WORLD JOURNAL OF PHARMACY AND PHARMACEUTICAL SCIENCES
Dhrubo et al.
World Journal of Pharmacy and Pharmaceutical Sciences
SJIF Impact Factor 2.786
Volume 3, Issue 10, 1331-1341.
Research Article
ISSN 2278 – 4357
TALE OF OXYGEN ATOM BY DIATOMIC OXYGEN GAS (O2) ON
THE EARTH AND TRIATOMIC OZONE GAS (O3) ABOVE THE
EARTH
Hardik H. Chaudhary, Limbesh B. Aal and Dr. Dhrubo Jyoti Sen
Department of Quality Assurance & Pharmaceutical Chemistry, Shri Sarvajanik Pharmacy
College, Gujarat Technological University, Arvind Baug, Mehsana-384001, Gujarat, India.
Article Received on
26 July 2014,
Revised on 16 August 2014,
Accepted on 27 September
2014
ABSTRACT
The photochemical mechanisms of Ozone in the Earth's stratosphere is
created by ultraviolet light striking oxygen molecules containing two
oxygen atoms (O2), splitting them into individual oxygen atoms
(atomic oxygen); the atomic oxygen then combines with unbroken O 2
*Correspondence for
Author
Dr. Dhrubo Jyoti Sen
to create ozone, O3. The ozone molecule is unstable (although, in the
stratosphere, long-lived) and when ultraviolet light hits ozone it splits
Department of Quality
into a molecule of O2 and an atom of atomic oxygen, a continuing
Assurance & Pharmaceutical
process called the ozone-oxygen cycle. Chemically, this can be
Chemistry, Shri Sarvajanik
described as
Pharmacy College, Gujarat
Technological University,
O2 + ℎν (UV) → 2O
Arvind Baug, Mehsana-
O + O2 ↔ O3
384001, Gujarat, India
Diatomic Oxygen
Triatomic Ozone
About 90% of the ozone in our 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 2-8 parts per million. If all of the ozone were compressed to the
pressure of the air at sea level, it would be only 3 millimeters thick. 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
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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 (< 200 nm) and ozone (> 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 depression, 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. 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.
Keywords: UV-A, UV-B, UV-C, Oxygen, Ozone, Ozone hole, CFC, Stratosphere.
INTRODUCTION
Three forms (or allotropes) of oxygen are involved in the ozone-oxygen cycle: oxygen atoms
(O or atomic oxygen), oxygen gas (O2 or diatomic oxygen), and ozone gas (O3 or triatomic
oxygen). Ozone is formed in the stratosphere when oxygen molecules photo dissociate after
intaking an ultraviolet photon whose wavelength is shorter than 240 nm. This converts a
single O2 into two atomic oxygen radicals. The atomic oxygen radicals then combine with
separate O2 molecules to create two O3 molecules. These ozone molecules absorb UV light
between 310 and 200 nm, following which ozone splits into a molecule of O2 and an oxygen
atom. The oxygen atom then joins up with an oxygen molecule to regenerate ozone. [1] This is
a continuing process that terminates when an oxygen atom "recombines" with an ozone
molecule to make two O2 molecules. 2 O3 → 3 O2
The overall amount of ozone in the stratosphere is determined by a balance between
photochemical production and recombination. Ozone can be destroyed by a number of free
radical catalysts, the most important of which are the hydroxyl radical (OH·),nitric
oxide radical (NO·), chlorine atom (Cl·) and bromine atom (Br·). The dot is a common
notation to indicate that all of these species have an unpaired electron and are thus extremely
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reactive. All of these have both natural and man-made sources; at the present time, most of
the OH· and NO· in the stratosphere is of natural origin, but human activity has dramatically
increased the levels of chlorine and bromine. These elements are found in certain stable
organic compounds, especially chlorofluorocarbons (CFCs), which may find their way to the
stratosphere without being destroyed in the troposphere due to their low reactivity. [2]
Once in the stratosphere, the Cl and Br atoms are liberated from the parent compounds by the
action of ultraviolet light, e.g.
CFCl3 + electromagnetic radiation → Cl· + ·CFCl2
Figure-1: Protection of earth by UV
The Cl and Br atoms can then destroy ozone molecules through a variety of catalytic cycles.
