Depletion Of The Ozone Layer Biology Essay


Ozone layer, a layer in earth's upper atmosphere holds a significant importance. It is the layer which prevents harmful ultraviolet rays from sun reaching the surface of the earth and makes life possible without any difficulties.

Human advancement in technology has made a severe effect on this life saving layer. The harmful gases that are released in the atmosphere by humans are causing deterioration of this layer at an alarming rate. It has caused many holes in this layer. Depletion of this layer will leave earth defenceless from the harmful rays from the sun causing death of plants and in turn, animals including humans. The rate at which this layer is vanishing is a matter of consideration and should be looked upon immediately or else it'll be too late to do something about it.

Steps are being taken to check this depletion but it is on a very small scale and this devastating situation must be brought into light as soon as possible, raising awareness is the best way to counter this..

About ozone

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Ozone is a gas which is tri atomic molecule, made up of 3 atoms of oxygen. It is a pale blue gas , soluble in inert non-polar solvents as CCl4 , or fluorocarbons. This gas is diamagnetic, i.e , its electrons are all paired.It is an irritating, corrosive, colourless gas with a smell something like burning electrical wiring. In fact, ozone is easily produced by any high-voltage electrical arc (spark plugs, Van de Graff generators, Tesla coils, arc welders). Each molecule of ozone has three oxygen atoms and is produced when oxygen molecules (O2) are broken up by energetic electrons or high energy radiation.

As far as its structure is concerned it is a bent molecule. The O-O distances are 127.2 pm. And the O-O-O angle is 116.78°.

A diagrammatic view of ozone is in the figure below [1]

Ozone being a powerful oxidizing agent, is unstable at high concentrations and decomposes to oxygen...

Ozone layer

The ozone layer" refers to the ozone within stratosphere, where over 90% of the earth's ozone resides.. .

The ozone layer absorbs 97-99% of the sun's high frequency ultraviolet light which is potentially damaging to life on earth. Every 1% decrease in the earth's ozone shield is projected to increases the amount of UV light exposure to the lower atmosphere by 2%. Because this would cause more ozone to form in the lower atmosphere, it is uncertain how much of UV light would actually reach the earth's surface. Recent UV measurements from around the northern hemisphere indicate small UV increases in rural areas and almost no increase in areas near large cities.

Units used to measure ozone concentration

When describing the amount or concentration of gas, scientists resort to several different units:

Dobson unit (DU) - the principle unit for measuring ozone concentration. One DU is about twenty-seven million molecules per square centimetre (the palm of your hand covers an area of roughly a hundred square centimetres). The ozone concentration over the US is about 300 DU and the Antarctic hole during the late spring can drop to 117 DU.

Mixing ratios: within a specified volume, it is a fraction of the number of molecules of a particular gas divided by the total number of molecules in that given space. Terms of usually abbreviated, like ppmv for parts-per-million or ppbv which is parts-per-billion. For example the concentration of HCl at 3 km is said to be about 0.1 ppbv; this means that if you selected a volume of air that contained 10 billion molecules of air, one of those molecules would be an HCl molecule.

Factors influencing Ozone concentrations

Stratospheric sulphate aerosols: large explosive volcanoes are able to place a significant amount of aerosols into the lower stratosphere, as well as some chlorine. Because more than 90% of a volcanic plume is water vapour most of the other compounds, including volcanic chlorine, get ''rained-out'' of the stratosphere. The effects of a large volcano on global weather are significant, which in turn can affect localized weather patterns such as the Antarctic ozone hole. Many observations have linked the 1991 Mt. Pinatubo eruption to a 20% increase in the ozone hole that following spring. The effects of a large volcanic eruption on total global ozone are more modest (less than 3%) and last no more than 2-3 years.

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Stratospheric winds: every 26 months the tropical winds in the lower stratosphere change from easterly to westerly and then back again, an event called the Quasi-biennial Oscillations (QBO). The QBO causes ozone values at particular latitude to expand and contract roughly 3%. Since stratospheric winds move ozone, not destroy it, the loss of one latitude is the gain of another and globally the effects cancel out.

