Depletion of the Ozone Layer

Depletion of the Ozone Layer

CHEM 151 Research Paper

July 11, 1996

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