OZONE DEPLETION
LINKS TO OTHER WEBPAGES USED IN THIS LAB:
[1] Use this site to find your
latitude and longitude! [http://www.mit.edu: 8001/geo]
[2]
Use this site to determine ozone concentrations, any time, any place! [http://jwocky.gsfc.nasa.gov/teacher/ozone_overhead.html]
[3] Use this site to investigate
the different satellites that are used to measure ozone [http://jwocky.gsfc.nasa.gov/]
Objectives:
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To understand how the earth's atmosphere is layered
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To know the chemical reactions which create and destroy stratospheric ozone.
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To learn how stratospheric ozone is monitored.
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To explore ways to reduce the use of ozone-depleting chemicals.
Activities:
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Compare the thickness of the atmosphere to the diameter of the earth.
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Plot a temperature vs. altitude profile of the atmosphere.
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View the development of the 1997 Antarctic and Arctic Ozone holes.
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View the 1978-1994 history of the ozone hole.
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Plot values for northern and southern hemisphere ozone depletion.
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Answer questions about ozone-depleting chemicals, health, and ecosystems.
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Obtain ozone levels over a specific area using latitude and longitude measurements.
Outline
THE EARTH'S ATMOSPHERE
Composition of the Atmosphere
Earth's atmosphere, which is held by gravity, is a mixture of gases
with three primary components: nitrogen (78%), oxygen (21%), and argon
(0.9%). The remaining one tenth of a percent consists of about a dozen
other gases. The most important of these gases are carbon dioxide and water
vapor because of their role in earth's heat budget and weather.
Spectrum of sunlight
and earth radiation
The atmosphere serves as a great protective shield for the earth. We
cannot endure the full unimpeded blast of the sun, even given its 150 million
kilometer distance. Radiant energy from the sun is composed of about 8%
short wavelength radiation (gamma-ray, x-ray, and ultraviolet), 47% visible
wavelength light, and 45% infrared wavelength radiation.
Distribution of Solar and Earth radiant energy by wavelength
(Figure from R.W. Christopherson, Geosystems, Prentice Hall)
Layers of the Atmosphere
The atmosphere is a thin layer relative to the size of the planet.
About 97% of our atmosphere is within 30 km of earth's surface. During
daylight hours, the atmosphere blocks out a portion of the incoming short-wavelength
(UV) solar radiation. The ozone layer absorbs the UV radiation which warms
the stratosphere. During hours of darkness, on the other hand, the warmed
stratosphere prevents excessive heat energy loss from the troposphere.
The result is a great moderation of temperatures.
About half of the solar energy that arrives at the upper layers of the
atmosphere reaches earth's surface. The radiant sun energy warms the earth
surface. The earth's surface in turn gives off that heat energy in the
form of long wavelength infrared radiation. The air resting near the ground
is warmed by earth radiation and by conduction from the warmed surface.
The temperature of air at rest tends to decline with increasing distance
above the surface. The average rate of decline is about 6.4°C per 1000
meters (3.5 °F/1000 ft). This rate is called the environmental temperature
lapse rate or lapse rate.
Do Exercise 1:
Ozone Depletion
What is the ozone layer?
The ozone layer is a concentration of ozone molecules (O3)
in the upper portion of the stratosphere between 24-48 km above the earth's
surface. This layer provides a protected shield against lethal solar UV
radiation. The ozone layer, which is presumed to have been relatively stable
for millions of years, has been shown to be in decline in the latter half
of this century. The culprit appears to be chlorine molecules which react
with the ozone to form chlorine monoxide and oxygen. The source of this
chlorine is the wide use of chlorofluorocarbons (CFCs) as propellants and
refrigerants.
How does ozone form in the stratosphere?
A chemical reaction driven by the absorption of UV solar radiation
breaks molecular oxygen (O2) into free oxygen (O) atoms. Free
oxygen (O) then bonds with molecular oxygen (O2) to form ozone
(O3).
Ozone is naturally destroyed when UV solar radiation converts O3
back to O and O2. Ozone is also destroyed naturally by reacting
with nitrous oxide (N2O) and naturally occurring oxides of chlorine
and hydrogen which reaches the stratosphere from the atmosphere below.
NASA audio clip about ozone
Creation of Ozone
Destruction of Ozone
How do CFCs destroy the Ozone layer?
Gases produced in high quantities by humans on earth can cause the ozone
to be destroyed in the stratosphere. CFCs are the most problematic ozone-depleting
compound.
