Ozone gets destroyed, leaving a “hole” in the Ozone layer.  Actually its not even a hole, its just an area where there are  O2 and O molecules hanging around that used to be Ozone:

 

Now its not like there is zero Ozone in that hole, there is some because there are Oxygen atoms left over from Ozone breaking down, and they can in fact form back into Oxygen molecules and then into Ozone if they get the right amount of UV (and don’t react with other things like Hydrogen and Methane).  It would be more accurate to call it a “depression” than an actual hole.  There is still “air” there, just not much Ozone in it.  In the early spring the hole starts to grow as the polar region comes out of darkness and the Sun’s rays can start destroying Ozone.  As summer progresses, the inclination to the sun becomes more direct, and the Sun’s rays start hitting the Oxygen layer, creating Ozone.  The reason that the Ozone hole over the south pole is bigger is because earth’s orbit is elliptical which tends to amplify the destruction cycle in the southern hemisphere and diminish it in the northern hemisphere.

So, do people in far northern communities (or far south in the southern hemisphere) need to panic?  In fact, the hole would have to get very big for that to be a problem.  The UV rays we are exposed to, even at very high latitudes don’t pass through the Ozone hole to get to earth surface:

So is the Ozone hole a complete hoax?  Pollutants like CFC’s could make the holes larger in theory, but the fact is that the holes are natural in the first place, and they fluctuate daily as the earth spins, seasonally as the earth’s inclination to the sun changes, annually as the earth’s orbit takes it closer and farther away from the sun, and from fluctuations in the sun’s output of UV in the first place.

NASA is keeping close tabs on the Ozone hole in the Antarctic.  Here’s a graphic that shows how the ozone hole grows quickly and then recovers again annually:

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