ozone depletion

All the powerful nations, or rather, conscious nations of the world are laying stress on two main things time and again because they are too important to be pushed aside or shelved even for a day. These two things are—economic slowdown and global warming.

If economic slowdown has affected our material well-being and has had a negative impact on the pace of our all­round development, global warming has reminded us of the damage we have done to the very existence of nature that has sustained us throughout the ages of our evolution with regard to our living as well as our thought pattern. When a man went on a progressing spree, he first did damage to nature unknowingly and later on, realised his mistake.

But the realisation that dawned upon him could not keep him from fiddling with the bounty of nature. He continued squandering the precious resources, of which a large proportion was to remain absent forever from the planet once consumed. For example, fossil fuels cannot be produced by man, however hard he tries. They are the product of millions of years which it took our earth to assume the shape it has got today. It is unfortunate that all of us know this fact, but our complacency never lets us say ‘no’ wasting irreversible energy for the sake of our luxurious living. Our lame excuse has always been—development. Behind the facade of development, we have started treading the path that leads to destruction. The price we are going to pay for unscrupulous practice, as it has been so far, will be catastrophic. Global warming, if unchecked, will surely result in the end of our existence itself.

Ozone Layer

In fact, global warming is the result of atmosphere filled with greenhouse gases. It is feared that greenhouse gases will cause melting of Arctic ice, raising ocean levels and altering the climate the world over. The terms, global warming, and climate change, are often used interchangeably. Gases such as carbon dioxide, methane, nitrous oxide, and refrigerants create a greenhouse effect by trapping heat in the lower atmosphere. As a result, the earth becomes warmer, because the sun’s rays are allowed to enter the lower atmosphere and the heat from its rays remains engulfed and fails to escape.

We should remember that the earth’s atmosphere consists of different layers. The layer that is closest to the surface of the earth is called the troposphere. It extends from the earth’s surface up to about 10 kilometres. The ozone layer is located above the troposphere in the Greenland were reported to have shown the stratosphere (10 kilometres to approximately 50 kilometres high). Stratospheric ozone (a form of oxygen) is the earth’s natural protection for all forms of life. It shields the earth from harmful ultraviolet-B (UV-B) radiation. UV-B radiation is equally harmful to humans, animals and plants. The ozone layer is being destroyed by certain industrial chemicals, refrigerants and pesticides used on crops. We have to be aware of the fact that ozone depletion damages get all the more alarming, if the stratosphere is very cold, as has been the case for the past few years. How much concern global warming has caused, can be gauged from the signing of ‘The Montreal Protocol on Substances that Deplete the Ozone Layer’ in 1987 under the auspices of United Nations Environment Programme (UNEP) by all the Governments of the world.

If we look into the past record, we will find that in 1995, 2,500 scientists prepared a report called the Second Assessment of Intergovernmental Panel on Climate Change (IPCC). Its reports stress that global warming or climate change is an ugly reality and real culprits are certainly human emissions of greenhouse gases. 1998 was the hottest year since accurate records began in the 1840s, and ten of the hottest years have undeniably occurred during the last 15-17 years.

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Ozone depletion describes two distinct but related observations : a slow but steady decline of about 4% per decade in the total volume of ozone in stratosphere since the late 1970s and a much larger, but a seasonal decrease in stratospheric ozone over the earth’s polar regions during the same period. The often repeated or commonly referred ‘ozone hole’ is nothing else but a term for the latter phenomenon.

Three forms (or allotropes) of oxygen are involved in the ozone-oxygen cycle : oxygen atoms (or atomic oxygen), oxygen gas (O2 or diatomic oxygen and ozone gas (O3 or triatomic oxygen). Ozone is formed in the stratosphere when oxygen molecules photodissociate after absorbing an ultraviolet photon whose wavelength is shorter than 240 nm. This produces two oxygen atoms. The atomic oxygen then combines with O2 to create O3. Ozone molecules absorb ultraviolet light between 310 and 200 nm, following which ozone splits into a molecule of O2 and an oxygen atom. The oxygen atom then joins up with an oxygen molecule to regenerate ozone. This is a continuous process which comes to an end when an oxygen atom recombines with an ozone molecule to make two Oz molecules:

O + O3 → 2O2

The overall amount of ozone in the stratosphere is determined by a balance between photochemical production and recombination. Ozone can be destroyed by a number of free radical catalysts, the most important of which are the hydroxyl radical (OH.), the nitric oxide radical (NO.) and atomic chlorine (Cl.) and Bromine (Br.). All of these have both natural and anthropogenic (man-made) sources. We should take note of the fact that at present, most of the (OH.) and (NO.) in the stratosphere is of natural origin. But the levels of chlorine and bromine have been dramatically increased by human activity. These elements are found in certain stable organic compounds, particularly in chlorofluorocarbons (CFCs). These CFCs may find their way to the stratosphere without being destroyed in the troposphere as they have low reactivity property. Once in the stratosphere, the Cl and Br atoms are liberated from the parent compounds by the action of ultraviolet light :

CFCl3 + hv → CFCl2 + Cl

(h is Planck’s constant, v is frequency of electromagnetic radiation)

The Cl and Br atoms can then destroy ozone molecules through a variety of catalytic cycles. For example, a chlorine atom reacts with an ozone molecule, taking an oxygen atom with it and forms ClO. It leaves a normal oxygen molecule.

