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Ozone
and its Effect
Dr. S.M. Alam
Ozone is the most reactive form of oxygen and its molecular
consist of three atoms of oxygen. It is an
unstable, pale bluish gas with a characteristic fresh, penetrating
odour. Among the strongest oxidizing agents known, it is found
in the atmosphere in minute quantities. It is produced continuously
in the outer layers
of the atmosphere by the action of solar ultraviolet radiation
on the oxygen. Ozone is produced in the
laboratory by passing dry air between two plate electrodes
connected to an alternating current of several
thousand volts. It is usedcommercially as a disinfectant for
air and water and as a bleaching agent for waxes, oils and
other organic compounds. In the atmosphere, like an infection
that grows more and more virulent, the continent-size hole
in Earth's ozone layer keeps getting bigger and bigger. Each
year since the late 1970s,
much of the protective layer of stratospheric ozone above
Antarctica has disappeared, creating what is
popularly known as the ozone hole. The Antarctic hole now
measures about 9 million square miles, nearly the size of
North America. Less dramatic, but still significant, depletion
of ozone levels has been recorded
around the globe. With less ozone in the atmosphere, more
ultraviolet radiation strikes Earth,
causing more skin cancer, eye damage, and possible harm to
crops
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Ozone
has strong oxidizing properties and causes injury and premature
mortality of plant tissues. This element inhibited photosynthesis,
respiration and nutrient uptake and subsequently may lead to
reduced yields of agricultural crops. Several studies have shown
that the root: shoot, root ratio decreases with increasing ozone
uptake in the plant and that plants trend to compensate for
the adverse effects of some ozone exposure by increasing dry
matter partitioning to leaf blades. Studies of Pakistan rice
cultivars, on the other hand, have found substantially greater
ozone impacts on rice yields. Mean concentration levels of 40-43
ppb during the study periods were accompanied by a yield reduction
of as much as 37-57 %, depending on experimental design. The
studies in the US and in Europe have shown that sensitivity
to ozone varies greatly between species and may also vary considerably
between different cultivars. Others environmental factors, especially
the humidity of soil and air, influence sensitivity to ozone.
According to an estimate exposure of plant to ozone levels of
0.05 to 0.07 parts per million in the Troposphere for seven
hours per day during one season, has caused crop loss amounting
to 3.1 billion U.S. dollars in the United States alone. Rise
in temperature will cause damage to the crop production, because
the growing of food grain crops mainly depend on temperature.
Thus, increase in temperature of the planet has the potential
of changing the overall pattern of food production globally.
Results have also shown that concentration of carbon monoxide
varied from 70 parts per billion over America to a high level
of 115 parts per billion over the Eastern Atlantic ocean and
yet still higher level of 140 parts per billion over the Mediterranean.
The 1985 report of the discovery of an "ozone hole"
over Antarctica focused attention on the idea that humans can
have a significant impact on the global environment. This discovery,
along with evidence that ozone is being lost at nearly all latitudes
outside the tropics, has prompted much research into the causes
of ozone depletion and the biological effects of increased ultraviolet
radiation exposure. The potential effects on humans and the
environment have led to international resolutions designed to
gradually phase out production of ozone-depleting substances.
As a result, the scientific and industrial communities have
collaborated to find safe and economical replacements for chlorofluorocarbons
(CFCs), the man made chemicals implicated in ozone loss.
Ozone is a molecule composed of three oxygen atoms, designated
by the chemical symbol O3. Although ozone is found in small
amounts at all altitudes in the atmosphere, due to chemical,
dynamical, and radiative processes, it is not evenly distributed.
Approximately 90 per cent of all ozone is contained in the region
of the atmosphere known as the stratosphere, which lies between
15 and 50 km above the Earth's surface. The region below the
stratosphere where our weather takes place is known as the troposphere.
The diagram "Vertical Temperature Structure of the Earth's
Atmosphere" shows the different layers of the atmosphere
and indicates that these layers are defined by whether the temperature
is increasing or decreasing with height. The region of the stratosphere
that contains higher concentrations of ozone is generally referred
to as the ozone layer. The history of ozone layer research dates
back to 1930, when the first theory of how the ozone layer is
formed was presented. Reports give a brief synopsis of important
discoveries and events leading to an understanding of stratospheric
ozone and the discovery of destructive capabilities of chlorofluorocarbons
(CFCs) on ozone.
Ozone is always present in the earth atmosphere in trace quantities.
The role of high concentration of ozone in the Stratosphere
is vital in as much as it acts as a very effective filter for
absorbing ultra violet rays which are an integral component
of sunlight. In Stratosphere, it acts as a shielding layer to
protect us from the effect of ultraviolet rays, whereas on earth
surface it acts as our enemy. The ultraviolet rays falling on
the surface of the earth resulting in very serious health hazard
human beings in the form of skin cancer. At levels as low as
10 ppm, it causes headaches, eye irritation coughing and chest
discomfort. Despite its low concentration, ozone plays a critical
role in chemical and biological processes by filtering ultraviolet
radiation in the 220-320 nm wavelength range. The region of
concern for biological effects is the ultraviolet-B (UV-B) range
from 280 to 320 nm. The effectiveness of ozone absorption decreases
exponentially as the wavelength of radiation increases. All
radiation consisting of wavelengths shorter than 280 nm is absorbed
in the upper atmosphere; wavelengths longer than 320 nm are
not significantly absorbed by ozone. Therefore, biological systems
are vulnerable to wavelengths in the transitional region of
280 to 320 nm due to ozone losses. Lower ozone amounts result
in greater amounts of UV-B reaching the surface, which can lead
to damaging effects on humans, plants, and animals. Thus, ozone
located in the stratosphere is crucial to life on Earth, but
ironically, ozone found at the surface of the Earth can be harmful
to humans, plants, and animals. For example, high ozone amounts
at ground level are known to cause respiratory problems in humans
and can lower yields of certain crops. The location of ozone
defines whether ozone is beneficial or harmful to humans and
the environment.
