Piece Of The Sky Is Missing

[T]his goodly frame, the earth, seems to me a sterile promontory; this most excellent canopy, the air, look you, this brave o'erhanging firmament, this majestical roof fretted with golden fire, why, it appears no other thing to me than a foul and pestilent congregation of vapors.

WILLIAM SHAKESPEARE,

I'd always wanted a toy electric train. But it wasn't until I was 10 that my parents could afford to buy me one. What they got me, secondhand but in good condition, wasn't one of those bantamweight, finger-long, miniature scale models you see today, but a real clunker. The locomotive alone must have weighed five pounds. There was also a coal tender, a passenger car, and a caboose. The all-metal interlocking tracks came in three varieties: straight, curved, and one beautifully crossed mutation that permitted the construction of a figure-eight railway. I saved up to buy a green plastic tunnel, so I could see the engine, its headlight dispelling the darkness, triumphantly chugging through.

My memories of those happy times are suffused with a smell—not unpleasant, faintly sweet, and always emanating from the transformer, a big black metal box with a sliding red lever that controlled the speed of the train. If you had asked me to describe its function, I suppose I would have said that it converted the kind of electricity in the walls of our apartment to the kind of electricity that the locomotive needed. Only much later did I learn that the smell was made by a particular chemical—generated by the electricity as it passed through air—and that the chemical had a name: ozone.

The air all around us, the stuff we breathe, is made of about 20 percent oxygen—not the atom, symbolized as O, but the molecule, symbolized as O2, meaning two oxygen atoms chemically bound together. This molecular oxygen is what makes us go. We breathe it in, combine it with food, and extract energy. Ozone is a much rarer way in which oxygen atoms combine. It is symbolized as O3, meaning three oxygen atoms chemically bound together.

My transformer had an imperfection. A tiny electric spark had been sputtering away, breaking the bonds of oxygen molecules as they happened by: O2 + energy ^ O + O

(The arrow means is changed into.) But solitary oxygen atoms (O) are unhappy, chemically reactive, anxious to combine with adjacent molecules—and they do: O + O2 + M ^ O3 + M

Here, M stands for any third molecule; it doesn't get used up in the reaction but is required to help it along. M is a catalyst.

There are plenty of M molecules around, chiefly molecular nitrogen.

That's what was going on in my transformer to make ozone. It also goes on in automobile engines and in the fires of industry, producing reactive ozone down here near the ground, contributing to smog and industrial pollution. It doelfn't smell so sweet to me anymore. The biggest ozone danger is not too much of it down here, but too little of it up there.

It was all done responsibly, carefully, with concern for the environment. By the 1920s, refrigerators were widely perceived to be a good thing. For reasons of convenience, public health, the ability of producers of fruit, vegetables, and milk products to market at sizable distances, and tasty meals combined, everyone wanted to have one. (No more lugging blocks of ice; what could be bad about that?) But the working fluid, whose heating and cooling provided the refrigeration, was either ammonia or sulfur dioxide— poisonous and evil-smelling gases. A leak was very ugly. A substitute was badly needed—one that was liquid under the right conditions, that would circulate inside the refrigerator but would not hurt anything if the refrigerator leaked or was converted into scrap metal. For these purposes it would be nice to find a material that was also neither poisonous nor flammable, that doesn't corrode, burn your eyes, attract bugs, or even bother the cat. But in all of Nature, no such material seemed to exist.

So chemists in the United States and Weimar and Nazi Germany invented a class of molecules that had never existed on Earth before. They called them chlorofluorocarbons (CFCs), made up of one or more carbon atoms to which are attached some chlorine and/or fluorine atoms. Here's one: (C for carbon, Cl for chlorine, F for fluorine.) They were wildly successful, far exceeding the expectations of their inventors. Not only did they become the chief working fluid in refrigerators, but in air conditioners as well. They found widespread applications in aerosol spray cans, insulating foam, and industrial solvents and cleansing agents (especially in the microelectronics industry). The most famous brand name is Freon, a trademark of DuPont. It was used for decades and no harm seemed ever to come from it. Safe as safe could be, everyone figured. That's why, after a while, a surprising amount of what we took for granted in industrial chemistry depended on CFCs.

By the early 1970s a million tons of the stuff were manufactured every year. So, it's the early 1970s, let's say, and you're standing in your bathroom, spraying under your arms. The CFC aerosol comes out in a fine deodorant-carrying mist. The pro-pellant CFC molecules don't stick to you. They bounce off into the air, swirl near the mirror, careen off the walls. Eventually, some of them trickle out the window or under the door and, as time passes—it may take days or weeks—they find themselves in the great outdoors. The CFCs bump into other molecules in the air, off buildings and telephone poles, and, carried up by convection currents and by the global atmospheric circulation, are swept around the planet. With very few exceptions, they do not fall apart and do not chemically combine with any of the other molecules they encounter. They're practically inert. After a few years, they find themselves in the high atmosphere. A Piece of the Sky Is Missing • 103

Ozone is naturally formed up there at an altitude of around 25 kilometers (15 miles). Ultraviolet light (UV)

from the Sun— corresponding to the spark in my imperfectly insulated electric-train transformer—breaks O2

molecules down into O atoms. They recombine and reform ozone, just as in my transformer.

A CFC molecule survives at those altitudes eflfi the average for a century before the UV makes it give up its chlorine. Chlorine is a catalyst that destroys ozone molecules but is not destroyed itself. It takes a couple of years before the chlorine is carried back into the lower atmosphere and washed out in rainwater. In that time, a chlorine atom may preside over the destruction of 100,000 ozone molecules.

The reaction goes like this:

So the net result is:

Two ozone molecules have been destroyed; three oxygen molecules have been generated; and the chlorine atoms are available to do further mischief.

So what? Who cares? Some invisible molecules, somewhere high up in the sky, are being destroyed by some other invisible molecules manufactured down here on Earth. Why should we worry about that? Because ozone is our shield against ultraviolet light from the Sun. If all the ozone in the upper air were brought down to the temperature and pressure around you at this moment, the layer would be only three millimeters thick—about the height of the

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