Chlorofluorocarbons (CFCs)

CFCs are highly stable, essentially inert chemicals in the troposphere, with correspondingly long residence times. For example, CFC-11 has an atmospheric lifetime of 60 years, CFC-12 120 years, CFC-113 90 years, and CFC-114 200 years. The atmospheric concentration of total CFCs in the early 1990s was about 0.7 ppb (parts per billion), and was increasing about 5–6% per year. Because of continued releases from CFC-containing equipment and products already in use, CFC emissions to the lower atmosphere have continued since their manufacture was banned in 1990. However, CFC concentrations in the troposphere declined in 2000 for the first time since the compounds were introduced. Model calculations show that it will take 20–40 years to return to pre-1980 levels.

Because of their long life spans and resistance to chemical activity, CFCs slowly wend their way into the stratosphere, 5–11 mi (8–17 km) above the earth's surface, where they are exposed to intense ultraviolet and other short-wave radiation. CFCs degrade in the stratosphere by photolytic breakdown, releasing highly reactive atoms of chlorine and fluorine, which then form simple compounds such as chlorine monoxide (ClO). These secondary products of stratospheric CFC decomposition react with ozone (O₃), and result in a net consumption of this radiation-shielding gas.

Ozone is naturally present in relatively large concentrations in the stratosphere. Stratospheric O₃ concentrations typically average 0.2–0.3 ppm, compared with less than 0.02–0.03 ppm in the troposphere. (Ozone, ironically, is toxic to humans, and tropospheric O₃ is a component of the photochemical smog that pollutes the air in urban areas.) Stratospheric O₃ is naturally formed and destroyed during a sequence of photochemical reactions called the Chapman reactions. Ultraviolet radiation decomposes O₂ molecules into single oxygen atoms, which then combine with O₂ to form O₃. Ultraviolet light then breaks the O₃ molecules back into O₂ and oxygen atoms by photodissociation. Rates of natural ozone creation and destruction were essentially equal, and the concentration of stratospheric ozone was nearly constant, prior to introduction of ozone-depleting compounds by human activity. Unlike the Chapman reactions, reactions with trace chemicals like ions or simple molecules of chlorine, bromine, and fluorine, results in rapid one-way depletion of ozone. CFCs account for at least 80% of the total stratospheric ozone depletion. Other man-made chemical compounds, including halogens containing bromide and nitrogen oxides, are responsible for most of the remaining 20%.

The stratospheric O₃ layer absorbs incoming solar ultraviolet (UV) radiation, thereby serving as a UV shield that protects organisms on Earth's surface from some of the deleterious effects of this high-energy radiation. If the ultraviolet radiation is not intercepted, it disrupts the genetic material, DNA, which is itself an efficient absorber of UV. Damage to human and animal DNA can result in greater incidences of skin cancers, including often-fatal melanomas; cataracts and other eye damage such as snow blindness; and immune system disorders. Potential ecological consequences of excessive UV radiation include inhibition of plant productivity in regions where UV light has damaged pigments, including chlorophyll.