In the simplest example of such a cycle, a chlorine atom reacts with an ozone molecule,
taking an oxygen atom with it (forming ClO) and leaving a normal oxygen molecule. The
chlorine monoxide (i.e., the ClO) can react with a second molecule of ozone (i.e., O3) to yield
another chlorine atom and two molecules of oxygen. The chemical shorthand for these gasphase reactions is:
 Cl· + O3 → ClO + O2: The chlorine atom changes an ozone molecule to ordinary oxygen
 ClO + O3 → Cl· + 2 O2: The ClO from the previous reaction destroys a second ozone
molecule and recreates the original chlorine atom, which can repeat the first reaction and
continue to destroy ozone. [3]
The overall effect is a decrease in the amount of ozone, though the rate of these processes can
be decreased by the effects of null cycles. More complicated mechanisms have been
discovered that lead to ozone destruction in the lower stratosphere as well. A single chlorine
atom would keep on destroying ozone (thus a catalyst) for up to two years (the time scale for
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transport back down to the troposphere) were it not for reactions that remove them from this
cycle by forming reservoir species such as hydrogen chloride (HCl) and chlorine nitrate
(ClONO2). On a per atom basis, bromine is even more efficient than chlorine at destroying
ozone, but there is much less bromine in the atmosphere at present. As a result, both chlorine
and bromine contribute significantly to overall ozone depletion.
[4]
Laboratory studies have
shown that fluorine and iodine atoms participate in analogous catalytic cycles. However, in
the Earth's stratosphere, fluorine atoms react rapidly with water and methane to form strongly
bound HF, while organic molecules containing iodine react so rapidly in the lower
atmosphere that they do not reach the stratosphere in significant quantities. On average, a
single chlorine atom is able to react with 100,000 ozone molecules before it is removed from
the catalytic cycle. This fact plus the amount of chlorine released into the atmosphere yearly
by chlorofluorocarbons (CFCs) and hydrofluorocarbons (HCFCs) demonstrates how
dangerous CFCs and HCFCs are to the environment. [5]
Chemicals causing hazards to the atmosphere
CFCs and related compounds in the atmosphere
Chlorofluorocarbons (CFCs) and other halogenated ozone depleting substances (ODS) are
mainly responsible for man-made chemical ozone depletion. The total amount of effective
halogens (chlorine and bromine) in the stratosphere can be calculated and are known as
the equivalent effective stratospheric chlorine (EESC). CFCs were invented by Thomas
Midgley, Jr. in the 1920s. They were used in air conditioning and cooling units, as aerosol
spray propellants prior to the 1970s, and in the cleaning processes of delicate electronic
equipment. They also occur as by-products of some chemical processes. No significant
natural sources have ever been identified for these compounds—their presence in the
atmosphere is due almost entirely to human manufacture. As mentioned above, when such
ozone-depleting chemicals reach the stratosphere, they are dissociated by ultraviolet light to
release chlorine atoms. The chlorine atoms act as a catalyst, and each can break down tens of
thousands of ozone molecules before being removed from the stratosphere. Given the
longevity of CFC molecules, recovery times are measured in decades. It is calculated that a
CFC molecule takes an average of about five to seven years to go from the ground level up to
the upper atmosphere, and it can stay there for about a century, destroying up to one hundred
thousand ozone molecules during that time. 1,1,1-Trichloro-2,2,2-trifluoroethane, also known
as CFC-113a, is one of four man-made chemicals newly discovered in the atmosphere by a
team at the University of East Anglia. CFC-113a is the only known CFC whose abundance in
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the atmosphere is still growing. Its source remains a mystery, but illegal manufacturing is
suspected by some. CFC-113a seems to have been accumulating unabated since 1960.
Between 2010 and 2012, emissions of the gas jumped by 45%.[6]
Computer modeling
Scientists have been increasingly able to attribute the observed ozone depletion to the
increase of man-made (anthropogenic) halogen compounds from CFCs by the use of complex
chemistry transport models and their validation against observational data. These models
work by combining satellite measurements of chemical concentrations and meteorological
fields with chemical reaction rate constants obtained in lab experiments. They are able to
identify not only the key chemical reactions but also the transport processes that bring
CFC photolysis products into contact with ozone.
