Everywhere, All the Time

Why aren't the laws of 
ozone destruction 
universal?

The hallmark of ideal physical laws is that they would apply everywhere, all the time, under all circumstances, and some laws of physics come close. The laws of chemistry, being properties of materials, cannot meet that stringent criterion. For example, the chemical equation telling how hydrogen and oxygen combine to form water is correct for most terrestrial circumstances, but becomes utterly meaningless inside the sun, where the temperature is high enough to dissociate water molecules, or indeed to strip the electrons from atoms. Still, the general idea is that the chemicals listed on the left side of the equation combine (or dissociate) to become the products on the other side _ everywhere, all the time, under all reasonable circumstances.

The putative thinning of the ozone layer is said to be caused by chlorine in the stratosphere, and the doomsday scenario states that all of that chlorine comes from man- made chlorofluorocarbons (CFCs, called freon). The Nobel Prize in Chemistry was given in 1995 to Sherwood and Molina for showing the chemical equations that describe the destruction of ozone by chlorine in a sort of catalytic cycle. All they require is the presence of chlorine and oxygen, some ultraviolet light (UV) and any other molecule M that can carry away excess energy.

The ozone doomsayers take the Sherwood-Molina equations as if they are written in a stone slab. Presumably, then, they work everywhere, all the time. At noon? Yes, of course. At 3:20 in the afternoon? Yes, of course. At midnight? Well, no, because there's no ultraviolet rays, but so long as there is UV present. Do they work in the stratosphere? Yes, of course. Do they work in the stratosphere over New York City? Yes. What's so hard to understand: they work everywhere. Do they work at the South Pole? Yes, they work everywhere: what's so hard to understand? Do they work over the South Pole in January? Yes. What am I speaking, gibberish?

Setting aside the question of whether stratospheric chlorine is entirely due to CFCs, if the Sherwood-Molina equations are correct, they apply everywhere, all the time. Chlorine chews up ozone in the manner described, at all times at all places: everywhere, all the time. Wherever there is chlorine in the stratosphere and solar UV to make the process work, the equations say that ozone is being depleted: everywhere, all the time. The chlorine is uniformly distributed, and the UV flux is uniform, at least on the side of the earth facing the sun. The rate of destruction ought to be the same at the equator as at the poles, the same at the North Pole as at the South Pole, the same in winter as in summer.

But uniform destruction of ozone is precisely what does not happen. Every October, the ozone over Antarctica thins out very dramatically for a couple of months. The records date back to 1958, when the ozone depth was thinned to only 150 Dobson units, and well before the vast majority of CFCs had even been manufactured. The ozone concentration then returns to normal levels. (Ozone is continuously produced by interaction of untraviolet light with oxygen.) At the North Pole, and ozone thinning is very subtle, if it occurs at all. There are wide annual fluctuations, and the very slight observed decrease of the average may well be due to slight decreases in temperature that have been observed at high altitude. At the equator, there is no measurable destruction of ozone either in the short term or in the long term.

There may or may not be a relationship between the concentration of chlorine in the stratosphere and the putative destruction of ozone. If the destruction is real, and if chlorine is a causative agent (as the Sherwood-Molina equations say), and if the majority of the chlorine in the stratosphere comes from freon, then the freon that leaks out of your air conditioner may play a non-trivial role in ozone destruction. But first, we need to state two obviously general truths: there is something special about the place (Antarctica) and there is something special about the time (October). That is a far cry from “everywhere, all the time!”