Diagram:
Step 1: CFC accumulate in stratosphere
Step 2: Sunlight breaks chlorine (Cl) atom from the CFC; Cl attacks
Ozone (O3) breaking it into to O and O2 .
Step 3: Chlorine atom combines with free oxygen (O) to form Chlorine
monoxide. Chlorine monoxide breaks down into Cl and O. Cl is free to attack
more ozone molecules.
Repetition of the step 2 and 3 chemical reaction allows one chlorine
atom to destroy millions of ozone molecules.
What is the ozone hole and when does it form?
The ozone hole is the destruction of the ozone layer in the stratosphere
over the polar regions. First recognized in the southern hemisphere, it
now develops under certain conditions in the northern hemisphere. Ozone
depletion reaches to lower latitudes (especially the southern tip of South
America and New Zealand).
The Antarctic ozone hole opens and closes seasonally. In the southern
hemisphere winter (June-Sept.), super cooling of the stratosphere produces
an icy cloud layer where CFCs accumulate. When the sun returns in the austral
spring (Sept.-Nov.), stratospheric ice particles are the sites of chemical
reactions where chlorine atoms freed from the CFC molecule begin to attack
ozone molecules. By the beginning of the austral summer (Dec.), the stratospheric
ice melts and the ozone hole heals itself by the formation of ozone.
Animations created with images from the NASA-TOMS
web-site.
View
the 1996 Antarctic Ozone Hole
The concentration of ozone in the ozone hole over Antarctica has decreased
from 1978 to the present.
The size of the ozone hole has increased from 1978 to the present.
Antarctic Ozone Hole and Concentrations for 1998
In 1998, the ozone hole over the Antarctic was the largest observed since
the hole was first observed in the late 1970's. The hole extends over approximately
26 million square kilometers. By October 1998 the level of ozone dropped
to 92 Dobson units. That is the lowest level of ozone measured with the
exception of when the levels dropped in 1991 to 88 Dobson units due to
the eruption of Mount Pinatubo in the Philippines.
View
of October 5, 1998 Ozone Hole
Arctic Ozone Hole
Anomalously low ozone over the Arctic by: P. A. Newman, J. F. Gleason,
R. D. McPeters, and R. S. Stolarski NASA Goddard Space Flight Center, Greenbelt,
Maryland
GEOPHYSICAL RESEARCH LETTERS, VOL. 24, NO. 22, PAGES 2689-2692, NOVEMBER
15, 1997
Abstract:
Total ozone observations from the Total Ozone Mapping Spectrometer (TOMS)
instruments during March 1997 reveal an extensive region of low column
densities in the Arctic region centered near the north pole. Values were
below 250 Dobson units for nearly a two week period during this period,
and were correlated with the position of the northern lower stratospheric
polar vortex. The March 1997 average total ozone column densities were
more than 30% lower than the average of column densities observed during
the 1979-1982 March period.
Figure 1: March average of total ozone from 63N to 90N. The 71 and 72
data are Nimbus 4 BUV, the 1979 to 1993 data are Nimbus 7 TOMS, the 1994
data are Meteor 3 TOMS, the 1996 data are NOAA 9 SBUV/2 data, and the 1997
data are Earth Probe TOMS data.
Figure 2: March monthly average total ozone polar stereographic images
for 1971 and 1972 (Nimbus 4 BUV); 1979, 1980, 1990, 1993 (Nimbus 7 TOMS);
1996 (NOAA 9 SBUV-2); and 1997 (EP TOMS).
Figure 3: The daily minimum total ozone values observed between 40N
and 90N during 1997 (dots), and 1993 (thick line). The range of minimum
values for the years 1978 to 1994 is indicated by the grey shading, while
the average minimum value is indicated by the thick white line. Record
low values were measured in Late March and early April in the polar region.
How is atmospheric ozone monitored?
Atmospheric ozone can be measured through ground-based instruments
placed at strategic points such as Antarctica or by airborne instruments,
such as those mounted on balloons, high-altitude aircraft, rockets, and
satellites.
Daily global stratospheric ozone concentration data are best collected
by various instruments onboard several different satellites. The most continuous
measurement of stratospheric ozone concentration was collected by the Total
Ozone Mapping Spectrometer (TOMS) and the Solar Backscatter Ultraviolet
(SBUV) instruments aboard the Nimbus-7 satellite. Nimbus-7 was operational
between October 31, 1978 and May 6, 1993. Between 1991 and 1994 real-time
global ozone monitoring data were obtained from the TOMS instrumentation
flying onboard the Russian Meteor-3 satellite. Operational for less than
a year, instrumentation onboard the Japanese Advanced Earth Observing Satellite
(ADEOS) "MIDORI" satellite was lost on June 30, 1997.