The chlorine monoxide can react with a second molecule of ozone or O3 to produce another chlorine atom and two molecules of oxygen :

Cl + O3 → ClO + O2

Cl + O3 → ClO + 2O2

The overall impact is a decrease in the amount of ozone. Scientists have discovered more complicated mechanisms that lead to ozone depletion in the lower stratosphere as well. A single chlorine atom would keep on destroying ozone for up to two years, but some reactions remove single chlorine atoms from this cycle by forming reservoir species such as hydrogen chloride (HCl) and chlorine nitrate (CINO3).

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If we judge it on a per-atom basis, bromine is even more efficient as far as destroying ozone is concerned. Fortunately, there is much less bromine in the atmosphere at present. Both chlorine and bromine largely contribute to the overall ozone depletion. Though laboratory studies have established that fluorine and iodine atoms participate in similar catalytic cycles yet in the earth’s stratosphere, fluorine atoms react rapidly with water and methane to form strongly-bound HF, while organic molecules which contain iodine, react so rapidly in the lower atmosphere that they do not reach the stratosphere in significant quantities. We can evaluate the amount of damage done by chlorine to the environment if we know the fact that a single chlorine atom is able to react with 1,00,000 ozone molecules. We also get at once, conscious of the hazards caused to the earth by CFCs.

The most pronounced decrease in ozone has been in the lower stratosphere. However, the ozone hole is usually measured by the reduction in the total column ozone above a point on the earth’s surface, not in terms of ozone concentrations at these levels.

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Reductions of up to 70% in the ozone column observed in the austral (southern hemispheric) spring over Antarctica and first reported in 1985 are continuing. Through the 1990s, total column ozone in September and October continued to be 40-50% lower than ozone-hole values. In the Arctic, the amount lost is more variable year-to-year than in the Antarctic. The greater declines, up to 30%, are in the winter and spring, when the stratosphere is colder. Ozone depletion is enhanced substantially by the reactions that take place on polar stratospheric clouds (PSCs). PSCs form more rapidly in the extreme cold of Antarctic stratosphere. It is because of this that ozone holes first formed and are deeper, over Antarctica. Early models failed to notice PSCs and predicted a gradual global depletion. This was the reason that sudden Antarctic ozone hole came as a surprise to many scientists. In middle latitudes, it is preferable to talk of ozone depletion rather than holes. Ozone depletion also explains much of the observed reduction in stratospheric and upper tropospheric temperatures. The source of the warmth of the stratosphere is the absorption of UV radiation by ozone, hence reduced ozone leads to cooling.

As the ozone layer absorbs UV-B (Ultraviolet light from the sun) ozone layer depletion is thought to give rise to UV-B levels, which could lead to damage, including increase in skin cancer cases. This was the reason for the Montreal Protocol. Ozone, while a minority constituent in the earth’s atmosphere, is responsible for most of the absorption of UV-B radiation.

Since the adoption and strengthening of the Montreal Protocol has led to reductions in the emissions of CFCs, atmospheric concentration of the most significant compounds have been declining. These substances are being gradually removed from the atmosphere. The complete recovery of the Antarctic ozone layer, however, will not occur until 2050. The decrease in ozone-depleting chemicals has also been significant, affected by a decrease in bromine-containing chemicals. In October 2008, the Ecuadorian Space Agency published a report called ‘HIPERION’. It was a study of the last 28 years, data from 10 satellites and dozens of ground instruments around the world. The study found that the UV radiation reaching equatorial latitudes was far greater than expected. UV radiation climbed in some very populous cities up to 24 UVI. The WHO UV Index considers 11 as an extreme index and a great risk to health. The report concluded that the ozone depletion around mid latitudes on the planet is already endangering large populations in these areas. Later, the Peruvian space agency ‘CONIDA’ made its own study and found the facts stated by the Ecuadorian study quite correct.

The same CO2 radiative forcing that produces near-surface global warming is expected to cool the stratosphere. This cooling, in turn, is expected to produce a relative increase in polar ozone (O3) depletion and the frequency of ozone holes. Conversely, ozone depletion represents a radiative forcing of the climate system. There are two opposing effects : reduced ozone causes the stratosphere to absorb less solar radiation, thus cooling the stratosphere, while warming the troposphere. The resulting colder stratosphere emits less long-wave radiation downward. The troposphere is cooled as a result.

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