A chemical constituent of the smog that plagues most of the world's big cities has been identified for the first time by scientists at the National Bureau of Standards.

The chemical is a simple mixture of carbon, oxygen and hydrogen called dioxirane. Its existence was predicted two years ago by smog chemists, but only now has it been isolated in the laboratory. So unstable is dioxirane at normal air temperatures that its capture and identification could only be done at supercold temperatures under strict controls.

The discovery was made by scientists Richard D. Suenram and Frank J. Lovas.

Photochemical smog forms from the mixing of automobile exhausts, the nitrogen and oxygen in the air and the play of sunlight on all of them to form ozone.

The ozone reacts further with the whole chemical mix to create toxic substances like formaldehyde and nitrosamines that make the smog dangerous to human health.

"This discovery might do nothing to change smog conditions anywhere," said Dr. John T. Herron, an NBS chemist who confirmed the discovery by Suenram and Lovas. "But it is definitely a step forward in understanding the chemistry of smog, which we don't understand as much as we think."

Once ozone begins reacting with automobile exhausts it forms a family of unstable compounds called free radicals, which accelerate a chain reaction that forms smog. Dioxirane appears to be a key intermediate in the chain reaction.

"Dioxirane is quite unstable, even at temperatures down to 170 degrees below zero Fahrenheit," Lovas said. "At moderate, temperatures like those in the air it probably doesn't last longer than one millionth of asec-

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