OZONE DEPLETION

• Ozone is a natural gas, which is an allotrope of oxygen consisting of three atoms of oxygen bound together in a non-linear fashion. The chemical symbol of ozone is O_3.
• Ozone is found in two different layers of the atmosphere. Ozone in the troposphere is not good because it pollutes the air and forms smog which makes it difficult to breathe. Ozone in the stratosphere protects life on Earth by absorbing the sun’s harmful ultraviolet (UV) rays.
• The ozone layer is very important because the configuration of the ozone molecule, and its chemical properties are such that ozone efficiently absorbs ultraviolet light, thus acting like a sun-screen.
• UV rays cause direct damage to the genetic material or DNA of animal, and plant cells. Exposure of mammals to UV light has been shown to act on the immune system, thereby making the body more susceptible to diseases.
• Ozone protects oxygen at lower altitudes from being broken up by the action of ultraviolet light and also keeps most of the ultraviolet radiation from reaching the earth’s surface.
• Ozone helps in reducing the risks of mutation & harm to plant, and animal life. Excess UV rays can cause skin cancer and will also harm all plants & animals. Life on Earth could not exist without the protective shield of the ozone layer.

Change in Equilibrium

• The equilibrium between the formation & destruction of ozone has been upset by the influx of several substances into the atmosphere which react with ozone and destroy it.
• The rate at which ozone is being destroyed is much faster than the rate at which it is being formed.
• It implies that there is a significant decrease in the concentration of ozone in a particular region of the atmosphere, so the name ozone depletion.
• The best example of such an ozone depletion is the atmosphere over the Antarctic which has only about 50% of the ozone that originally occurred there.
• The actual realization of ozone depletion came only in 1985.

Source

• Chlorofluorocarbons (CFCs) are made up of chlorine, fluorine & carbon.
• CFCs are used as refrigerants, propellants in aerosol sprays, foaming agents in plastic manufacturing, fire extinguishing agents, solvents for cleaning electronic & metallic components, for freezing foods etc.
• 2/3^rd of CFC is used as refrigerants while 1/3^rd is used as blowing agents in foam insulation products.
• CFCs have wide & varied applications due to their properties like non-corrosiveness, non-inflammability, low toxicity and chemical stability.
• In fact, the residence time of CFCs in the atmosphere is estimated to be between 40-150 years.
• During this period, the CFCs move upwards by random diffusion from the troposphere to the stratosphere.
• The CFCs enter into the atmosphere by gradual evaporation from their source.
• CFCs can escape into the atmosphere from a discarded refrigerator. Since the CFCs are thermally stable they can survive in the troposphere.
• But in the stratosphere, they are exposed to UV radiation.
• The molecules of CFCs when exposed to UV radiation break up, freeing chlorine atoms.
• A free chlorine atom reacts with an ozone molecule to form chlorine monoxide. The molecules of chlorine monoxide combine with an atom of oxygen.
• This reaction results in the formation of an oxygen molecule & reformation of the free chlorine atom.
• The depletion of ozone is catalytic. The element that destroys ozone is being reformed at the end of the cycle.
• A single chlorine atom destroys thousands of ozone molecules before encountering reactive nitrogen or hydrogen compounds that eventually return chlorine to its reservoirs.

CFC Substitutes

• The substitute for CFCs should be safe, low cost, increased energy efficiency of CFC replacement technology, effective refrigerants with low ozone layer depletion potential(ODP) & low global warming potential.
• CFC-12 (R-12) is a widely used refrigerant. HFC 134a (R-134a) is the most promising alternative (R-143a) & (R-152a) can also be used.

Nitrogen Oxides

• The sources of nitrogen oxides are mainly explosions of thermonuclear weapons, industrial emissions & agricultural fertilizers.
• Nitric Oxide (NO) catalytically destroys ozone.

Nitric oxide + ozoneà Nitrogen dioxide + Oxygen

Nitrogen dioxide + Monoxideà Nitric oxide+ oxygen

• Nitrous oxide is released from solid through denitrification of nitrates under anaerobic conditions & nitrification of ammonia under aerobic conditions.
• This nitrous oxide can gradually reach the middle of the stratosphere, where it is photolytically destroyed to yield nitric oxide which in turn destroys ozone.