Natural variations in ozone do occur, but recent levels of ozone
loss over the poles and lower latitudes cannot be explained
by natural variability alone. Manmade CFC compounds were developed
in the early 1930s for a variety of industrial and commercial
applications, but it was not until the 1970s that these and
other chlorine-containing substances were suspected of having
the potential to destroy atmospheric ozone. In 1985 a team of
British researchers first reported unusually low ozone levels
over Halley Bay, Antarctica, which were caused by chemical reactions
with chlorine and nitrogen compounds. Research was initiated
that found CFCs to be largely responsible for the anomalously
low levels during the polar springtime. This polar ozone depletion
at lower stratospheric altitudes is what has been termed the
"ozone hole". The primary concern over ozone depletion
is the potential impacts on human health and ecosystems due
to increased UV exposure. Increases in skin cancer and cataracts
in human populations are expected in a higher UV environment.
Lower yields of certain cash crops may result due to increased
UV-B stress. Higher UV-B levels in the upper ocean layer may
inhibit phytoplankton activities, which can impact the entire
marine ecosystem. In addition to direct biological consequences,
indirect effects may arise through changes in atmospheric chemistry.
Increased UV-B will alter photochemical reaction rates in the
lower atmosphere that are important in the production of surface
layer ozone and urban smog.
Concern over these potential effects has prompted the international
community to enact policies aimed at reducing the production
of ozone-depleting chemicals. An important event in the history
of international ozone policy was the Montreal Protocol on substances
that deplete the ozone layer, which called for the phase out
and reduction of certain substances over a multiyear time frame.
Discoveries of more extensive ozone loss and rapid formulation
of replacement substances for chlorine-containing compounds
have led to refinements of the original Protocol. Updates set
forth at London and Copenhagen have called for accelerated phase
out and replacement schedules.
Chlorofluorocarbons (CFCs), along with other chlorine- and bromine-containing
compounds, have been implicated in the accelerated depletion
of ozone in the Earth's stratosphere. CFCs were developed in
the early 1930s and are used in a variety of industrial, commercial,
and household applications. These substances are non-toxic,
non-flammable, and non-reactive with other chemical compounds.
These desirable safety characteristics, along with their stable
thermodynamic properties, make them ideal for many applications--as
coolants for commercial and home refrigeration units, aerosol
propellants, electronic cleaning solvents, and blowing agents.
Production and use of chlorofluorocarbons experienced nearly
uninterrupted growth as demand for products requiring their
use continued to rise.
Not until 1973 was chlorine found to be a catalytic agent in
ozone destruction. Catalytic destruction of ozone removes the
odd oxygen species [atomic oxygen (O) and ozone (O3)], while
leaving chlorine unaffected. This process was known to be potentially
damaging to the ozone layer, but conclusive evidence of stratospheric
ozone loss was not discovered until 1984. Announcement of polar
ozone depletion over Antarctica in March 1985 prompted scientific
initiatives to discover the ozone depletion processes, along
with calls to freeze or diminish production of chlorinated fluorocarbons.
A complex scenario of atmospheric dynamics, solar radiation,
and chemical reactions was found to explain the anomolously
low levels of ozone during the polar springtime. Recent expeditions
to the Arctic regions show that similar processes can occur
in the northern hemisphere, but to a somewhat lesser degree
due to warmer temperatures and erratic dynamic patterns.
A primary objective for researchers in addressing this issue
has been analysis of measurements and trends in ozone and chlorofluorocarbon
levels. Global monitoring of ozone levels from space by the
Total Ozone Mapping Spectrometer (TOMS) instrument has shown
statistically significant downward trends in ozone at all latitudes
outside the tropics. Measurements at several ground-based stations
have shown corresponding upward trends in CFCs in both the northern
and southern hemisphere. Despite rapid phase out of CFCs, ozone
levels are expected to be lower than pre-depletion levels for
several decades due to the long tropospheric lifetimes of CFCs.
These compounds are carried into the stratosphere, where they
can undergo hundreds of catalytic cycles involving ozone before
being scavenged by other chemical species. Replacement compounds
for CFCs have also been evaluated for their Ozone Depletion
Potential (ODP). Hydrochlorofluorocarbons (HCFCs) still contain
chlorine atoms, but the presence of hydrogen makes them reactive
with chemical species in the troposphere. This greatly reduces
the prospects of the chlorine reaching the stratosphere, as
chlorine will be removed by chemical processes in the lower
atmosphere. Hydrofluorocarbons (HFCs), potential replacements
for CFCs that contain no chlorine, have been evaluated for potential
effects of fluorine compounds on ozone destruction.
Possible increases in surface ozone may be a cause of concern
regarding y the future prospects for world agricultural production.
The possible impact of global climate change on the world’s
food production is gaining increased attention, and various
attempts have been made to estimate the changes in agricultural
production that could result from climate changes in different
parts of the world. Thus, it has recently been estimated that
climate change may increase the yield of cereals (rice, wheat
and corn) by a few per cent in the nest 50 years in the world. |
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