Depletion of the Ozone Layer

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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,...
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