Ozone layer | Wikipedia audio article


The ozone layer or ozone shield is a region
of Earth’s stratosphere that absorbs most of the Sun’s ultraviolet radiation. It contains
high concentration of ozone (O3) in relation to other parts of the atmosphere, although
still small in relation to other gases in the stratosphere. The ozone layer contains
less than 10 parts per million of ozone, while the average ozone concentration in Earth’s
atmosphere as a whole is about 0.3 parts per million. The ozone layer is mainly found in
the lower portion of the stratosphere, from approximately 15 to 35 kilometers (9.3 to
21.7 mi) above Earth, although its thickness varies seasonally and geographically.The ozone
layer was discovered in 1913 by the French physicists Charles Fabry and Henri Buisson.
Measurements of the sun showed that the radiation sent out from its surface and reaching the
ground on Earth is usually consistent with the spectrum of a black body with a temperature
in the range of 5,500–6,000 K (5,227 to 5,727 °C), except that there was no radiation
below a wavelength of about 310 nm at the ultraviolet end of the spectrum. It was deduced
that the missing radiation was being absorbed by something in the atmosphere. Eventually
the spectrum of the missing radiation was matched to only one known chemical, ozone.
Its properties were explored in detail by the British meteorologist G. M. B. Dobson,
who developed a simple spectrophotometer (the Dobsonmeter) that could be used to measure
stratospheric ozone from the ground. Between 1928 and 1958, Dobson established a worldwide
network of ozone monitoring stations, which continue to operate to this day. The “Dobson
unit”, a convenient measure of the amount of ozone overhead, is named in his honor.
The ozone layer absorbs 97 to 99 percent of the Sun’s medium-frequency ultraviolet light
(from about 200 nm to 315 nm wavelength), which otherwise would potentially damage exposed
life forms near the surface.In 1976 atmospheric research revealed that the ozone layer was
being depleted by chemicals released by industry, mainly chlorofluorocarbons (CFCs). Concerns
that increased UV radiation due to ozone depletion threatened life on Earth, including increased
skin cancer in humans and other ecological problems, led to bans on the chemicals, and
the latest evidence is that ozone depletion has slowed or stopped. The United Nations
General Assembly has designated September 16 as the International Day for the Preservation
of the Ozone Layer. Venus also has a thin ozone layer at an altitude
of 100 kilometers from the planet’s surface.==Sources==
The photochemical mechanisms that give rise to the ozone layer were discovered by the
British physicist Sydney Chapman in 1930. Ozone in the Earth’s stratosphere is created
by ultraviolet light striking ordinary oxygen molecules containing two oxygen atoms (O2),
splitting them into individual oxygen atoms (atomic oxygen); the atomic oxygen then combines
with unbroken O2 to create ozone, O3. The ozone molecule is unstable (although, in the
stratosphere, long-lived) and when ultraviolet light hits ozone it splits into a molecule
of O2 and an individual atom of oxygen, a continuing process called the ozone-oxygen
cycle. Chemically, this can be described as: O2 + ℎνuv → 2 O
O + O2 ↔️ O3About 90 percent of the ozone in the atmosphere is contained in the stratosphere.
Ozone concentrations are greatest between about 20 and 40 kilometres (66,000 and 131,000
ft), where they range from about 2 to 8 parts per million. If all of the ozone were compressed
to the pressure of the air at sea level, it would be only 3 millimetres (1⁄8 inch) thick.==Ultraviolet light==Although the concentration of the ozone in
the ozone layer is very small, it is vitally important to life because it absorbs biologically
harmful ultraviolet (UV) radiation coming from the sun. Extremely short or vacuum UV
(10–100 nm) is screened out by nitrogen. UV radiation capable of penetrating nitrogen
is divided into three categories, based on its wavelength; these are referred to as UV-A
(400–315 nm), UV-B (315–280 nm), and UV-C (280–100 nm).
UV-C, which is very harmful to all living things, is entirely screened out by a combination
of dioxygen (about 200 nm) by around 35 kilometres (115,000 ft)
altitude. UV-B radiation can be harmful to the skin and is the main cause of sunburn;
excessive exposure can also cause cataracts, immune system suppression, and genetic damage,
resulting in problems such as skin cancer. The ozone layer (which absorbs from about
200 nm to 310 nm with a maximal absorption at about 250 nm) is very effective at screening
out UV-B; for radiation with a wavelength of 290 nm, the intensity at the top of the
atmosphere is 350 million times stronger than at the Earth’s surface. Nevertheless, some
UV-B, particularly at its longest wavelengths, reaches the surface, and is important for
the skin’s production of vitamin D. Ozone is transparent to most UV-A, so most
of this longer-wavelength UV radiation reaches the surface, and it constitutes most of the
UV reaching the Earth. This type of UV radiation is significantly less harmful to DNA, although
it may still potentially cause physical damage, premature aging of the skin, indirect genetic
damage, and skin cancer.==Distribution in the stratosphere==
The thickness of the ozone layer varies worldwide and is generally thinner near the equator
and thicker near the poles. Thickness refers to how much ozone is in a column over a given
area and varies from season to season. The reasons for these variations are due to atmospheric
circulation patterns and solar intensity. The majority of ozone is produced over the
tropics and is transported towards the poles by stratospheric wind patterns. In the northern
hemisphere these patterns, known as the Brewer-Dobson circulation, make the ozone layer thickest
in the spring and thinnest in the fall. When ozone in produced by solar UV radiation in
the tropics, it is done so by circulation lifting ozone-poor air out of the troposphere
and into the stratosphere where the sun photolyzes oxygen molecules and turns them into ozone.
