Ozone is a toxic, usually colorless gas (that can be blue when in high concentration) with a characteristic acrid odor. A variant of normal oxygen, it has three oxygen atoms per molecule rather than the usual two.
Ozone strongly absorbs ultraviolet radiation at wavelengths of 220 through 290 nm with peak absorption at 260.4 nm. Ozone will also absorb infrared radiation at wavelengths in the range 9-10 μg. Ozone occurs naturally in the ozonosphere (ozone layer), which surrounds the earth, protecting living organisms at the earth's surface from ultraviolet radiation. The ozonosphere is located in the stratosphere from 6–31 miles (10–50 km) above the earth's surface, with the highest concentration between 7.5 and 12 miles (12 and 20 km). The concentration of ozone in the ozonosphere is 1 molecule per 100,000 molecules, or if the gas were at standard temperature and pressure, the ozone layer would be 0.12 inch (3 mm) thick. However, the ozone layer absorbs about 90 percent of incident ultraviolet radiation.
Ozone in the stratosphere results from a chemical equilibrium between oxygen, ozone, and ultraviolet radiation. Ultraviolet radiation is absorbed by oxygen and produces ozone. Simultaneously, ozone absorbs ultraviolet radiation and decomposes to oxygen and other products. Ozone layer depletion occurs as a result of complex reactions in the atmosphere between organic compounds that react with ozone faster than the ozone is replenished. Compounds of most concern include the byproducts of ultraviolet degradation of chlorofluorocar bons (CFCs), chlorine and fluorine.
Ozone is also a secondary air pollutant at the surface of the earth as a result of complex chemical reactions between sunshine and primary pollutants, such as hydrocarbons and oxides of nitrogen. Ozone can also be generated in the presence of oxygen from equipment that gives off intense light, electrical sparks, or creates intense static electricity, such as photocopiers and laser printers. Human olfactory senses are very sensitive to ozone, being able to detect ozone odor at concentrations between 0.02 and 0.05 parts per million. Toxic symptoms for humans from exposure to ozone include headaches and drying of the throat and respiratory tracts. Ozone is highly toxic to many plant species and destroys or degrades many building materials, such as paint, rubber, and some plastics. The threshold limit value (TLV) for air quality standards is 0.1 ppm or 0.2 mg O3 per m3 of air.
Industrial uses of ozone include chemical manufacturing and air, water, and waste treatment. Industrial quantities of ozone are typically generated from air or pure oxygen by means of silent corona discharge. Ozone is used in water treatment as a disinfectant to kill pathogenic microorganisms or for oxidation of organic and inorganic compounds. Combinations of ozone and hydrogen peroxide or ultraviolet radiation in water can generate powerful oxidants useful in breaking down complex synthetic organic compounds. In wastewater treatment, ozone can be used to disinfect effluents, or to decrease their color and odor. In some industrial applications, ozone can enhance biodegradation of complex organic molecules. Industrial cooling tower treatment with ozone prevents transmission of airborne pathogenic organisms and can reduce odor.
See also Radiation ; Ultraviolet radiation.
Gillespie, Alexander. Climate Change, Ozone Depletion and Air Pollution. Leiden, UK, and Boston, MA: Nijboff/Brill, 2005.
United States Environmental Protection Agency (EPA). “Air: Air Pollutants: Ground Level Ozone.” http://www.epa.gov/ebtpages/airairpollutantsgroundlevelozone.html (accessed November 12, 2010).
United States Environmental Protection Agency (EPA). “Pollutants/Toxics: Ozone.” http://www.epa.gov/ebtpages/pollozone.html (accessed November 12, 2010).
Gordon R. Finch