Greenhouse gases: to the degree that greenhouse gases might heat the planet and alter weather patterns, the magnitude of the stratospheric winds will certainly be affected. Some of the more popular scenarios of global warming predict cooler stratospheric temperatures, leading to more polar stratospheric clouds and more active chlorine in the area of the Antarctic ozone hole.

Sunspot cycle: ozone is created by solar UV radiation. The amount of UV radiation produced by the sun is not constant but varies by several percent in a roughly 11year cycle. This 11year cycle is related to magnetic changes within the sun which increase the solar UV output, and is heralded by increase sunspots which appear on the surface of the sun. Comparisons of yearly ozone concentrations show a small 11 year variation in global ozone of about 2%. Episodes of unusual solar activity, solar storms and large solar flares, could certainly alter this value.

Stratospheric chlorine, coming mostly from man-made halocarbons. Careful subtracting of other natural factors yields a net decrease of 3% per decade in global ozone, 1978-1991; due most likely to catalytic degradation by stratospheric chlorine.

Decrease in global ozone The measurement period is from November 1978 through November 1987, and combines depletion due to natural and man-made causes. This analysis and graphic comes from the United Nations Environmental Protection Agency (UNEP).

Depletion of ozone

Ozone depletion refers to two types of deterioration of ozone concentration. 1st one is the decline of about 4% of the total volume of ozone in earth's stratosphere. The 2nd one is a much larger phenomenon of seasonal decrease of earth's ozone concentration over the polar caps. The latter is commonly referred as ozone hole.

There is substantial decrease in the ozone concentration in the lower atmosphere. Up to 70% reduction has been observed in the south hemispherical spring over Antarctica first observed in 1985 and is continuing. Reactions that take place in the polar stratospheric clouds play an important role in enhancing ozone depletion. 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. Some stratospheric cooling is also predicted from increases in greenhouse gases such as CO2; however the ozone-induced cooling appears to be dominant.

Predictions of ozone levels remain difficult. The World Meteorological Organization Global Ozone Research and Monitoring Project-Report No. 44 comes out strongly in favour for the Montreal Protocol, but notes that a UNEP 1994 Assessment overestimated ozone loss for the 1994-1997 period.

The Ozone Hole

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. The ozone hole occurs during the Antarctic spring, from September to early December, as strong westerly winds start to circulate around the continent and create an atmospheric container. Within this polar vortex, over 50% of the lower stratospheric ozone is destroyed during the Antarctic spring.

As explained above, the primary cause of ozone depletion is the presence of chlorine-containing source gases (primarily CFCs and related halocarbons). In the presence of UV light, these gases dissociate, releasing chlorine atoms, which then go on to catalyse ozone destruction. The Cl-catalyzed ozone depletion can take place in the gas phase, but it is dramatically enhanced in the presence of polar stratospheric clouds (PSCs).

These polar stratospheric clouds (PSC) form during winter, in the extreme cold. Polar winters are dark, consisting of 3 months without solar radiation (sunlight). The lack of sunlight contributes to a decrease in temperature and the polar vortex traps and chills air. Temperatures hover around or below -80 °C. These low temperatures form cloud particles. There are three types of PSC clouds; nitric acid trihydrate clouds, slowly cooling water-ice clouds, and rapid cooling water-ice(nacreous) clouds; that provide surfaces for chemical reactions that lead to ozone destruction.

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The photochemical processes involved are complex but well understood. The key observation is that, ordinarily, most of the chlorine in the stratosphere resides in stable "reservoir" compounds, primarily hydrochloric acid (HCl) and chlorine nitrate (ClONO2). During the Antarctic winter and spring, however, reactions on the surface of the polar stratospheric cloud particles convert these "reservoir" compounds into reactive free radicals (Cl and ClO). The clouds can also remove NO2 from the atmosphere by converting it to nitric acid, which prevents the newly formed ClO from being converted back into ClONO2.

The role of sunlight in ozone depletion is the reason why the Antarctic ozone depletion is greatest during spring. During winter, even though PSCs are at their most abundant, there is no light over the pole to drive the chemical reactions. During the spring, however, the sun comes out, providing energy to drive photochemical reactions, and melt the polar stratospheric clouds, releasing the trapped compounds. Warm temperature at the end of spring, break up the vortex around mid-December. As warm, ozone-rich air flows in from lower latitudes, the PSCs are destroyed, the ozone depletion process shuts down, and the ozone hole closes.