Then, the ozone-rich air is carried to higher latitudes and drops into lower layers of the
atmosphere.Research has found that the ozone levels in the United States are highest in
the spring months of April and May and lowest in October. While the total amount of ozone
increases moving from the tropics to higher latitudes, the concentrations are greater
in high northern latitudes than in high southern latitudes, due to the ozone hole phenomenon.
The highest amounts of ozone are found over the Arctic during the spring months of March
and April, but the Antarctic has their lowest amounts of ozone during their summer months
of September and October,==Depletion==The ozone layer can be depleted by free radical
catalysts, including nitric oxide (NO), nitrous oxide (N2O), hydroxyl (OH), atomic chlorine
(Cl), and atomic bromine (Br). While there are natural sources for all of these species,
the concentrations of chlorine and bromine increased markedly in recent decades because
of the release of large quantities of man-made organohalogen compounds, especially chlorofluorocarbons
(CFCs) and bromofluorocarbons. These highly stable compounds are capable of surviving
the rise to the stratosphere, where Cl and Br radicals are liberated by the action of
ultraviolet light. Each radical is then free to initiate and catalyze a chain reaction
capable of breaking down over 100,000 ozone molecules. By 2009, nitrous oxide was the
largest ozone-depleting substance (ODS) emitted through human activities.
The breakdown of ozone in the stratosphere results in reduced absorption of ultraviolet
radiation. Consequently, unabsorbed and dangerous ultraviolet radiation is able to reach the
Earth’s surface at a higher intensity. Ozone levels have dropped by a worldwide average
of about 4 percent since the late 1970s. For approximately 5 percent of the Earth’s surface,
around the north and south poles, much larger seasonal declines have been seen, and are
described as “ozone holes”. The discovery of the annual depletion of ozone above the
Antarctic was first announced by Joe Farman, Brian Gardiner and Jonathan Shanklin, in a
paper which appeared in Nature on May 16, 1985.===Regulation===To support successful regulation attempts,
the ozone case was communicated to lay persons “with easy-to-understand bridging metaphors
derived from the popular culture” and related to “immediate risks with everyday relevance”.
The specific metaphors used in the discussion (ozone shield, ozone hole) proved quite useful
and, compared to global climate change, the ozone case was much more seen as a “hot issue”
and imminent risk. Lay people were cautious about a depletion of the ozone layer and the
risks of skin cancer. In 1978, the United States, Canada and Norway
enacted bans on CFC-containing aerosol sprays that damage the ozone layer. The European
Community rejected an analogous proposal to do the same. In the U.S., chlorofluorocarbons
continued to be used in other applications, such as refrigeration and industrial cleaning,
until after the discovery of the Antarctic ozone hole in 1985. After negotiation of an
international treaty (the Montreal Protocol), CFC production was capped at 1986 levels with
commitments to long-term reductions. This allowed for a ten-year phase-in for developing
countries (identified in Article 5 of the protocol). Since that time, the treaty was
amended to ban CFC production after 1995 in the developed countries, and later in developing
countries. Today, all of the world’s 197 countries have signed the treaty. Beginning January
1, 1996, only recycled and stockpiled CFCs were available for use in developed countries
like the US. This production phaseout was possible because of efforts to ensure that
there would be substitute chemicals and technologies for all ODS uses.On August 2, 2003, scientists
announced that the global depletion of the ozone layer may be slowing down because of
the international regulation of ozone-depleting substances. In a study organized by the American
Geophysical Union, three satellites and three ground stations confirmed that the upper-atmosphere
ozone-depletion rate slowed down significantly during the previous decade. Some breakdown
can be expected to continue because of ODSs used by nations which have not banned them,
and because of gases which are already in the stratosphere. Some ODSs, including CFCs,
have very long atmospheric lifetimes, ranging from 50 to over 100 years. It has been estimated
that the ozone layer will recover to 1980 levels near the middle of the 21st century.
A gradual trend toward “healing” was reported in 2016.Compounds containing C–H bonds (such
as hydrochlorofluorocarbons, or HCFCs) have been designed to replace CFCs in certain applications.
These replacement compounds are more reactive and less likely to survive long enough in
the atmosphere to reach the stratosphere where they could affect the ozone layer. While being
less damaging than CFCs, HCFCs can have a negative impact on the ozone layer, so they
are also being phased out. These in turn are being replaced by hydrofluorocarbons (HFCs)
and other compounds that do not destroy stratospheric ozone at all.
The residual effects of CFCs accumulating within the atmosphere lead to a concentration
gradient between the atmosphere and the ocean. This organohalogen compound is able to dissolve
into the ocean’s surface waters and is able to act as a time-dependent tracer. This tracer
helps scientists study ocean circulation by tracing biological, physical and chemical
pathways===
Implications for astronomy===As ozone in the atmosphere prevents most energetic
ultraviolet radiation reaching the surface of the Earth, astronomical data in these wavelengths
have to be gathered from satellites orbiting above the atmosphere and ozone layer. Most
of the light from young hot stars is in the ultraviolet and so study of these wavelengths
is important for studying the origins of galaxies. The Galaxy Evolution Explorer, GALEX, is an
orbiting ultraviolet space telescope launched on April 28, 2003, which operated until early
2012.==See also==United Nations Environment Programme

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