Today, the National Oceanographic and Atmospheric Administration (NOAA)
has accurate SBUV instruments aboard several weather satellites which measure
total ozone levels to an accuracy of ±1%. The Total Ozone Mapping
Spectrometer onboard an NASA's Earth Probe Satellite (TOMS/EP) was launched
in 1996.
How is total stratospheric ozone measured?
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Incoming solar radiation (insolation) intensity is reduced as it passes
through the atmosphere proportional to the concentration and thickness
of the ozone layer. Total ozone is calculated by comparing incoming solar
radiation to radiation backscatter in the ultraviolet wavelength spectrum.
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Ozone concentration is measured in Dobson units (Named after one of the
first scientists to measure ozone distribution in the stratosphere). One
Dobson unit (DU) is equivalent to 2.69 x 10E16 molecules per square centimeter,
which is the amount of gas in one square centimeter at 1 atmosphere of
pressure (i.e. at the earth's surface) and at zero degrees Celsius. The
average concentration of ozone in the stratosphere is 250 to 300 DU in
the tropics and between 300 to 475 DU in the temperate midlatitude region.
Reduction of stratospheric ozone below normal levels is known as "Ozone
depletion."
Do Exercise 2:
Policy issues and health affects
What chemicals lead to the ozone destruction?
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Several gases and by-products of human activities on Earth create ozone-depleting
chemicals that reach the stratosphere. These compounds include nitrous
oxides from jet exhaust and fertilizers, methane from agriculture and burning
fuel, halons in fire extinguishers, and chlorofluorocarbons (CFCs) in sprays,
coolants, and foams. CFC is a compound containing chlorine, fluorine, and
carbon. It's manufacture and use is now globally limited. Halon is a compound
consisting of bromine, fluorine, and carbon. Halons cause ozone depletion
because they contain bromine. Bromine is many times more effective at destroying
ozone than chlorine.
What policies govern ozone-depleting chemicals?
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The "Montreal Protocol on Substances That Deplete the Ozone Layer" (MP)
is an international treaty ratified in 1987 that governs the protection
of stratospheric ozone. The treaty and its amendments (1990 and 1992) control
the production and use of major ozone-depleting chemical . A complete phaseout
of the production of eight ozone-depleting chemicals was to have taken
place on January 1, 1996. Developing countries may produce at 15 percent
1986 levels.
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The U.S. Clean Air Act tightly governs atmospheric emissions including
ozone-depleting substances in the U.S. The U.S Environmental Protection
Agency (EPA) is empowered to enforce these regulations and has set up programs
to recycle refrigerants.
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EPA Home page "U.S. production of ozone-depleting gases has declined significantly
since 1988, and has now reached levels (measured by their ozone depletion
potential) comparable to those of 30 years ago. Because of international
agreements to decrease production and ultimately to phase out production
of CFCs and halons, scientists expect that total chlorine and bromine concentrations
in the troposphere will peak by 1996 and begin a slow decline soon thereafter.
Concentrations are expected to peak in the stratosphere 3-5 years later.
Increasing ozone losses are predicted for the remainder of the decade,
with gradual recovery by the mid-21st century."
What is being done about CFC use?
Safe alternatives for ozone-depleting chemicals are being searched out.
One substitute for CFCs are hydrogenated CFCs (HCFCs). HCFC are less likely
to reach the stratosphere because they are broken down in the troposphere.
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The EPA recycling program has reduced the release of refrigerants. The
CFC phaseout has led to the develop of energy efficient air-conditioning
and refrigeration equipment.
What are the health risks of a depleted ozone
layer?
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Some projections suggest that UV radiation reaching the surface may increase
5-20% in the next 30 years. UV causes skin cancer, increase cataracts,
weakening of the immune system by mutations. Most alarming is potential
destruction of oceanic plankton at the base of the food chain amd potential
reduction of photosynthesis in plants.
Do Exercise 3
Some ozone resources:
NASA TOMS home page
TAKE THE OZONE
HOLE TOUR
from the Centre of Atmospheric Science, Cambridge University
Multimedia:
Mixing
in the Antarctic Circumpolar Vortex
TOVSTOMS
Ozone
Earth Probe TOMS Animation of
Northern Hemisphere, 1997 [choose 'multimedia', ADEOS GIF animation
[N. Hemisphere]
video
Encyclopedia
Britannica Online
The EPA's
Stratospheric Ozone home page