Other Elements

• Bromine-containing compounds called HBFCs-Hydrobromo fluorocarbons (both used in fire extinguishers & methyl bromide).
• Each bromide atom destroys a hundred times more ozone molecules than that of chlorine atom.

Bromine + Oxygen à Bromine monoxide + Oxygen

Bromine monoxide + chlorine monoxide à oxygen + Bromine + chlorine

• Bromine combines with ozone forming bromine monoxide & oxygen.
• The BrO further reacts with chlorine monoxide(ClO) to give oxygen & free atoms can further react with ozone.
• Carbon tetrachloride – cheap, highly toxic solvent & methyl chloroform –used as a cleaning solvent for clothes, metals and a propellant in a wide range of consumer products, such as correction fluid, dry cleaning sprays and other aerosols.

Sulphuric acid particles: These particles free chlorine from molecular reservoirs and convert reactive nitrogen into inert forms thus preventing the formation of chlorine reservoirs.

Monitoring: Some organisations help in monitoring the atmosphere & form a network of information communication about the atmosphere, including ozone layer monitoring are:

• World Meteorological Organisation(WMO)
• Integrated Global Ocean Services Systems(IGOSS)
• World Weather Watch(WWW)
• Global Climate Observing System(GCOS)

1. Polar stratospheric clouds

There are three different types of stratospheric clouds as follows.

1)  Nacreous clouds extend from 10-100km in length & several kilometers in thickness. They are also called mother-of-pearl clouds due to their glow with a sea-shell-like iridescence.

2) The second type of cloud contains nitric acid instead of pure water.

3) The third type of clouds have the same chemical composition as nacreous clouds but form at a slower rate, which results in a larger cloud with no iridescence.

• The chlorine released by the breakdown of CFCs exists initially as pure chlorine or as chlorine monoxide but these two forms react further to form compounds chlorine nitrate & HCl that are stable.
• The stable compounds HCl & ClONO₂ are reservoirs of chlorine and hence chlorine takes part in reactions of any sort, it has to be freed.
• There is a correlation between the cycle of ozone depletion & the presence of polar stratospheric clouds(PSCs) i.e., the ice particles of the cloud provided substrates for chemical reactions which freed chlorine from its reservoirs.
• Usually, the reaction between HCl & ClONO₂ is very slow, but this reaction occurs at a faster rate in the presence of a suitable substrate which is provided by the stratospheric clouds at the poles.
• It results in the formation of molecular chlorine & nitric acid. The molecular chlorine formed in the above reaction can be broken down to atomic chlorine & the ozone depletion reaction would continue. The PSCs not only activate chlorine but also absorb reactive nitrogen. If nitrogen oxides were present they would combine with chlorine monoxide to form a reservoir of chlorine nitrate (ClONO₂).
• Stratospheric chlorine monoxide reacts with itself forming a dimer Cl₂O_2. This dimer is easily dissociated by sunlight, giving rise to free chlorine atoms which can further react to destroy ozone.
• Every spring, a hole as big as the USA develops in the ozone layer over Antarctica, at the south pole. A smaller hole develops each year over the Arctic, at the north pole and there are signs that the ozone layer is getting thinner all over the planet.

2.  Ozone depletion in Antarctica

• The Antarctic stratosphere is colder. The low temperature enables the formation of polar stratospheric clouds (PSCs), below 20km.
• Ozone absorbs sunlight, causing the characteristic increase in temperature with an increase in altitude in the stratosphere. If ozone is depleted, the air becomes cooler, further adding to the favourable conditions for the formation of PSCs and stabilization of the vortex. The vortex is a ring of rapidly circulating air that confines the ozone depletion in the Antarctic region.
• Typical happenings in the winter months leading to ozone depletion over Antarctica.
• The longevity of the Antarctica vortex is another factor, enhancing favourable conditions for the depletion of ozone. The vortex remains, in fact throughout the polar winter, well into mid-spring whereas the vortex in the Arctic disintegrates by the time the polar spring arrives.
• In June, Antarctic weather begins, the vortex develops & the temperature falls enough for the clouds to form.
• During July & August PSCs denitrify & dehydrate the stratosphere through precipitation, hydrochloric acid & chlorine nitrate react on cloud surfaces to free chlorine & winter temperatures drop to their lowest point.
• In September sunlight returns to the centre of the vortex as the austral spring begins & PSCs disappear because of increasing temperature. ClO-ClO & ClO-BrO catalytic cycles destroy ozone.
• During October lowest levels of ozone are reached.
• In November, the Polar Vortex breaks down, the ozone-rich areas from the mid-latitudes replenish the Antarctic stratosphere & ozone-poor air spreads over the southern hemisphere.

