SCIENCE OF OZONE DEPLETION

THE ATMOSPHERE AND OZONE LAYER

The small blue and green planet we call home is a very  special and unique place .  We live on the only planet in our solar system and possibly in the galaxy where life is known to exist. All life exists within thin film of air, water, and soil about 15 km deep. This spherical shell of life is known as the biosphere. The bioshpere can be divided into three layers; the atmosphere(air), the hydrosphere (water), and the lithosphere (rock and soil) . It is the unique attributes of the Earth’s atmosphere that allow it to be a habitable place for humans, animals, microbes and plants as we know them

The atmosphere is a mixture of gases and particles that surround our planet. When seen from space, the atmosphere appears as thin seam of dark blue light on a curved horizon.

The atmosphere extends a few hundred kilometers above the Earth. It is made of layers that surround the Earth like rings. However,99% of its total mass lies in two regions within the first 50 km above the Earth’s surface; the troposphere and the stratosphere. The stratosphere extends out, beyond the troposphere to about 50 km above the Earth.

What is Ozone?
Ozone is a form of oxygen. Oxygen occurs in three different forms in the atmosphere; as oxygen atoms (O), as oxygen molecules (O2) and as zone(O3).

Ozone’s unique physical properties allow the ozone layer to act as our planet’s sunscreen, providing an invisible filter to help protect all life forms from the sun’s damaging UV (ultraviolet)rays. Most incoming UV radiation is absorbed by ozone and prevented from reaching the Earth’s surface. Without the protective effect of ozone, life on Earth would not have evolved the way it has

What is Ultraviolet radiation:

Ultraviolet radiation is the one form of radiant energy coming out from the sun. The sun emits a range of energy known as the electromagnetic spectrum. The various forms of energy, or radiation, are classified according to wavelength (measured in nanometers where one nm is a millionth of a millimeter). The shorter the wave-length, the more energetic the radiation. In order of decreasing energy, the principal forms of radiation are gamma rays, x-rays, UV (ultraviolet radiation), visible light, infrared radiation, microwaves, and radio waves. Ultraviolet, which is invisible, is so named because it occurs next to violet in the visible light spectrum. The three categories of UV radiation are :

· UV-A between 320 and 400 nm

· UV-B between 280 and 320 nm

· UV-C between 200 and 280 nm

Of these UV-B and C being highly energetic and are dangerous to life on earth. UV-A being less energetic is not dangerous. Fortunately, UV-C is absorbed strongly by oxygen and also by ozone in the upper atmosphere. UV-B is also absorbed by ozone layer in the Stratosphere and only 2-3% of it reaches the earth’s surface. The ozone Layer, therefore, is highly beneficial to plant and animal life on earth in filtering out the dangerous part of sun’s radiation and allowing only the beneficial part to reach earth. Any disturbance or depletion of this layer would result in an increase UV-B and UV-C radiation reaching the earth’s surface leading to dangerous consequences.

What is Ozone Depletion?

Ozone depletion occurs when the natural balance between the production and destruction of stratospheric ozone is tipped in favour of destruction.Although natural phenomenon can cause temporary ozone loss, chlorine and bromine released from synthetic compounds is now accepted as the main cause of a net loss of stratospheric ozone in many parts of the world since 1980.There is strong evidence that global ozone depletion is occuring. The evidence is in the observations of the Antratic ozone “hole”and atmospheric records indicating seasonal declines in global ozone levels.

What is Antractic Hole ?

The terms “ozone hole” refers to a large and rapid decrease in the abundance of ozone molecules, not the complete absence of them.

The Antarctic “ozone hole” occurs during the southern spring between September and November. It was first reported by the British Antarctic Survey Team in May 1985. The Team found that for the period between September and mid November, ozone concentrations over Halley Bay, Antarctica, had declined 40% from levels during the 1960s. Severe depletion had been occuring since the late 1970s.

A recent assessment made by a panel of UNEP experts gives a detailed account of the impacts of ozone depletion on human health, animals, plants, microorganisms, materials and air quality.

Effects on Human and Animal Health

Increased penetration of solar UV-B radiation is likely to have profound impact on human health with potential risks of eye diseases, skin cancer and infectious diseases. UV radiation is known to damage the cornea and lens of the eye. Chronic exposure to UV-B could lead to cataract of the cortical and posterior subcapsular forms. UV-B radiation can adversely affect the immune system causing a number of infectious diseases. In light skinned human populations, it is likely to develop nonmelanoma skin cancer (NMSC). Experiments on animals show that UV exposure decreases the immune response to skin cancers, infectious agents and other antigens

Effects on Terrestrial Plants

It is a known fact that the physiological and developmental processes of plants are affected by UV-B radiation. Scientists believe that an increase in UV-B levels would necessitate using more UV-B tolerant cultivar and breeding new tolerant ones in agriculture. In forests and grasslands increased UV-B radiation is likely to result in changes in species composition (mutation) thus altering the bio-diversity in different ecosystems. UV-B could also affect the plant community indirectly resulting in changes in plant form, secondary metabolism, etc. These changes can have important implications for plant competitive balance, plant pathogens and bio-geochemical cycles.

