Chlorinated Fluorocarbons (CFCs)

CFCs were invented in 1928 by Thomas Midgley of the DuPont Corporation and have become widely used because of their many advantageous physical and chemical properties.

Chlorine-containing molecules, such as the CFC's, CF₂, CL₂, CCl₃F, that have long enough lifetimes will diffuse to the stratosphere where they are acted on by ultraviolet radiation.

A chlorine atom is ripped off, and the destruction of ozone begins. We might ask why in the stratosphere and why over Antarctica? The measurements show some decrease at other latitudes, but why is the effect so pronounced specifically over Antarctica?

To answer these questions, we need to understand a little about the meteorology and chemistry of the stratosphere in the Southern Hemisphere (Figures 7 and 8). In July the South Pole has the polar night, or polar winter, so the sun remains below the horizon all day and the whole atmosphere from the surface to the lower stratosphere gets very cold. Temperatures in the stratosphere drop to -90 degrees Celsius and clouds of ice particles begin to form. The ice crystals in these clouds provide a local surface for heterogeneous chemistry to take place.

Hydrochloric acid (HCl) combines with chlorine nitrate to form Cl₂ and NONO₂. On the surface of the cloud ice crystals, H₂O from the ice combines with the chlorine nitrate to form HOCl and NONO₂. When the sun rises over the horizon in early spring (about the end of August or first part of September), atomic chlorine is released by the first rays of solar energy. However, the nitrogen oxides, which are the only known scavengers of chlorine, are held on the ice crystals.

The free chlorine is left uninhibited to convert ozone to diatomic oxygen by the reactions in Figure 4 until additional warming releases the nitrogen oxides. The time between the release of chlorine and the release of the nitrogen oxides allows chlorine to destroy ozone without interruption until the ozone is all gone. In summary, stratospheric ice cloud particles under extremely cold conditions in the absence of sufficient sunlight to release the nitrogen compounds are what allow free chlorine to convert ozone to diatomic oxygen in very large amounts.

NASA scientists have clearly demonstrated this connection by flying through these clouds and measuring concentrations of chlorine, chlorine nitrate, and ozone. The chlorine - chlorine nitrate - ozone depletion linkage is very well established. Three stratospheric chemists have won the Nobel prize in chemistry for their work in clarifying these conditions. Despite this overwhelming scientific evidence for the linkage between increasing concentrations of CFCs and ozone depletion, articles continue to appear in otherwise very respectable business magazines questioning this connection. Such articles typically cite some scientist with no published work in this area as their authorities. This is a clear case of the need to carefully and critically review the evidence for claims that go counter to the scientific consensus.

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