Causes of zone depletions

Causes of zone depletions on earth

1. Causes of Zone Depletions:
Manufactured compounds are also capable of altering atmospheric ozone levels. Chlorine,
released from CFCs, (15k JPG) and bromine (Br), released from halons, are two of the most
important chemicals associated with ozone depletion. Halons are primarily used in fire
extinguishers. CFCs are used extensively in aerosols, air conditioners, refrigerators, and cleaning
solvents. Two major types of CFCs are trichlorofluorocarbon (CFC13), or CFC-11,and
dichlorodifluoromethane (CF2Cl2), or CFC-12. Trichlorofluorocarbon is used in aerosols, while
dichlorodifluoromethane is typically used as a coolant.
CFCs were originally created to provide a substitute for toxic refrigerant gases and to reduce the
occupational hazard of compressor explosions. Near Earth's surface, chloroflourocarbons are
relatively harmless and do not react with any material, including human skin. For 50 years they
appeared to be the perfect example of a benign technical solution to environmental and
engineering problems, with no negative side effects. While CFCs remain in the troposphere, they
are virtually indestructible. They are not water soluble and cannot even be washed out of the
atmosphere by rain. We now understand that the very quality that made them seem so safe –their
stability – is what makes them so dangerous to the Earth system. CFCs remain in the troposphere
for more than 40 years before their slow migration to the stratosphere is complete. Even if we
were to end their production and use at this very moment, they will continue to contribute to
ozone destruction far into the future.
In the stratosphere, high energy UV radiation causes the CFC molecules to break down through
photodissociation. Atomic chlorine, a true catalyst for ozone destruction, is released in the
process. Chlorine initiates and takes part in a series of ozone-destroying chemical reactions and
emerges from the process unchanged. The free chlorine atom initially reacts with an unstable
oxygen containing compound (such as ozone) to form chlorine monoxide (ClO):

Cl + O3 → ClO + O2
The ClO molecule then reacts with atomic oxygen to produce molecular oxygen (O2) and more
atomic chlorine. The regenerated Cl atom is then free to initiate a new cycle.
ClO + O → Cl + O2

2. As shown in figures ,



This destructive chain of reactions will continue over and over again, limited only by the amount
of chlorine available to fuel the process.

3. Chlorine occurs naturally in the oceans. However, the majority of chlorine in the atmosphere has
originated with man-made chemicals. Without the dissociation of manufactured
chlorofluorocarbons, there would be almost no chlorine in the stratosphere. CFC-12
concentrations were less than 100 parts per trillion by volume when they were first measured in
the 1960s. Between 1975 and 1987, concentrations more than doubled from less than 200 parts
per trillion by volume to more than 400 parts per trillion by volume. The amount of chlorine in
the stratosphere also increased by a factor of 2 to 3. Scientists believe that continued buildup of
CFCs could lead to severe ozone loss (61k JPG) worldwide. Thus, ongoing studies are essential
to provide a necessary understanding of the causes of ozone depletion.The history of CFCs
demonstrates that human activities can have unexpected long-term effects on the environment.
Effects of Ozone Depletion:
Increased UV:
Ozone, while a minority constituent in Earth's atmosphere, is responsible for most of the
absorption of UVB radiation. The amount of UVB radiation that penetrates through the ozone
layer decreases exponentially with the slant-path thickness and density of the layer. When
stratospheric ozone levels decrease, higher levels of UVB reach the Earth’s surface. UV-driven
phenolic formation in tree rings has dated the start of ozone depletion in northern latitudes to the
late 1700s.
In October 2008, the Ecuadorian Space Agency published a report called HIPERION, a study of
the last 28 years data from 10 satellites and dozens of ground instruments around the world
among them their own, and found that the UV radiation reaching equatorial latitudes was far
greater than expected, with the UV Index climbing as high as 24 in some very populated cities;
the WHO considers 11 as an extreme index and a great risk to health. The report concluded that
depleted ozone levels around the mid-latitudes of the planet are already endangering large
populations in these areas. Later, the CONIDA, the Peruvian Space Agency, published its own
study, which yielded almost the same findings as the Ecuadorian study.
Increased production of vitamin D:
Vitamin D is produced in the skin by ultraviolet light. Thus, higher UVB 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.

4. 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 affect 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 magnify all of the effects of UV on human health, both positive
(including production of Vitamin D) and negative (including sunburn, skin cancer, and
cataracts). In addition, increased surface UV leads to increased tropospheric ozone, which is a
health risk to humans.
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 fully understood, but it appears that both
UVB and UVA are involved. 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 UVB
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 nonmelanoma skin cancer over a period of seven years, along
with decreased ozone and increased UVB levels.
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 UVB, therefore plant growth can be
directly affected by UVB radiation.
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 "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

5. observed skin damage, in the same way that human skin cancer rates have been on the increase in
recent decades.
Overview of Ozone Depletion on Environment: