Monday, January 01, 2007

Remembering the Montreal Protocol

As its 20th anniversary approaches, what can the landmark agreement on controlling CFCs teach those who want to control greenhouse gases?

By David Rotman

Chemists Mario Molina (above left) and Sherwood Rowland, shown in 1976, calculated that CFCs used in aerosols, refrigeration, and air conditioning were destroying the ozone layer. Credit: Associated Press

Until the early 1970s, it could be said that, like politics, all chemistry was local. That changed in dramatic fashion with a series of discoveries concerning the global effects of a family of chemicals called chlorofluorocarbons, or CFCs. These compounds had played a key role in the midcentury chemical revolution, allowing such innovations as safe refrigeration, cheap aerosol deodorants, and widespread air conditioning. First commercialized by ­DuPont in the early 1930s under the trade name Freon, CFCs appeared to be the perfect industrial chemical: nontoxic, nonflam­mable, and odorless. But in 1973, a pair of chemists at the University of California, Irvine--Sherwood Rowland and his postdoctoral fellow Mario Molina--began to explore the fate of the CFC gases that were being emitted into the atmosphere. Molina began the investigation of CFCs in October of that year, and by Christmas, the researchers had their answer: the CFCs were breaking down in the atmospheric ozone layer, which begins 15 kilometers above the earth, ends roughly 30 kilometers later, and absorbs much of the sun's deadly ultraviolet radiation.

The researchers found that the CFCs wafted up through the lower atmosphere intact, too stable to react with the swirling brew of chemicals around them. But once they reached the mid-­stratosphere, above most of the protective layer of ozone, the intense solar radiation broke the CFC molecules apart, releasing chlorine. Two simple reactions gave Rowland and Molina concern: Cl + O₃ = ClO + O₂, and ClO + O = Cl + O₂. That is, chlorine (Cl) reacted with ozone (O₃), generating chlorine monoxide (ClO), which in turn reacted with an oxygen atom to release another chlorine; the net result was that the chlorine was destroying ozone without depleting itself. "When we found the chain reactions" occurring in the ozone layer, remembered Rowland this fall, the fate of CFCs "suddenly went from a scientific curiosity to an environmental worry."

The next decade was a contentious one for ­Rowland and Molina, as many in the general public, the chemical industry, and even the scientific community expressed skepticism that a nontoxic gas sprayed out of a can (in the early 1970s, recalls Rowland, roughly two-thirds of CFCs were used as propellants in aerosol products, such as deodorants) could have a significant impact on the composition of the atmosphere--much less on the viability of life on earth. "If you came off the street, it seemed ludicrous that underarm deodorants might have an effect in a global way," Rowland says.

In 1978 the United States banned the use of CFCs in most spray-can applications. But in the early 1980s, models of the atmospheric chemistry involving CFCs became more and more complex, and various questions arose over the science.

In 1985, Rowland and Molina were vindicated. British scientists using ground-based instruments spotted a gaping "hole" in the ozone layer above the Antarctic. Subsequently, NASA reported that there was a thinning of the ozone layer over the populated areas of the Northern Hemisphere. These findings proved that Rowland and Molina's chemistry had been correct. They also provided startling evidence that industrial chemicals, emitted largely over the industrialized population centers of North America and Europe, could change the atmosphere on a global scale.