What is ozone and where is it in the atmosphere?
is a gas that is naturally present in our atmosphere. Each ozone
molecule contains three atoms of oxygen
is denoted chemically as O3.
Ozone is found primarily in two regions of the atmosphere. About 10%
ozone is in the troposphere, the region closest to Earth (from the
surface to about 10–16 kilometers
miles)). The remaining ozone (about 90%) resides in the stratosphere
between the top of the troposphere
about 50 kilometers (31 miles) altitude. The large amount of ozone in
the stratosphere is often referred to
the “ozone layer.”
How is ozone formed in the atmosphere?
is formed throughout the atmosphere in multistep chemical processes
that require sunlight. In the
the process begins with an oxygen molecule (O2)
being broken apart by ultraviolet radiation from
Sun. In the lower atmosphere (troposphere), ozone is formed by a
different set of chemical reactions that
naturally occurring gases and those from pollution sources.
Why do we care about atmospheric ozone?
in the stratosphere absorbs a large part of the Sun’s biologically
harmful ultraviolet radiation.
ozone is considered “good” ozone because of this beneficial role.
In contrast, ozone formed at
surface in excess of natural amounts is considered “bad” ozone
because it is harmful to humans, plants,
animals. Natural ozone near the surface and in the lower atmosphere
plays an important beneficial role in
removing pollutants from the atmosphere.
How is total ozone distributed over the globe?
distribution of total ozone over the Earth varies with location on
timescales that range from daily to
The variations are caused by large-scale movements of stratospheric
air and the chemical production
destruction of ozone. Total ozone is generally lowest at the equator
and highest in polar regions.
How is ozone measured in the atmosphere?
amount of ozone in the atmosphere is measured by instruments on the
ground and carried aloft on
aircraft, and satellites. Some instruments measure ozone locally by
continuously drawing air samples
a small detection chamber. Other instruments measure ozone remotely
over long distances by using ozone’s
optical absorption or emission properties.
What are the principal steps in stratospheric ozone depletion caused
initial step in the depletion of stratospheric ozone by human
activities is the emission, at Earth’s surface,
gases containing chlorine and bromine. Most of these gases accumulate
in the lower atmosphere because
are unreactive and do not dissolve readily in rain or snow. Natural
air motions transport these accumulated
to the stratosphere, where they are converted to more reactive gases.
Some of these gases then
in reactions that destroy ozone. Finally, when air returns to the
lower atmosphere, these reactive
and bromine gases are removed from Earth’s atmosphere by rain and
What emissions from human activities lead to ozone depletion?
industrial processes and consumer products result in the emission of
(ODSs) to the atmosphere. ODSs are manufactured halogen source gases
worldwide by the Montreal Protocol. These gases bring chlorine and
the stratosphere, where they destroy ozone in chemical reactions.
the chlorofluorocarbons (CFCs), once used in almost all refrigeration
and air conditioning
and the halons, which were used in fire extinguishers. Current ODS
abundances in the
are known directly from air sample measurements.
What are the reactive halogen gases that destroy stratospheric ozone?
from human activities and natural processes represent a large source
of chlorine- and brominecontaining
that enter the stratosphere. When exposed to ultraviolet radiation
from the Sun, these halogen
gases are converted to more reactive gases containing chlorine and
bromine. Some reactive gases act as
reservoirs that convert to form the most reactive gases, namely
chlorine monoxide (ClO) and bromine
(BrO). The most reactive gases participate in catalytic reactions
that efficiently destroy ozone. Most
emit some reactive halogen gases that readily dissolve in water and
are usually washed out of the
before they can reach the stratosphere.
What are the chlorine and bromine reactions that destroy
gases containing chlorine and bromine destroy stratospheric ozone in
“catalytic” cycles made up of two
more separate reactions. As a result, a single chlorine or bromine
atom can destroy many thousands of ozone
before it leaves the stratosphere. In this way, a small amount of
reactive chlorine or bromine has a large
on the ozone layer. A special situation develops in polar regions in
the late winter/early spring season where
enhancements in the abundance of the most reactive gas, chlorine
monoxide, leads to severe ozone depletion.
Why has an “ozone hole” appeared over Antarctica when
are present throughout the stratosphere?
substances are present throughout the stratospheric ozone layer
because they are transported
distances by atmospheric air motions. The severe depletion of the
Antarctic ozone layer known as the
hole” occurs because of the special atmospheric and chemical
conditions that exist there and nowhere
on the globe. The very low winter temperatures in the Antarctic
stratosphere cause polar stratospheric
(PSCs) to form. Special reactions that occur on PSCs, combined with
the relative isolation of polar
air, allow chlorine and bromine reactions to produce the ozone hole
in Antarctic springtime.
