OZONE IN THE ATMOSPHERE
1. What is ozone and where is it in the atmosphere?
Ozone is a gas that is naturally present in our atmosphere. Each ozone molecule contains three
atoms of oxygen and is denoted chemically as O3. Ozone is found primarily in two regions of the
atmosphere. About 10% of atmosphere ozone are in the troposphere, the region closest to Earth
(from the surface to about 10 - 16 kilometers (6 - 10 miles). The remaining ozone (90%) resides
in the atmosphere, primarily between the top of the troposphere and about 50 kilometers (31
miles) altitude. The large amount of ozone in the stratosphere is often referred to as the "ozone
layer"
2. How is ozone formed in the atmosphere?
Ozone is formed throughout the atmosphere in multistep chemical processes that require
sunlight. In the stratosphere, the process begins with the breaking apart of the oxygen molecule
(O2) by ultraviolet radiation from the Sun. In the lower atmosphere (troposphere), ozone is
formed in a different set of chemical reactions involving hydrocarbons and nitrogen- containing
gases.
3. Is there ozone hole over the Arctic?
Significant reductions in ozone content in the stratosphere above the Arctic have been observed
during the late winter and early spring (January - March) in 6 of the last 9 years. However, these
reductions, typically 20 - 25 %, are much smaller than those observed currently each spring over
the Antarctic (the ozone hole).
The difference between the ozone content in the two Polar Regions is cause by the dissimilar
weather patterns. The Antarctic continent is a very large landmass surrounded by oceans. This
symmetrical condition produces very low stratosphere temperatures within a meteorologically
isolated region, the so - called polar vortex, which extends from about 65°S to the pole. The cold
temperatures lead in turn to the formation of clouds, known as polar stratospheric clouds. These
clouds provide chemical changes that promote production of chemically active chlorine and
bromine that rapidly destroy ozone. The conditions that maintain elevated levels of chemically
active chlorine and bromine persist into September and October in Antarctica, when sunlight
returns over the region to initiate ozone depletion.
In recent years, there has been a string of unusually cold winters in the Arctic, compared with
those in the preceding 30 years. The cold and persistent conditions have led to enhanced ozone
depletion, since the atmospheric concentrations of ozone depletion gases have also been
relatively large during these years. However, the cause of the observed change in meteorological
conditions is not yet understood. Such conditions might persists over the coming years, further
enhancing ozone depletion. But it is also possible that, in the next few years, they could revert to
the conditions characteristic of a decade ago. In the latter case, chemical ozone depletion in the
Arctic would be expected to diminish.
Therefore, although there has been significant ozone depletion in the Arctic in recent years, it is
difficult to predict what may lie ahead, because the future climate of the Arctic stratosphere
cannot be predicted with confidence.
4. When did Antarctic ozone hole first appear?
The Springtime Antarctic ozone hole is a new phenomenon that appeared in the early 1980s.
The observed average amount of ozone during September, October, and November over the
British Antarctic Survey station at Halley, Antarctica, first revealed notable decreases in the
early 1980s, compared with the preceding data obtained starting in 1957. The ozone hole is
formed each year when there is a sharp decline (currently up to 60%) in the total ozone over
most of Antarctica for a period of about three months (September - November) during spring in
the Southern Hemisphere. Late summer (January - March) ozone amounts show no such sharp
decline in the 1980s and 1990s. Observations from other stations in Antarctica and from satellite
- based instruments reveal similar decreases in springtime amount ozone overhead. Balloon -
borne ozone instruments show dramatic, changes in the way ozone is distributed at with altitude.
Before the stratosphere was affected by human - produced chlorine and bromine, the naturally
occurring springtime ozone levels over Antarctica were about 30 - 40 % lower than springtime
ozone levels over the Arctic. Dobson first observed this natural difference between Antarctic and
Arctic conditions in the late 1950s. It stems from the exceptionally cold temperatures and
different winter wind patterns within the Antarctic Stratosphere as compared with the Arctic.
This is not at all the same phenomenon as the marked downward trend in total ozone in recent
years.
Changes in stratospheric meteorology cannot explain the ozone hole. Measurement shows the
wintertime Antarctic stratospheric temperatures of past decades had not changed prior to the
development of the ozone hole each September. Ground, aircraft, and satellite measurements
have provided, in contrast, clear evidence of the importance of the chemistry of chlorine and
bromine originating from human - made, compounds in depleting Antarctic ozone in recent
years.
