The ozone layer diminishes more each year. As the area of polar ozone depletion (commonly called the ozone hole) gets larger, additional ultraviolet rays are allowed to pass through. These rays cause cancer, cataracts, and lowered immunity to diseases.1 What causes the depletion of the ozone layer? In 1970, Crutzen first showed that nitrogen oxides produced by decaying nitrous oxide from soil-borne microbes react catalytically with ozone hastening its depletion. His findings started research on “global biogeochemical cycles” as well as the effects of supersonic transport aircraft that release nitrogen oxide into the stratosphere.2 In 1974, Molina and Rowland found that human-made chlorofluorocarbons used for making foam, cleaning fluids, refrigerants, and repellents transform into ozone-depleting agents.3 Chlorofluorocarbons stay in the atmosphere for several decades due to their long tropospheric lifetimes. These compounds are carried into the stratosphere where they undergo hundreds of catalytic cycles with ozone.4 They are broken down into chlorine atoms by ultraviolet radiation.5 Chlorine acts as the catalyst for breaking down atomic oxygen and molecular ozone into two molecules of molecular oxygen.
The basic set of reactions that involve this process are: Cl + O3 –>ClO + O2 and ClO + O –>Cl + O2 The net result: O3 + O –>2O2 Chlorine is initially removed in the first equation by the reaction with ozone to form chlorine monoxide. Then it is regenerated through the reaction with monatomic oxygen in the second equation. The net result of the two reactions is the depletion of ozone and atomic oxygen.6 Chlorofluorocarbons (CFCs), halons, and methyl bromide are a few of the ozone depletion substances (ODS) that break down ozone under intense ultraviolet light. The bromine and fluorine in these chemicals act as catalysts, reforming ozone (O3) molecules and monatomic oxygen into molecular oxygen (O2). In volcanic eruptions, the sulfate aerosols released are a natural cause of ozone depletion. The hydrolysis of N2O5 on sulfate aerosols, coupled with the reaction with chlorine in HCl, ClO, ClONO2 and bromine compounds, causes the breakdown of ozone. The sulfate aerosols cause chemical reactions in addition to chlorine and bromine reactions on stratospheric clouds that destroy the ozone.8 Some ozone depletion is due to volcanic eruptions.
Analysis of the El Chichon volcanic eruption in 1983 found ozone destruction in areas of higher aerosol concentration (Hofmann and Solomon, “Ozone Destruction through Heterogeneous Chemistry Following the Eruption of El Chichon”). They deduced that the “aerosol particles act as a base for multiphase reactions leading to ozone loss.”9 Chlorine and bromine cooperates with stratospheric particles such as ice, nitrate, and sulfate to speed the reaction. Sulfuric acid produced by eruptions enhances the destructiveness of the chlorine chemicals that attack ozone. Volcanically perturbed conditions increase chlorine’s breakdown of ozone. Also, chlorine and bromine react well under cold temperatures 15-20 kilometers up in the stratosphere where mos of the ozone is lost. This helps explain why there is less ozone in the Antarctic and Arctic polar regions.10, 11 The Antarctic ozone hole is the largest.
A 1985 study reported the loss of large amounts of ozone over Halley Bay, Antarctica. The suspected cause was the catalytic cycles involving chlorine and nitrogen.12 Halons, an especially potent source of ozone depleting molecules, are used in fire extinguishers, refrigerants, chemical processing. They are composed of bromine, chlorine, and carbon. Most of the bromine in the atmosphere originally came from halons. Bromine is estimated to be 50 times more effective than chlorine in destroying ozone.13 Insect fumigation, burning biomass, and gasoline usage all release methyl bromide into the air. Some is recaptured before reaching the stratosphere by soil bacteria and chemicals in the troposphere. The remainder breaks down under exposure to sunlight, freeing bromine to attack the stratospheric ozone.
Annual atmospheric releases of methyl bromide include 20 to 60 kilotons from fumigation (fifty percent of the methyl bromide used as a soil fumigant is released into the atmosphere), 10 to 50 kilotons from biomass burning, and .5 to 1.5 kilotons from leaded gasoline automobile exhaust each year. Marine plant life also releases methyl bromide, but most is recaptured in seawater reactions.14, 15 Hydrochlorofluorocarbons(HCFCs) and hydrofluorocarbons(HFCs) are being used as substitutes to replace chlorofluorocarbons. They “still contain chlorine atoms that are responsible for the catalytic destruction of ozone but they contain hydrogen which makes them vulnerable to the reaction with hydroxyl radicals (OH) in the lower atmosphere. The reactions in the troposphere remove the chlorine before it reaches the stratosphere where ozone depletion occurs.
16 Some of the HFCs and HCFCs being used to replace CFCs are HFC-134a, HCFC-22, HCFC-141b and HCFC-123. HFC-134a replaces CFC- 12 in most refrigeration uses. HCFC-22 is marketed as a coolant for commercial and residential air-conditioning systems. HCFC- 141b and HCFC-123 are used for making urethane and other foams.1 Each year since the 1970s, the stratospheric ozone above Antarctica disappears during September and reforms in November when ozone-rich air comes in from the north. Because new chemicals that do not destroy ozone are replacing ozone-depleting chemicals, the ozone hole is projected to disappear by the middle of the 21st century.18
1. Monastersky, R. (1992, September 19). UV hazard: Ozone lost versus ozone gained. Science News, 142, pp. 180-181. 2. Lipkin, R. (1995, October 21). Ozone Depletion research wins Nobel. Science News, 148, pp. 262 3. Lipkin (ibid.) 4. Consortium for International Earth Science Information Network(CIESIN) (1996, June, Version: 1.7). Chlorofluorocarbons and Ozone Depletion. http://www.ciesin.org/TG/OZ/cfcozn.html 5. CIESIN (1996, June, Version: 1.7). Production and Use of Chlorofluorocarbons. http://www.ciesin.org/TG/OZ/prodcfcs.html 6. CIESIN (1996, June, Version: 1.7). Ozone Depletion Processes. http://www.ciesin.org/TG/OZ/ozndplt 7. US Environmental Protection Agency (1996). Ozone Depletion Glossary. http://www.epa.gov/ozone/defns.html 8. National Oceanic and Atmospheric Administration (1994). Scientific Assessment of Ozone Depletion-Executive Summary. http://www.al.noaa.gov/WWWHD/pubdocs/Assessment94/executive- summary.html#A 9. CIESIN (1996, June, Version 1.7). Ozone Depletion Processes. (ibid.) 10. National Oceanic and Atmospheric Administration (1994). Scientific Assessment of Ozone Depletion-Executive Summary. (ibid.) 11. Kerr, Richard A. (1994, October 14). Antarctica Ozone Hole Fails to Recover. Science, 266, pp.217 12. Kerr, Richard A. (ibid.) 13. US Environmental Protection Agency. Ozone Depletion Glossary. (ibid.) 14. Adler, T. (1995, October, 28). Methyl Bromide doesn’t stick around. Science News, 148, pp. 278 15. National Oceanic and Atmospheric Administration (1994). Scientific Assessment of Ozone: 1994-Executive Summary. (ibid.) 16. CIESIN (1996, June, Version: 1.7). Ozone Depletion Processes. (ibid. 17. CIESIN (1996, June, Version: 1.7). Ozone Depletion Processes. (ibid.) 18. Monastersky, R. (1995, October 14). Ozone hole reemerges above Atlantic. Science News, 148, pp. 245-246