How severe is the depletion of the Antarctic ozone layer?
depletion of the Antarctic ozone layer was first reported in the
mid-1980s. Antarctic ozone depletion
seasonal, occurring primarily in late winter and early spring (August
to November). Peak depletion occurs in
October when ozone is often completely destroyed over a range of
altitudes, thereby reducing total ozone
as much as two-thirds at some locations. This severe depletion
creates the “ozone hole” apparent in images
Antarctic total ozone made using satellite observations. In most
years the maximum area of the ozone hole
exceeds the size of the Antarctic continent.
Is there depletion of the Arctic ozone layer?
significant depletion of the Arctic ozone layer now occurs in most
years in the late winter/early spring period
to March). However, the maximum depletion is less severe than that
observed in the Antarctic and is
variable from year to year. A large and recurrent “ozone hole,”
as found in the Antarctic stratosphere,
not occur in the Arctic.
How large is the depletion of the global ozone layer?
of the global ozone layer began gradually in the 1980s and reached a
maximum of about 5% in
early 1990s. The depletion has lessened since then and now is about
3.5% averaged over the globe. The
depletion exceeds the natural year-to-year variations of global total
ozone. The ozone loss is very small
the equator and increases with latitude toward the poles. The larger
polar depletion is attributed to the late
spring ozone destruction that occurs there each year.
Do changes in the Sun and volcanic eruptions affect the ozone layer?
factors such as changes in solar radiation, as well as the formation
of stratospheric particles after volcanic
do influence the ozone layer. However, neither factor can explain the
average decreases observed in
total ozone over the last three decades. If large volcanic eruptions
occur in the coming decades, ozone
will increase for several years afterwards.
Are there regulations on the production of ozone-depleting gases?
the production and consumption of ozone-depleting substances are
controlled under a 1987 international
known as the “Montreal Protocol on Substances that Deplete the
Ozone Layer” and by its
Amendments and Adjustments. The Protocol, now ratified by all 197
United Nations members,
legally binding controls on national production and consumption of
Production and consumption of all principal ODSs by developed and
developing nations will be almost
phased out before the middle of the 21st century.
Has the Montreal Protocol been successful in reducing ozone-depleting
as a result of the Montreal Protocol, the overall abundance of
ozone-depleting substances (ODSs) in the
has been decreasing for about a decade. If the nations of the world
continue to comply with the
of the Montreal Protocol, the decrease will continue throughout the
21st century. Those gases that
still increasing in the atmosphere, such as halon-1301 and HCFC-22,
will begin to decrease in the coming
if compliance with the Protocol continues. Only after midcentury will
the effective abundance of ODSs
to values that were present before the Antarctic ozone hole was
observed in the early 1980s.
Does depletion of the ozone layer increase ground-level ultraviolet
ultraviolet radiation at Earth’s surface increases as the amount of
overhead total ozone decreases, because
absorbs ultraviolet radiation from the Sun. Measurements by
ground-based instruments and estimates
using satellite data provide evidence that surface ultraviolet
radiation has increased in large geographic
in response to ozone depletion.
Is depletion of the ozone layer the principal cause of climate
ozone depletion itself is not the principal cause of climate change.
Changes in ozone and climate are directly
because ozone absorbs solar radiation and is also a greenhouse gas.
Stratospheric ozone depletion and
in global tropospheric ozone that have occurred in recent decades
have opposing contributions to climate
The ozone depletion contribution, while leading to surface cooling,
is small compared with the contribution
all other greenhouse gas increases, which leads to surface warming.
The total forcing from these other
gases is the principal cause of observed and projected climate
change. Ozone depletion and climate
are indirectly linked because both ozone depleting substances and
their substitutes are greenhouse gases.
Have reductions of ozone-depleting substances under the Montreal
protected Earth’s climate?
All ozone-depleting substances are also greenhouse gases that
contribute to climate forcing when they
in the atmosphere. Montreal Protocol controls have led to a
substantial reduction in the emissions of
substances (ODSs) over the last two decades. These reductions have
provided the added benefit
reducing the human contribution to climate change while protecting
the ozone layer. Without Montreal Protocol
the climate forcing contribution from annual ODS emissions could now
be 10-fold larger than its present
which would be a significant fraction of the climate forcing from
current carbon dioxide (CO2)
How is ozone expected to change in the coming decades?
recovery of the ozone layer from the effects of ozone-depleting
expected near the middle of the 21st century, assuming global
compliance with the Montreal
Recovery will occur as ODSs and reactive halogen gases in the
stratosphere decrease in
coming decades. In addition to responding to ODSs, future ozone
amounts will increasingly
influenced by expected changes in climate. The resulting changes in
depend strongly on the geographic region. During the long recovery
period, large volcanic
could temporarily reduce global ozone amounts for several years.
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