Eli5: how can earth ozone layers heal itself even with our high pollution level?
In: 5
Ozone is constantly generated in the upper atmosphere by the effects of radiation, mostly short-wave UV from the sun. This UV is higher energy than the UVA and UVB that reach the Earth’s surface (and, appropriately, is called UVC in this context). That range of energy happens to be just right for it to be absorbed by oxygen molecules (O2, the regular kind of oxygen we breathe here on the ground) in the upper atmosphere.
When O2 absorbs such a UV photon, it gains a bunch of energy, enough to “kick” the two oxygens apart. That produces two separate individual oxygen atoms. O2, the bound form of oxygen, is already pretty reactive, but pure atomic oxygen is a whole other matter and is ready to party with just about any molecule. And one of the options is…well, another O2 molecule. That produces ozone (O3).
Ozone absorbs longer-wave ultraviolet than oxygen does. Without ozone, oxygen would absorb UVC, but UVB and UVA would reach the surface almost totally unabsorbed. Ozone absorbs UVB as well, reducing the amount that reaches the ground; the remaining UVB is mostly what causes sunburns and skin cancer (since it’s high enough energy to easily damage biological molecules like DNA).
Ozone is unstable, though. The reaction 2 O3 -> 3 O2 is pretty favored from a pure energy perspective, although it’s kind of slow, in the same way that, say, a boulder going from the top of a mountain to the bottom releases a lot of energy but it takes a while to get there. (Chemistry and biology students will know what I’m talking about here as *thermodynamics* versus *kinetics*.) So even though the production of ozone is slow, the breakdown is also slow, and the combination of the two leads to a steady equilibrium of ozone in the upper atmosphere…
…or it did until some bozos called humans came along.
—–
During the 1900s, a class of chemicals called CFCs (chlorofluorocarbons, so-named because they have chlorine, fluorine, and carbon in their structures) became common. They have some convenient properties, and they’re relatively non-toxic, and they became common in uses like aerosol cans and refrigerators. They ended up being released in moderate amounts into the atmosphere.
Normally, you’d think this wouldn’t be a problem. In the scheme of things they were a pretty mild, relatively non-toxic pollutant.
But unfortunately, CFCs have two properties that make them bad news for ozone:
* First, CFCs *catalyze* the conversion of ozone back to O2. They don’t change the energies involved, but they speed up the reaction by a huge amount. That shifts the balance of ozone and O2 strongly in the direction of O2 if CFCs are present. Ozone is still produced by the same processes that always produced it, but it’s broken down more quickly so there’s less ozone around on average.
* And second, CFCs are extremely long-lived. They take decades to break down in the atmosphere, which is long enough for them to be mixed up into the layer where the ozone layer is, and stay there for a very long time.
After decades of CFC use, researchers noticed, hey, wasn’t there an ozone layer there a minute ago? A large “hole” of much lower ozone layers began appearing, particularly near the poles (for complicated reasons, CFCs do a better job damaging ozone in colder temperatures). Everyone freaked out, [one of the most successful environmental programs ever was launched](https://en.wikipedia.org/wiki/Montreal_Protocol), and we stopped using CFCs for most applications in 1989 (today we use their non-ozone-depleting cousins the fluorocarbons; it turns out the chlorine is the problem).
But because CFCs last a loooooooooong time, CFC levels in the upper atmosphere have only just come down. Now that CFCs aren’t naturally destroying ozone anymore, ozone – which was being naturally produced all along, but was quickly destroyed by CFCs – can start to stick around for longer, and levels of ozone in the ozone layer are starting to rise back towards their natural levels. They should return to close to their original values in another few decades.
So the answer to your question is: we don’t pollute the atmosphere with the specific chemicals that destroy the ozone layer anymore. (Or at least, we pollute it with far less of them.)
Latest Answers