The stratospheric ozone layer occurs from 12 to 50 km above the earth; the actual ozone concentration in the layer is in the order of ppmv (Francis 1994). Ozone can be both formed and destroyed by reactions with NOx; of recent concern is the enhanced destruction of stratospheric ozone by chlorofluorocarbons (CFCs) and other manmade oxidizing air pollutants. The natural ozone layer fulfills several functions related to absorbing a significant fraction of the ultraviolet (uv) component of sunlight and terrestrial infrared radiation, and it also emits infrared radiation.
Several potential deleterious effects result from decreasing the stratospheric ozone concentration. Of major concern is increased skin cancer in humans resulting from greater UV radiation reaching the earth's surface. Additional potential concerns include the effects on some marine or aquatic organisms, damage to some crops, and alterations in the climate (Francis 1994). While environmental engineers are uncertain about all seasonal and geographic characteristics of the natural ozone layer and quantifying these effects, the effects are recognized via precursor pollutant control measures included in the 1990 CAAAs.
Precursor pollutants that reduce stratospheric ozone concentrations via atmospheric reactions include CFCs and nitrous oxide. Principal CFCs include methylchloroform and carbon tetrachloride; these CFCs are emitted to the atmosphere as a result of their use as aerosol propel-lants, refrigerants, foam-blowing agents, and solvents. Example reactions for one CFC (CFC-12) and ozone follow (Francis 1994):
Cl- denotes atomic chlorine. ClO is also chemically reactive and combines with atomic oxygen as follows:
Because of the preceding cycling of Cl-, environmental engineers estimate that a single Cl atom can destroy an average of 100,000 ozone molecules (Francis 1994).
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