Refrigerants for the 21st Century
8. CFCs and the Ozone Layer

By 1972 world-wide production of CFC-11 and CFC-12 had reached 0.3 and 0.5 million tons/year, respectively, and was expected to increase by more than 9%/year in future years. As pointed out earlier, the generally desirable chemical stability of these and other fluorocarbons meant that they would accumulate in the earth's atmosphere as they escape from from their valuable uses. Subsequently, it was established that the atmospheric lives of CFC-11 and CFC-12 are 50 and 102 years, respectively.

With such long lives, there is adequate time for CFC-11 or 12 to diffuse into the stratosphere. Here they are bombarded by the sun's intensive radiation, causing dissociation to produce the highly reactive chlorine radical (Cl).

Courtesy NOAA

Dr. Sherwood Rowland and a post-doctoral research scientist, Mario Molina, at the University of California - Irvine in 1974, speculated that the Cl radical so generated in the stratosphere could destroy O3 in a chain reaction. The mechanism would be:

Cl + O3 --> ClO + O2

(from CFC photolysis)

O + ClO --> Cl + O2

(from O2 photolysis)

Of course, there is other atmospheric chemistry which could remove the Cl radical, namely its reaction with hydrocarbons, such as methane, or with water (H2O) to produce HCl:

CH4 + Cl --> HCl

H2O + Cl --> HCl

The HCl should be removed as hydrochloric acid (aqueous HCl) - acid rain. Moreover, even HCl can act as a source of the Cl radical in the stratosphere by virtue of its reaction with the hydroxyl radical (OH) known to be present throughout the atmosphere - even in the troposphere.

However, laboratory measurements have shown that the kinetics of the O3 - Cl radical reaction is quite a bit faster than those that take the Cl radical literally down the HCl "sink". It has been estimated that one Cl radical can destroy up to 100,000 O3 molecules before it is depleted.

Fortunately, the fluorine radical (F) does not significantly impact O3 since its reaction with CH4 to produce HF is much faster, and the OH radical does not attack HF. However, the Br radical, which can form readily in the stratosphere from, say CF3Br (halon 1301), is even more aggressive toward O3 than the Cl radical.

The pioneering research by Sherwood Rowland and Mario Molina eventually (in 1995) was honored with the Nobel Prize In Chemistry*. However, their initial hypothesis caused considerable controversy.

* - With Paul Crutzen of the Max Planck Institute (Germany) for his 1970 research showing that man-made nitrogen oxides, including those emitted by supersonic aircraft in the stratosphere, also can destroy O3.