5. Why has an ozone hole appeared over Antarctica when CFCs and Halons are released
mainly in the Northern Hemisphere?
The Earth's atmosphere is continuously stirred over the globe by winds. As a result, ozone -
depletion gases get mixed throughout the atmosphere, including Antarctica, regardless of where
they are emitted. The special meteorological conditions in Antarctica cause these gases to be
more effective there in depleting ozone compared to anywhere else.
Human emissions of chlorofluorocarbons (CFCs) and halons (bromine - containing gases) have
occurred mainly in the Northern Hemisphere. About 90% have been released in the latitudes
corresponding to Europe, Russia, Japan, and North America. Gases such as CFCs and halon,
which are insoluble in water and relatively unreactive, are mixed within a year or two throughout
the lower atmosphere. The CFCs and halons in this well - mixed air rise from the lower
atmosphere into the stratosphere mainly in tropical latitudes. Winds then move this air poleward
- both north and south - from the tropics, so that air throughout the global stratosphere contains
nearly equal amounts of chlorine and bromine.
In the Southern Hemisphere, the South Pole is a part of a very large landmass (Antarctic) that is
completely surrounded by ocean. This is reflected in the meteorological conditions that allow the
formation in winter of a very cold region in the stratosphere over the Antarctic continent, isolated
by a band of strong winds circulating around the edge of that region. The very low stratospheric
temperatures lead to the formation of clouds (polar stratospheric clouds) that are responsible for
chemical changes that promote production of chemically active chlorine and bromine. This
chlorine and bromine activation then leads to rapid ozone loss when sunlight returns to
Antarctica in September and October of each year, which then results in the Antarctic ozone hole
Similar condition does not exist over the Arctic. The wintertime temperatures in the Arctic
stratosphere are not persistently low for as many weeks as over Antarctica, which result in
correspondingly less ozone depletion in the Arctic.
6. Does most of the Chlorine in the stratosphere come from human or natural source?
Most of the chlorine in the stratosphere is there as a result of human activities. Many compounds
containing chlorine are released at the ground. Those that dissolve in water cannot reach
stratospheric altitudes in significant amounts because they are " washed out" of the atmosphere
in rain or snow. For examples, large quantities of chlorine are released from evaporated ocean
spray as sea salt (sodium chloride) particles. However, because sea salt dissolves in water, this
chlorine is taken up quickly in clouds or in ice, snow, or rain droplets and does not reach the
stratosphere. Another ground - level source of chlorine is from its use in swimming pools and as
household bleach. When released, this chlorine is rapidly converted to form that dissolve in
water and therefore are removed from the lower atmosphere. Such chlorine never reaches the
stratosphere in significant amounts. Volcanoes can emit large quantities of hydrogen chloride,
but this gas is rapidly converted to hydrochloric acid, which dissolves in rainwater, ice, and snow
and does not reach the stratosphere. Even in explosives volcanic plumes that rise high in the
atmosphere, nearly all the hydrogen chloride is removed by precipitation before reaching
stratospheric altitudes. Finally, although the exhaust from the Space Shuttle and from some
rockets does inject some chlorine directly into the stratosphere, the quantities are very small (less
than 1% of the annual input from halocarbons in the present stratosphere).
In contrast, the major ozone - depleting human - produced halocarbons - such as
chlorofluorocarbons (CFCs) and carbon tetrachloride (CC14) - are not soluble in water, do not
react with snow or other natural surfaces, and are not broken down chemically in the lower
atmosphere. Therefore, these and other human produced substances containing chlorine do reach
the stratosphere.
Several of evidence combine to establish human - produced halocarbons as the primary source of
stratospheric chlorine.First, measurement has shown that the chlorinated species that rise to the
stratosphere are primarily manufactured compounds [mainly CFCs carbon tetrachloride, mehyl
chloroform, and the hydrochlorofluorocarbon (HCFC) substitutes for CFCs], together with small
amounts of hydrochloric acid (HCI) and methyl chloride (CH3CI), which are partly natural in
origin. Second, researchers have measured nearly all known gases containing chlorine in the
stratosphere. They have found that the measured nearly all known - produced halocarbons, plus
the much smaller contribution from natural sources, could account for all of the stratospheric
chlorine measured between 1980 and 1988 corresponds to the known increases in concentrations
of human- produced halocarbons during that time.
