The Ozone Layer
The ozone layer is a region of concentration of the ozone molecule (O3) in the Earth's
atmosphere. The layer sits at an altitude of about 10-50 kilometers, with a maximum
concentration in the stratosphere at an altitude of approximately 25 kilometers. In
recent years, scientists have measured a seasonal thinning of the ozone layer primarily
at the South Pole. This phenomenon is being called the ozone hole.
The ozone layer naturally shields Earth's life from the harmful effects of the
Sun's ultraviolet (UV) radiation. A severe decrease in the concentration of ozone in
the ozone layer could lead to the following harmful effects:
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An increase in the incidence of skin cancer (ultraviolet radiation can
destroy acids in DNA).
A large increase in cataracts and Sun burning.
Suppression of immune systems in organisms.
Adverse impact on crops and animals.
Reduction in the growth of phytoplankton found in the Earth's oceans.
Cooling of the Earth's stratosphere and possibly some surface climatic
effect.
Ozone is created naturally in the stratosphere by the combining of atomic oxygen (O)
with molecular oxygen (O2). This process is activated by sunlight. Ozone is
destroyed naturally by the absorption of ultraviolet radiation,
O3 + UV >>> O2 + O
and by the collision of ozone with other atmospheric atoms and molecules.
O3 + O >>> 2O2
O3 + O3 >>> 3O2
Since the late 1970s, scientists have discovered that stratospheric ozone amounts over
Antarctica in springtime (September - November) have decreased by as much as
60%. Satellite measurements (NIMBUS 7 - Total Ozone Mapping Spectrometer)
have indicated a 2% decrease in ozone between 65 degrees North - 65 degrees South
per decade since 1978 (see figure below). A reduction of about 3% per year has been
measured at Antarctica where most of the ozone loss is occurring globally. During the
late 1990s, large losses of ozone were recorded above Antarctica year after year in the
months of September and August. In some years, spring levels of stratospheric ozone
were more than 60% lower than the levels recorded months prior to the seasonal
development of the hole.
This image shows the global (from 65° N latitude to 65° S
latitude) monthly average total ozone amount from 1979 to
2001. The green line shows measurements from Nimbus-7
TOMS instrument. The red line shows the results from the
Meteor-3 TOMS instrument. The blue line shows the results
from the Earth Probe TOMS instrument. (Source: NASA,
TOMS Multimedia).
It appears that human activities are altering the amount of stratospheric O3. The main
agent responsible for this destruction was humanmade chlorofluorocarbons or CFCs. First produced by General Motors
Corporation in 1928, CFCs were created as a replacement to the toxic
refrigerant ammonia. CFCs have also been used as a propellant in spray cans, cleaner
for electronics, sterilant for hospital equipment, and to produce the bubbles in
styrofoam. CFCs are cheap to produce and are very stable compounds, lasting up to
200 years in the atmosphere. By 1988, some 320,000 metric tons of CFCs were used
worldwide.
In 1987, a number of nations around the world met to begin formulating a global plan,
known as the Montreal Protocol, to reduce and eliminate the use of CFCs. Since
1987, the plan has been amended in 1990 and 1992 to quicken the schedule of
production and consumption reductions. By 1996, 161 countries were participating in
the Protocol. The Montreal Protocol called for a 100 % reduction in the creation and
use of CFCs by January 1, 1996 in the world's more developed countries. Less
developed countries have until January 1, 2010 to stop their production and
consumption of these dangerous chemicals.
CFCs created at the Earth's surface drift slowly upward to the stratosphere where
ultraviolet radiation from the Sun causes their decomposition and the release
of chlorine (Cl). Chlorine in turn attacks the molecules of ozone chemically
converting them into oxygen molecules.
Cl + O3 >>> ClO + O2
ClO + O = Cl + O2
A single chlorine atom removes about 100,000 ozone molecules before it is taken out
of operation by other substances. Chlorine is removed from the stratosphere by two
chemical reactions:
ClO + NO2 >>> ClONO2
CH4 + Cl >>> HCl + CH3
Normally, these two reactions would quickly neutralize the chlorine released into the
stratosphere. However, the presence of polar stratospheric clouds, rich in nitrogen,
and sunlight facilitates a series of reactions which prolongs the reactive life of
chlorine in the atmosphere. Interestingly, these polar stratospheric clouds require very
cold air (approximately -85° Celsius) for their formation. Stratospheric air of this
temperature occurs normally every year above Antarctica in the winter and early
spring months. Destruction of the ozone begins in Antarctica in the spring as this
region moves from 24 hours of night to 24 hours of day. These clouds are less
frequent in the Arctic stratosphere because winter cooling of the air in the stratosphere
is less severe.
NASA's Earth Probe -Total Ozone Mapping Spectrometer home page has the
latest images describing the current status of global stratosphere ozone levels in the
atmosphere.
The average areal coverage of the Antarctic ozone hole has now leveled off at about
24 million square kilometers (see figure below). Scientists believe that the ozone hole
over Antarctica will maintain this size till about 2018. After this date, the ozone hole
should begin to recover and be completely gone by about 2070.
Average areal coverage of the ozone hole from 1979 to 2004. In
this analysis, the ozone hole is defined as the area for which
ozone is less than 220 Dobson Units. The ozone hole has grown
from a few million square kilometers in 1981 to an area larger
than North America (about 24 million square kilometers) in the
year 2000. (Source: NASA, TOMS Multimedia).