Since the ozone layer absorbs UV-B ultraviolet light from the sun, ozone layer depletion is
expected to increase surface UV-B levels, which could lead to damage, including increase in
skin cancer. This was the reason for the Montreal Protocol. Although decreases in
stratospheric ozone are well-tied to CFCs and there are good theoretical reasons to believe
that decreases in ozone will lead to increases in surface UV-B, there is no direct observational
evidence linking ozone depletion to higher incidence of skin cancer and eye damage in
human beings. This is partly because UV-A, which has also been implicated in some forms of
skin cancer, is not absorbed by ozone, and it is nearly impossible to control statistics for
lifestyle changes in the populace. [7]
Figure-2: Protective ozone layer
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Increased UV
Ozone, while a minority constituent in Earth's atmosphere, is responsible for most of the
absorption of UV-B radiation. The amount of UV-B radiation that penetrates through the
ozone layer decreases exponentially with the slant-path thickness and density of the layer.
When stratospheric ozone levels begin depleting, higher levels of UV-B reaching the Earth’s
surface will become more frequent. This means that the less ozone there is, the less protection
there will be, hence more UV-B reaches the Earth. Correspondingly, a decrease in
atmospheric ozone is expected to give rise to significantly increased levels of UV-B near the
surface. Ozone-driven phenolic formation in tree rings has dated the start of ozone depletion
in northern latitudes to the late 1700s. Increases in surface UV-B due to the ozone hole can be
partially inferred by radiative transfer model calculations, but cannot be calculated from
direct measurements because of the lack of reliable historical (pre-ozone-hole) surface UV
data, although more recent surface UV observation measurement programs exist (e.g. at
Lauder, New Zealand). UV-215 and more energetic radiation is responsible for creation
ozone in the ozone layer from O2 (regular oxygen). Less energetic radiation, UV-215 through
UV-280, is only able to dissociate the single oxygen bond of ozone. Therefore, as a result of
reduction in stratospheric ozone, the amount of this radiation reaching the surface increases.
This less energetic radiation is however powerful enough to disrupt DNA bonding. [8]
Biological effects
The main public concern regarding the ozone hole has been the effects of increased surface
UV radiation on human health. So far, ozone depletion in most locations has been typically a
few percent and, as noted above, no direct evidence of health damage is available in most
latitudes. Were the high levels of depletion seen in the ozone hole ever to be common across
the globe, the effects could be substantially more dramatic. As the ozone hole over Antarctica
has in some instances grown so large as to reach southern parts of Australia, New
Zealand, Chile, Argentina, and South Africa, environmentalists have been concerned that the
increase in surface UV could be significant. Ozone depletion would change all of the effects
of UV on human health, both positive and negative. UV-B (the higher energy UV radiation
absorbed by ozone) is generally accepted to be a contributory factor to skin cancer and to
produce Vitamin D. In addition, increased surface UV leads to increased tropospheric ozone,
which is a health risk to humans. [9]
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Basal and squamous cell carcinomas
The most common forms of skin cancer in humans, basal and squamous cell carcinomas have
been strongly linked to UV-B exposure. The mechanism by which UV-B induces these
cancers is well understood—absorption of UV-B radiation causes the pyrimidine bases in the
DNA molecule to form dimers, resulting in transcription errors when the DNA replicates.
These cancers are relatively mild and rarely fatal, although the treatment of squamous cell
carcinoma
sometimes
requires
extensive
reconstructive
surgery.