7. Why do we care about atmospheric ozone?
Ozone in the stratosphere absorbs some of the Sun's biologically harmful ultraviolet radiation.
Because of this beneficial role, stratospheric ozone is considered " good ozone." In contrast,
ozone at Earth's surface that is formed from pollutants is considered "bad ozone" because it can
be harmful to humans and plant and animal life. Some ozone occurs naturally in the lower
atmosphere where it is beneficial because ozone helps remove pollutants from the atmosphere.
8. Is total ozone uniform over the globe?
No, the total amount of ozone above the surface of Earth varies with location on time scales that
range from daily to seasonal. Stratospheric winds and the chemical production and destruction of
ozone cause the variations. The total ozone is generally lowest at the equator and highest near the
poles because of the seasonal wind pattern in the stratosphere.
9. How is ozone measured in the atmosphere?
The amount of ozone in the atmosphere is measured by instruments on the ground and carried
aloft in balloons, aircraft, and satellites. Some measurement involves drawing air into an
instrument that contains a system for detecting ozone. Other measurements are based on ozone's
unique absorption of light in the atmosphere. In that case, sunlight is carefully measured after
passing through a portion of the atmosphere containing ozone.
10. Can the natural changes such as the sun's output and volcanic eruptions be responsible
for the observed changes in ozone?
Although there are natural forces that cause fluctuations in ozone amounts, there is no evidence
that natural changes are contributing significantly to the observed long - term trend of decreasing
ozone.
The formation of stratospheric ozone is initiated by ultraviolet (UV) light coming from the Sun.
As a result, the Sun's output affects the rate at which ozone is produced. The Sun's energy release
(both as UV light and as charged particles such as electrons and protons) does vary, especially
over the well - known 11-year sunspot cycle. Observations over several cycles (since the 1960s)
show that total global ozone levels vary by 1 - 2 % from the maximum to the minimum of a
typical cycle. However changes in the Sun's output cannot be responsible for the observed long
term changes in ozone, because the ozone downward trends are larger than 1 - 2 %. Since 1978
the Sun's energy output has gone through maximum values in about 1980 and 1991 and
minimum values in about 1985 and 1996. It is now increasing again toward its next maximum
around 2002; However, the trend in ozone was downward throughout that time. The ozone trends
presented in this and previous international scientific assessments have been obtained by
evaluating the long - term changes in ozone after accounting for the solar influence.
Major, explosive volcanic eruptions can inject material directly into the ozone layer,
Observations and model calculations show that volcanic particles cannot on their own deplete
ozone. It is only the interaction of human - produced chlorine with particle surfaces that
enhances ozone depletion in today's atmosphere. Specifically, laboratory measurements and
observations in the atmosphere have shown that chemical reactions on and within the surface of
volcanic particles injected into the lower stratosphere lead to enhanced ozone destruction by
increasing the concentration of chemically active forms of chlorine that arise from the human -
produced compounds like the chlorofluorocarbons (CFCs). The eruptions of Mt. Agung (1963),
Mt. Fuego (1974), El Chichon (1982) and particularly Mt.Pinatubo 1991) are examples. The
eruptions of Mt Pinaturbo resulted in a 30 to 40 fold increase in the total surface area of particles
available for enhancing chemical reactions. The effect of such natural events on the ozone layer
is then dependent on the concentration of chlorine containing molecules and particles available
in the stratosphere, in a manner similar to polar stratospheric clouds. Because the particles are
removed from the stratosphere in 2 to 5 years, the effect on - ozone is only episodes cannot account
for observed long - term changes. Observations and calculations indicate that the record-low ozone levels
observed in1992 - 1993 reflect the importance of the relatively large number of particles produced by the
Mt. Pinatubo eruption, couple with the relatively higher amount of human - produced stratospheric
chlorine in the 1990s compared to that at times of earlier volcanic eruptions.
11. How can Chloroflurocarbons(CFCs) get to the stratosphere if they're heavier than air?
CFCs reach the stratosphere because the Earth's atmosphere is always in motion and mixes the
chemicals added into it.