Most of the ozone that is destroyed is in the lower stratosphere, in contrast to the much smaller ozone depletion through homogeneous gas phase reactions, which occurs primarily in the upper stratosphere.

Causes of ozone depletion

The cause of ozone depletion is the increase in the level of free radicals such as hydroxyl radicals, nitric oxide radicals and atomic chlorine and bromine. The most important compound, which accounts for almost 80% of the total depletion of ozone in the stratosphere are chlorofluorocarbons (CFC). These compounds are very stable in the lower atmosphere of the Earth, but in the stratosphere, they break down to release a free chlorine atom due to ultraviolet radiation. A free chlorine atom reacts with an ozone molecule (O3) and forms chlorine monoxide (ClO) and a molecule of oxygen. Now chlorine monoxide reacts with an ozone molecule to form a chlorine atom and two molecules of oxygen. The free chlorine molecule again reacts with ozone to form chlorine monoxide. The process continues and the result is the reduction or depletion of ozone in the stratosphere.

The CFCs are widely used in air conditioning/ cooling units, and the cleaning process of delicate instruments. These compounds have no natural source and all of its presence in earth's atmosphere is entirely due to humans.

Effects And Consequences of Ozone depletion

The ozone layer, absorbing UVB rays from the sun, so, its depletion is expected to increase surface UVB levels, which can prove to be very harmful, including increased skin cancer rate.

Ozone, being a minor constituent in earth's atmosphere, is responsible for the absorption of most of the UVB rays coming from the sun. The ultraviolet rays that still penetrate the earth's atmosphere decrease exponentially with the density of the layer. So, depletion in ozone layer is expected to give rise to significantly increased levels of UVB near the surface.

Biological effects-

UVB (the higher energy UV radiation absorbed by ozone) is generally accepted to be a contributory factor to skin cancer. In addition, increased surface UV leads to increased tropospheric ozone, which is a health risk to humans.

1. Basal and Squamous Cell Carcinomas - The most common forms of skin cancer in humans, basal and squamous cell carcinomas, have been strongly linked to UVB exposure. The mechanism by which UVB induces these cancers is well understood-absorption of UVB radiation causes the pyrimidine bases in the DNA molecule to form dimers, resulting in transcription errors when the DNA replicates. These cancers are relatively mild and rarely fatal, although the treatment of squamous cell carcinoma sometimes requires extensive reconstructive surgery. By combining epidemiological data with results of animal studies, scientists have estimated that a one per cent decrease in stratospheric ozone would increase the incidence of these cancers by 2%.

2. 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 well understood, but it appears that both UVB and UVA are involved. Experiments on fish suggest that 90 to 95% of malignant melanomas may be due to UVA and visible radiation whereas experiments on opossums suggest a larger role for UVB.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 no melanoma skin cancer over a period of seven years, along with decreased ozone and increased UVB levels.

3. Cortical Cataracts - Studies are suggestive of an association between ocular cortical cataracts and UV-B exposure, using crude approximations of exposure and various cataract assessment techniques. A detailed assessment of ocular exposure to UV-B was carried out in a study on Chesapeake Bay Watermen, where increases in average annual ocular exposure were associated with increasing risk of cortical opacity. In this highly exposed group of predominantly white males, the evidence linking cortical opacities to sunlight exposure was the strongest to date. However, subsequent data from a population-based study in Beaver Dam, WI suggested the risk may be confined to men. In the Beaver Dam study, the exposures among women were lower than exposures among men, and no association was seen. Moreover, there were no data linking sunlight exposure to risk of cataract in African Americans, although other eye diseases have different prevalence among the different racial groups, and cortical opacity appears to be higher in African Americans compared with whites.

4. Increased Tropospheric Ozone - Increased surface UV leads to increased tropospheric ozone. Ground-level ozone is generally recognized to be a health risk, as ozone is toxic due to its strong oxidant properties. At this time, ozone at ground level is produced mainly by the action of UV radiation on combustion gases from vehicle exhausts.

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 light and they would be affected by its increase.