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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, 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

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