CFC molecules are indeed several times heavier than air. Nevertheless, thousands of
measurements from balloons, aircraft and satellites demonstrate that the CFAs are actually
present in the stratosphere. This is because winds and other air motions mix the atmosphere to
altitudes for above the top of the stratosphere much faster than molecules can settle according to
their weight. Gases such as CFCs that do not dissolve in water and that are relatively unreactive 13
in the atmosphere are mixed relatively quickly and therefore reach the atmosphere regardless of
their weight.
Measured changes in concentration of constituents versus altitude teach us more about the fate of
compounds in the atmosphere. For examples, the two gases carbon tetrafluoride (CF4, produced
mainly as a by - product of the manufacture of aluminum) and CFC -11 (CC13F, used in a
variety of human activities) are both heavier than air.
Carbon tetrafluoride is completely unreactive at altitudes up to at least 50 kilometers in the
atmosphere. Measurements show it to be nearly uniformly distributed throughout the
atmosphere, There have been fresh measurements over the past two decades of several other
completely unreceptive gases, both lighter than air (neon) and heavier than air (argon and
krypton), that show that they also mix upward through the stratosphere regardless of their
weight.
CCF-11 is unreactive in the lower atmosphere and is similarly uniformly mixed there, as shown
in the figure. However, the abundance of CFC -11 decreases as the gas reaches higher altitudes,
because it is broken down by high - energy solar ultraviolet radiation. Chlorine released from this
breakdown of CFC - 11 and other CFCs remains in the stratosphere for several years, where
every chlorine atom destroys many thousands of molecules of ozone.
12. What is the evidence that Chlorine and Bromine destroy stratospheric ozone?
Numerous laboratory investigations and analyses of worldwide measurements made in the
stratosphere have demonstrated that chlorine - and bromine - containing chemicals destroy ozone
molecules.
Research studies in the laboratory show that chlorine (CI) reacts very rapidly with ozone. They
also show that the reactive chemical chlorine monoxide (CIO) formed in that reaction can
undergo further processes that regenerate the original chlorine, allowing the sequence to be
repeated very many times (a chain reaction). Similar reactions also take place between bromine
and ozone.
But do this ozone - destroyed reactions occur in the " real world"? All the accumulated scientific
experience demonstrates that the same chemical reactions do take place in nature. Many other
reactions (including those of other chemical species) are often also taking place simultaneously
in the stratosphere. This makes the connections among the changes difficult to untangle.
Nevertheless, whenever chlorine (or bromine) and ozone are found together in the stratosphere,
the ozone- destroying reactions are taking place.
Sometimes a small number of chemical reactions are so dominant in the natural circumstance
that the connections are almost as clear as in laboratory experiments. Such a situation occurs in
the Antarctic stratosphere during the springtime formation of the zone hole. Independent
measurements made by instruments from the ground and from balloons, aircraft, and satellites
have provided a detailed understanding of the chemical reactions in the Antarctic stratosphere.
Large areas reach temperatures so low (less than - 80° C, or - 112 ° F) that stratospheric cloud
form, which is a rare occurrence, except during the polar winters. These polar stratospheric
clouds allow chemical reactions that transform chlorine species from those that do not cause
ozone depletion into those that do. Among the latter is chlorine monoxide, which initiates ozone
destruction in the presence of sunlight. The amount of reactive chlorine in such regions is
therefore much higher than that observed in the middle latitudes, which leads to much faster
chemical ozone destruction. The chemical reactions occurring in the presence of these clouds are
now well understood from studies under laboratory conditions that mimic those found naturally
in the atmosphere.
Scientists have repeatedly observed a large of chemical species over Antarctica since 1986.
Among the chemicals measured were ozone and chlorine monoxide, which is the reactive
chemical identified in the laboratory as one of the participants in the ozone destroying chain
reactions.
Similar reactions involving chlorine and bromine have also been shown to occur during winter
and spring in the Arctic polar regions, which leads to some chemical depletion of ozone in that
region. Because the Arctic is not usually as persistently cold as the Antarctic, fewer stratospheric
clouds form, and therefore is less ozone depletion in the Arctic.
THE OZONE DEPLETION PROCESS
13. What are the principal steps in stratospheric ozone depletion caused by human
activities.?
The initial step in the depletion of stratospheric ozone by human activities is the emission of
ozone depletion gases containing chlorine and bromine at Earth's surface. Most of these gases
accumulate in the lower atmosphere because they are unreactive and do not dissolve readily in
rain or snow. Eventually, the emitted gases are transported to the stratosphere where they are
converted to more reactive gases containing chlorine and bromine. These more reactive gases
then participate in reactions that destroy ozone. Finally, when air return to the lower atmosphere,
these reactive chlorine and bromine gases are removed from Earth's atmosphere by rain and
snow.
14. What emissions from human activities lead to ozone depletion?
Certain industrial processes and consumer products result in the atmospheric emission of
"halogen source gases." These gases contain chlorine and bromine atoms, which are known to be
harmful to the ozone layer. For example, the chlorofluorocarbons (CFCs) and
hydrochlorofluorocarbons (HCFCs), once used in almost refrigeration and air conditioning
systems, eventually reach the stratosphere where they are broken apart to release ozone-depleting
chlorine atoms. Other examples of human-produced ozone-depleting gases are the "halons."
Which are used in fire extinguishers and which contain ozone-depleting bromine atoms. the
production and consumption of all principal halogen source gases by human activities are
regulated worldwide under the Mnotreal Protocol.
15. What are the reactive halogen gases that destroy stratospheric ozone?
Emissions from human activities and natural processes are large sources of chlorine - and
bromine- containing gases for the stratosphere. When exposed to ultraviolet radiation from the
Sun, these halogen source gases are converted to more reactive gases also containing chlorine
and bromine. Important examples of the reactive gases that destroy stratospheric ozone are
chlorine monoxide (CIO) and bromine monoxide (BrO). These and other reactive gases
participate in "catalytic" reaction cycles that efficiently destroy ozone. Volcanoes can emit some
chlorine-containing gases, but these gases are ones that readily gases, but these gases are ones
that readily dissolves in rainwater and ice and are usually "washed out" of the atmosphere before
they can reach the stratosphere.
16. What are the chlorine and bromine reactions that destroy stratospheric ozone?
Reactive gases containing chlorine and bromine destroy stratospheric ozone in "catalytic" cycles
made up of two or more separate reactions. As a result, a single chlorine or bromine atom can
destroy many hundreds of ozone molecules before it reacts with another gas, breaking the cycle.
In this way, a small amount of reactive chlorine or bromine has a large impact on the ozone
layer. Special ozone destruction reactions occur in polar regions because the reactive gas
monoxide reaches very high levels there in the winter/spring season.
17. Why has an "ozone hole" appeared over Antarctica when ozone - depleting gases are
present throughout the stratosphere?
Ozone -depleting gases are present throughout the stratospheric ozone layer because they are
transported great distances by atmospheric air motions. The severe depletion of the Antarctic
ozone layer known as the "ozone hole" forms because of the special weather conditions that exist
there and nowhere else on the globe. The very cold temperatures of the Antarctic stratosphere
create ice clouds called polar stratospheric clouds (PSCs). Special reactions that occur on PSCs
and the relative isolation of polar stratospheric air allow chlorine and bromine reactions to
produce the ozone hole in Antarctic springtime.
STRATOSPHERIC OZONE DEPLETION
18. How severe depletion of the Antarctic ozone layer?
Severe depletion of the Antarctic ozone layer was first observed in the early 1980s.Antarctic
ozone depletion is seasonal, occurring primarily in late winter and spring (August - November).
Peak depletion occurs in October when ozone is often completely destroyed over a range of
altitudes, reducing overhead total ozone by as much as two - thirds at some locations. This severe
depletion creates the "ozone hole" in ../images of Antarctic total ozone made from space. In most
years the maximum area of the ozone hole usually exceeds the size of the Antarctic continent.
19. Is there depletion of the Arctic ozone layer?
Yes, significant of the Arctic ozone layer now occurs in some years in the late winter/spring
period (January-April). However, the maximum depletion is generally less severe than that
observed in the Antarctic and is more variable from year to year. A large and recurrent "ozone
hole," as found in the Antarctic stratosphere, does not occur in the Arctic.
(Source : David W. Fahey, 2002, “Twenty Questions & Answers about the Ozone Layer,
Scientific Assessment of Ozone Depletion : 2002”, NOAA Aeronomy Laboratory, USA)
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