Q.32. What is ozone hole? (AKTU. - 2011 - 12)
Ans. Ozone
hole was first discovered by Dr. Joe E Farman and his associates in British
Antarctic Survey. Then thining of ozone layer or reduction in concentration of
ozone, especially over Antartica is a major environmental problem. The thinning
of ozone layer is also known as ozone hole, which covers approximately 7
million sq. km. The British Antartica revealed decrease in the ozone layer in
1980s. The satellite data indicate that at certain times of the year, levels of
ozone in the stratosphere over Antarctica have decreased by more than 5%. The
hole in the ozone layer occurs yearly in late September, which is the beginning
of the spring in the southern hemisphere. The hole is roughly the size of the
United States and has increased in size every year since discovery. The
antarctic ozone hole is an area of the Antarctic stratosphere in which the
recent ozone levels have dropped to as low as 33% of their pre-1975 values.
Q.33. Illustrate the
photochemical reactions involved in the formation and destruction of ozone in
the stratosphere. (AKTU. - 2008-09, 11 - 12, 12 - 13)
Related Question -
Q. How ozone layer
formed and depleted in nature? (AKTU. - 2010 - 11)
Ans. Formation of Ozone: -
In
the stratosphere, O3 is formed naturally when
oxygen is dissociated by ultraviolet solar radiations in the wave-length region
of 80 to 240 nm.
O2 + hn ® O + O
O + O2 + M ® O3 + M
where M is
any third body molecule (most likely N2 or
O2 in the atmosphere) that remains unchanged in
the reaction.
Destruction of Ozone: - (AKTU. - 2010 - 11)
The ultraviolet radiations in
the region of 200 to 300 nm can also dissociate the ozone :
O3 +
hv ® O2 + O
In this (above) reaction, ozone
portrays the absorption of ultraviolet-B radiations and hence is responsible
for the removal of UV-B radiations (l = 280 to 320 nm) that would otherwise
reach the earth’s surface.
The ozone layer in the
stratosphere in being depleted as the ozone reacts with chlorine, water vapours
and nitrogen oxides released by:
(a) High altitude aircrafts.
(b) Chlorofluorohydrocarbons
which are widely used as aerosol spray propellants, refrigerants, fire
extinguishers etc.
(c) Nitrons oxide produced by
the action of bacteria (which are present in significant amounts due to use of
nitrate fertilizers) in soil.
(d) Huge buildup of gases and
chemicals emitted by industrial plants and automobiles.
The ozone layer over the
Antarctic has steadily weakned since measurements started in the early 1980s.
THe lans area under the ozone-depleted atmosphere has increased steadily to
more than 20 million sq km in the early 1990s and has varied between 20 to 29
million sq km since then. In 2000, the area of ozone hole reached a record 29
million sq km.
Q.34. How does ozone layer protect us. (AKTU. - 2012 - 13)
Ans. 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 would be 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 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
type of longer wavelength reaches the surface, and becomes the primary type of
UV from the Sun to do so. However, this type of UV radiation is significantly
less harmful to DNA, although it may still potentially cause indirect genetic
damage in skin.
