The upper layer of the atmosphere surrounded by ozone (15 to 30 Kms) is known as ozonosphere. Ozone layer is a protective stratospheric layer, also known as ozone umbrella.
Ozone concentration differs by about 10 ppm in stratosphere compared to 0.05 ppm in troposphere. It acts as a protective shield for the man, animals and plants by filtering out the lethal ultraviolet rays from reaching the earth’s surface.
If the ozone layer is not there, the harmful ultraviolet rays will reach to the lower atmosphere and its temperature will rise to that extent that the biological furnace of the biosphere will turn into a blast furnace.
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Formation of Ozone
The atmospheric oxygen absorbs UV radiation in the lower mesosphere at < 240nm and Photo dissociates into two oxygen atoms.
One of these oxygen atoms combines with 02 of upper stratosphere forming 03. Ozone is also capable of absorbing short wavelength UV radiations releasing oxygen atom.
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Reactions: 02 + hv {X < 240 nm) O + 02 + M 03 + hv (A, > 230-320 nm) O* + o,
Ozone Hole or Ozone Layer Depletion
The latest satellite measurement indicates an ozone loss at the rate of 8% just above the South Pole, at the centre of the ozone hole, 5% of the protective gas is depleting each day.
In 1980, a hole in the ozone umbrella was found out by a scientist named Chubachi Shigerui of Meteorological Research Institute of Japan. By the year 1985, this hole reached to the size of the American continent.
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Factors Affecting 03 Layer Depletion
The main cause of 03 layer depletion is the presence of chlorine containing gases (primarily CFCS and related hydrocarbons).
Due to the presence of UV light, these gases dissociate and release chlorine atom, which then leads to O3 destruction. The chlorine ion catalyzing O3 depletion can take place in the gaseous phase, but it is dramatically enhanced in the presence of polar stratospheric clouds.
During extreme cold in winters polar stratospheric clouds (Pscs) are formed. These clouds reside in vortex formed at poles since the polar winters are without solar radiation for 3 months, they are dark and cool (temp, around or below – 80°C).
Chlorine molecules are found in these PSC but are attached with some elements such as hydrogen and form HC1 and N03 to form C10N02.
During spring when the clouds start disappearing these Cl-compounds are dissociated and free radical of chlorine is released which attach on O3-layer to break it in 0 ; molecule/atom. Thus polar clouds act as reservoir for these chlorine gases.
Ozone hole is measured as the reduction in the total column of ozone, above a particular point on the earth’s surface which is normally expressed in Dobson unit.
Effect of Ozone Depletion
(a) Affects on Human Beings:
UV radiations cause irritation and develop red patches on skin. The prolonged cumulative exposure to UV-B rays causes skin-cancer, basal cell carcinoma, squamous cell carcinoma and malignant melanoma etc.
UV radiations cause blood vessels near the skin surface to carry more blood causing sun tan and skin ageing. It also gives birth to diseases like dizziness, visual impairment, changes in nucleic acid DNA and RNA etc.
Ozone-Depleting Substances, the Ozone Layer and UV Radiation: Past, Present and Future.
(a) Production of ozone-depleting substances (ODSs) before and after the 1987 Montreal Protocol and its Amendments from baseline scenario A1. Chloro- fluorocarbons (CFCs) are shown in black; additional ODSs from hydrochlorofluoro- carbons (HCFCs) are in gray.
Note: HCFCs which have been used as CFC replacements under the Protocol, lead to less ozone, destruction than CFCs.
(b) Combined effective abundances of ozone-depleting chlorine and bromine in the stratosphere. The range reflects uncertainties due to the lag time between emission at the surface and the stratosphere, as well as different hypothetical ODS emission scenarios.
(c) Total global ozone change (outside of the polar regions; 60°S60°N). Seasonal, quasibiennial oscillation (QBO), volcanic and solar effects have been removed. The black line shows measurements.
The gray region broadly represents the evolution of ozone predicted by models that encompass the range of future potential climate conditions. Pre-1980 values, to the left of the vertical dashed line, are often used as a benchmark for ozone and UV recover.
(d) Estimated change in UV erythemal (“sunburning”) irradiance for high sun. The gray areas show the calculated response to the ozone changes shown in (c). The hatched areas show rough estimates of what might occur due to climate-related changes in clouds and atmospheric fine particles (aerosols).
(b) Affect on Biotic Community:
Micro-phytoplankton, phytoplankton would die and that would in turn affect the zooplankton. Mortality rate of larvae of zooplanktons will increase.
(c) Affect on Plants:
Plants are sensitive to UV rays below 300 nm so they are badly affected. It causes lesions to plants usually confined to upper surfaces.
About 20-50% of chlorophyll reduces and mutations take place which harms plant proteins by changing their nature. In plants 03 enters through stomata, it causes damage to leaves reducing there photosynthetic rate.
Climatic Affects of Ozone Depletion
Ozone reduction in troposphere drastically changes the weather elements like temperature, wind pattern, precipitation etc. By absorbing UV radiation the ozone layer heats up the atmosphere. The absorbed heat causes a temperature inversion between 15 to 50 km.
An international agreement of 1987 at a conference in Montreal, signed by 34 countries (Montreal Protocol) called for a ban on use of CFCs reduction of about 50% by the end of 20th century.
In the London conference, developed countries agreed for 100% ban on CFCs use by 2003, in place of 50% as decided in Montreal protocol. The London amendment to the Montreal Protocol was signed in May 1992 and came into effect on 10 August, 1992.
The Environmental Protection Agency in its studies in 2003 concluded, that 85% reduction is necessary to prevent the CFC levels in the stratosphere.