By
combining
epidemiological data with results of animal studies, scientists have estimated that a one
percent decrease in stratospheric ozone would increase the incidence of these cancers by
2%.[10]
Malignant melanoma
Another form of skin cancer, malignant melanoma, is much less common but far more
dangerous, being lethal in about 15–20% of the cases diagnosed. The relationship between
malignant melanoma and ultraviolet exposure is not yet well understood, but it appears that
both UV-B and UV-A are involved. Experiments on fish suggest that 90 to 95% of malignant
melanomas may be due to UV-A and visible radiation whereas experiments on opossums
suggest a larger role for UV-B. Because of this uncertainty, it is difficult to estimate the
impact of ozone depletion on melanoma incidence. One study showed that a 10% increase in
UV-B radiation was associated with a 19% increase in melanomas for men and 16% for
women. A study of people in Punta Arenas, at the southern tip of Chile, showed a 56%
increase in melanoma and a 46% increase in non-melanoma skin cancer over a period of
seven years, along with decreased ozone and increased UV-B levels. [11]
Cortical cataracts
Studies are suggestive of an association between ocular cortical cataracts and UV-B
exposure, using crude approximations of exposure and various cataract assessment
techniques. A detailed assessment of ocular exposure to UV-B was carried out in a study on
Chesapeake Bay Watermen, where increases in average annual ocular exposure were
associated with increasing risk of cortical opacity. In this highly exposed group of
predominantly white males, the evidence linking cortical opacities to sunlight exposure was
the strongest to date. However, subsequent data from a population-based study in Beaver
Dam, WI suggested the risk may be confined to men. In the Beaver Dam study, the exposures
among women were lower than exposures among men, and no association was
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seen. Moreover, there were no data linking sunlight exposure to risk of cataract in African
Americans, although other eye diseases have different prevalences among the different racial
groups, and cortical opacity appears to be higher in African Americans compared with
whites. [12]
Increased tropospheric ozone
Increased surface UV leads to increased tropospheric ozone. Ground-level ozone is generally
recognized to be a health risk, as ozone is toxic due to its strong oxidant properties. The risks
are particularly high for young children, the elderly, and those with asthma or other
respiratory difficulties. At this time, ozone at ground level is produced mainly by the action
of UV radiation on combustion gases from vehicle exhausts. [13]
Increased production of vitamin D
Vitamin D is produced in the skin by ultraviolet light. Thus, higher UV-B exposure raises
human vitamin D in those deficient in it. Recent research (primarily since the Montreal
protocol), shows that many humans have less than optimal vitamin D levels. In particular, in
the U.S. population, the lowest quarter of vitamin D (<17.8 ng/ml) were found using
information from the National Health and Nutrition Examination Survey to be associated
with an increase in all cause mortality in the general population. While blood level of
Vitamin D in excess of 100 ng/ml appear to raise blood calcium excessively and to be
associated with higher mortality, the body has mechanisms that prevent sunlight from
producing Vitamin D in excess of the body's requirements. [14]
Figure-3: Ozone hole
Effects on non-human animals
A November 2010 report by scientists at the Institute of Zoology in London found that
whales off the coast of California have shown a sharp rise in sun damage, and these scientists
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"fear that the thinning ozone layer is to blame". The study photographed and took skin
biopsies from over 150 whales in the Gulf of California and found "widespread evidence of
epidermal damage commonly associated with acute and severe sunburn", having cells that
form when the DNA is damaged by UV radiation. The findings suggest "rising UV levels as a
result of ozone depletion are to blame for the observed skin damage, in the same way that
human skin cancer rates have been on the increase in recent decades." [15]
Effects on crops
An increase of UV radiation would be expected to affect crops. A number of economically
important species of plants, such as rice, depend on cyanobacteria residing on their roots for
the retention of nitrogen. Cyanobacteria are sensitive to UV radiation and would be affected
by its increase." Despite mechanisms to reduce or repair the effects of increased ultraviolet
radiation, plants have a limited ability to adapt to increased levels of UV-B, therefore plant
growth can be directly affected by UV-B radiation. [16]
Figure-4: Ozone layer depletion causing hazards
CONCLUSION
The ozone layer is a natural part of our atmosphere. It acts like a protective sun screen by
blocking harmful ultraviolet (UV) rays from the sun. Certain chemicals—such as
chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs), and halons—can damage
the ozone layer. These can cause holes to form in the layer and allow more UV rays to reach
the earth’s surface. Increased exposure to UV rays is dangerous for people, animals, and
plants. Chemicals that damage the ozone layer, known as ozone-depleting substances (ODS),
are used in:
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 commercial, home and vehicle air conditioners, and refrigerators,
 foam blowing agents,
 solvents,
 aerosol spray propellants,
 fire extinguishing agents, and
 chemical reactants.
To protect the ozone layer from these chemicals, we must also prevent the release of ozonedepleting substances to the atmosphere. Whenever possible, we must also replace them with
safer alternatives.
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