3. Arctic Ozone Depletion

• The ozone depletion was increasingly evident over the Arctic as well.
• The Arctic Ozone Depletion which swept across Britain in March was the greatest depletion of ozone ever seen in the northern hemisphere.
• In past it was caused by a dramatic cooling of the upper atmosphere in the northern latitudes over.
• The ozone depletion over the northern hemisphere has been increasing steadily since the winter of 1992.
• Apart from the buildup of ozone-depleting chemicals, the main cause is the increasing cold temperature in the arctic stratosphere which encourages the formulation of PSCs.

Measurement of Ozone: The ozone measurement instruments & techniques are varied. Some of them are the Dobson spectrophotometer & the filter ozonometer & Total Ozone Mapping Spectrometer (TOMS) in the Nimbus-7 satellite.

4. Environmental Effects of Ozone Depletion

A decrease in the quantity of total column ozone tends to cause increased penetration of solar UV-B radiation (290-315nm) to the earth’s surface. It has profound effects on human health, animals, plants, micro-organisms, materials & air quality.

Effects:

• UV radiation has been shown in experimental systems to damage the cornea & lens of the eye.
• Experiments in animals show that UV exposure decreases the immune response to skin cancers, infectious agents & other antigens can lead to unresponsiveness upon repeated challenges.
• Potential risks include the incidence & morbidity from eye diseases, skin cancer & infectious diseases.
• In susceptible populations, UV-B radiation is the key risk factor for the development of non-melanoma skin cancer.
• Psychological & developmental processes of plants are affected by UV-B radiation.
• In forests, and grasslands this is likely to result in changes in the composition of species, therefore there are implications for the biodiversity in different ecosystems.
• Response to UV-B also varies considerably among species, as well as cultivars of the same species.
• In agriculture, this will necessitate using more UV-B tolerant cultivars & breeding new ones.
• Indirect changes caused by UV-B such as changes in plant form, biomass allocation to parts of the plant timing of developmental phases & second metabolism may be equally or sometimes more important than the damaging effects of UV-B.
• Solar UV-B radiation was found to cause damage in the early developmental stages of fish, shrimp, crabs, amphibians & other animals.
• The most severe effects are decreased reproductive capacity & impaired larval development.
• Exposure to solar UV-B radiation has been shown to affect both orientation mechanisms & motility in phytoplankton, resulting in reduced survival rates for these organisms.
• An increase in solar UV radiation could affect terrestrial & aquatic biogeochemical cycles, thus altering both sources & sinks of greenhouse & chemically important trace gases.
• These potential changes would contribute to bio-sphere atmosphere feedback that reinforces the atmospheric build-up of these gases.
• Changes in the atmospheric concentrations of the hydroxyl radical(OH) may change the atmospheric lifetimes of climatically important gases such as methane & the CFC substitutes.
• Reduction in stratospheric ozone & the concomitant increase in UV-B radiation penetrating to the lower atmosphere result in higher photodissociation rates of key trace gases that control the chemical reactivity of the troposphere.
• This can increase both production & destruction of ozone & related oxidants such as hydrogen peroxide, which are known to have adverse effects on human health, terrestrial plants and outdoor materials.
• Increased tropospheric reactivity could also lead to increased production of particulates like cloud condensation nuclei, from the oxidation & subsequent nucleation of sulphur of both anthropogenic & natural origin.
• Synthetic polymers, naturally occurring bio-polymers as well as some other materials of commercial interest are adversely affected by solar UV radiation.
• Any increase in solar UV-B content due to partial ozone depletion will therefore accelerate the photodegradation rates of these materials, limiting their life outdoors.
• The application of these materials, particularly plastics in situations which demand routine exposure to sunlight is only possible through the use of light-stabilizers or surface treatment to protect them from sunlight.