Effects on Aquatic Ecosystems

While more than 30 percent of the world’s animal protein for human consumption comes from the sea alone, it is feared that increased levels of UV exposure can have adverse impacts on the productivity of aquatic systems. High levels of exposure in tropics and subtropics may affect the distribution of phytoplanktons which form the foundation of aquatic food webs. Reportedly a recent study has indicated 6-12 percent reduction in phytoplankton production in the marginal ice zone due to increases in UV-B. UV-B can also cause damage to early development stages of fish, shrimp, crab, amphibians and other animals, the most severe effects being decreased reproductive capacity and impaired larval development.

Effects on Bio-geo-chemical Cycles

Increased solar UV radiation could affect terrestrial and aquatic bio-geo-chemical cycles thus altering both sources and sinks of greenhouse and important trace gases, e.g. carbon dioxide (CO2), carbon monoxide (CO), carbonyl sulphide (COS), etc. These changes would contribute to biosphere-atmosphere feedbacks responsible for the atmosphere build-up of these gases. Other effects of increased UV-B radiation include: changes in the production and decomposition of plant matter; reduction of primary production changes in the uptake and release of important atmospheric gases; reduction of bacterioplankton growth in the upper ocean; increased degradation of aquatic dissolved organic matter (DOM), etc. Aquatic nitrogen cycling can be affected by enhanced UV-B through inhibition of nitrifying bacteria and photodecomposition of simple inorganic species such as nitrate. The marine sulphur cycle may also be affected resulting in possible changes in the sea-to-air emissions of COS and dimethylsulfied (DMS), two gases that are degraded to sulphate aerosols in the stratosphere and troposphere, respectively.

Effects on Air Quality

Reduction of stratospheric ozone and increased penetration of UV-B radiation result in higher photodissociation rates of key trace gases that control the chemical reactivity of the troposphere. This can increase both production and destruction of ozone and related oxidants such as hydrogen peroxide which are known to have adverse effects on human health, terrestrial plants and outdoor materials. Changes in the atmospheric concentrations of the hydroxyl radical (OH) may change the atmospheric lifetimes of important gases such as methane and substitutes of chlorofluoro carbons (CFCs). Increased tropospheric reactivity could also lead to increased production of particulates such as cloud condensation nuclei from the oxidation and subsequent nucleation of sulphur of both anthropogenic and natural origin (e.g. COS and DMS).

Effects on Materials

Increased levels of solar UV radiation is known to have adverse effects on synthetic polymers, naturally occurring biopolymers and some other materials of commercial interest. UV-B radiation accelerates the photodegradation rates of these materials thus limiting their lifetimes. Typical damages range from discoloration to loss of mechanical integrity. Such a situation would eventually demand substitution of the affected materials by more photostable plastics and other materials in future.

In 1974, two United States (US) scientists Mario Molina and F. Sherwood Rowland at the University of California were struck by the observation of Lovelock that the CFCs were present in the atmosphere all over the world more or less evenly distributed by appreciable concentrations. They suggested that these stable CFC molecules could drift slowly upto the stratosphere where they may breakdown into chlorine atoms by energetic UV-B and UB-C rays of the sun. The chlorine radicals thus produced can undergo complex chemical reaction producing chlorine monoxide which can attack an ozone molecule converting it into oxygen and in the process regenerating the chlorine atom again. Thus the ozone destroying effect is catalytic and a small amount of CFC would be destroying large number of ozone molecules. Their basic theory was then put to test by the National Aeronautic Space Authority (NASA) scientists and found to be valid, ringing alarm bells in many countries and laying the foundation for international action.

International Action

The first international action to focus attention on the dangers of ozone depletion in the stratosphere and its dangerous consequences in the long run on life on earth was focused in 1977 when in a meeting of 32 countries in Washington D.C. a World plan on action on Ozone layer with UNEP as the coordinator was adopted.

As experts began their investigation, data piled up and in 1985 in an article published in the prestigious science journal, “Nature” by Dr. Farman pointed out that although there is overall depletion of the ozone layer all over the world, the most severe depletion had taken place over the Antarctica. This is what is famously called as "the Antarctica Ozone hole". His findings were confirmed by Satellite observations and offered the first proof of severe ozone depletion and stirred the scientific community to take urgent remedial actions in an international convention held in Vienna on March 22, 1985. This resulted in an international agreement in 1987 on specific measures to be taken in the form of an international treaty known as the Montreal Protocol on Substances That Deplete the Ozone Layer. Under this Protocol the first concrete step to save the Ozone layer was taken by immediately agreeing to completely phase out chlorofluorocarbons (CFC), Halons, Carbon tetrachloride (CTC) and Methyl chloroform (MCF) as per a given schedule

Alternatives to currently used Ozone Depleting Substances

During the last few years intense research has yielded a large number of substitute chemicals as replacements to currently used chlorofluorocarbons (CFCs), Halons, CTC, and Methyl chloroform.