Smog And Depletion Of Ozone Layer

Differences between classical and photochemical smog

The major diffreences between classical and photochemical smog are given below:

Depletion Of Ozone Layer

Ozone Chemistry

Ozone,O₃ is the important species in the stratosphere,acting as a protective radiation shield for living organisms on the earth.The maximum ozone concentration is around 10 ppm in the stratosphere at an altitude of 25-30 km_.

Ozone is formed by a photochemical reaction,followed by a third body reaction.

$$O_2+hr(242mm) \rightarrow O+O$$

$$O+O_2+M(N_2 or O_2) \rightarrow O_3+M$$

The third body(M) aabsorbs the excess energy liberated by the above reaction and thereby stabilize the O₃ molecule.

Ozone strongly absorbs the ultraviolet light in the region 220 to 230 nm and thereby protects life on the earth from severe radiation damage.

The mechanism of ozone removal(destruction) is not well understood.It is believed that ozone is eliminated by reactiom with atomic oxygen,ractive hydroxyl radicals and mainly by nitric oxide.

1.$$O_3+O \rightarrow O_2+O_2$$

2.$$O_3+HO^• \rightarrow O_2+HOO^•$$

$$HOO^•+O \rightarrow HO^•+O_2$$

3.$$O_3+NO \rightarrow NO_2+O_2$$

$$NO_2+O \rightarrow NO+O_2$$

It may be noted that nitric oxide is produced in the stratosphere,below 30 km,by the reaction of nitrous solid(N₂O) with excited oxygen atoms,and above 30 km by ionizing radiation on nitrogen.

$$N_2O+O^• \rightarrow 2NO$$

$$N_2+hv \rightarrow N+N$$

$$O_2+N \rightarrow NO+O$$

Ozone Hole

In September,1980 scientists reported a large hole in the ozone layer over Antartica where ozone level dropped by 30%.Chlorofluorocarbons(CFCs),commonly known as "Freons" was the primary suspect for casing ozone depletion.It was established that one molecule of CFCs is capable of destroying one lakh O₃ molecules in the stratosphere.

Subsequently,a similar hole was discovered over the thickly populated northern hemisphere.A study of NASA scientistsrevealed that the amount of ozone over the northern hemisphere decreased by 3% between 1969 and 1986.

An international agreement made in 1987 at a conference in Montreal and signedby 34 countries(Montreal Protocol)called for a Freeze on use of CFC and a reduction of upto 50% by the end of 20th century.

Since the Montreal protocol,the ozone layer has been found to disappear at a faster rate than expected.In June,1992 Japanese scientists announced that thay had found that the ozone hole was 13 times wider in 1991.in June 1990 at London conference the developed countries agreed to 100% ban of CFCs by 2000 A.D. instead of 50% as per Montreal protocol,with a 10 year time-lag for the developing countries.The London amendment to the Montreal protocol was ratified in May 1992 and come into effect on August10,1992.

The extreme chemical stability and non-toxicity of CFCs enable them to persist for years in the atmosphere and to enter the atmosphere.In the stratosphere.CFCs are subjected to photochemical dissociation by intense UV radiation.

The net result is regeneration of Cl^• radical which sustains the chain reaction.It is known that one Cl atom/radical can destroy one lakh O₃ molecules.CFCs have lifetimes of the order of 100 years.

How does Ozone Hole Affects Us?

Ozone layer acts as an umbrella against the harmful radiations from the sun.The uv-radiation affects the life of human beings.Some of these are as follows:

i)Skin cancer:The most worrisome biological effect due to prolonged medium energy uv rays exposure is a type of skin cancer called "melanoma".

ii)Immune system attacked.

iii)Retardation of plant growth.

iv)Destruction of marine life.

v)Crack in the Larsen ice shelf.

How to overcome the Ozone depletion problem?

among the ban/low use of ozone destroying CFC's and no various other methods can be used to overcome this problem.These includes:

i)Use of HCFC's is more safer than use of CFC's.This is because hydrogen containing chlorofluoro carbons are destroyed in troposphere itself and not transported to strastosphere to cause ozone destruction.

ii)Recently,a bacteria has been identified which can eat CFC's.

iii)Injecting the alkenes,such as ethane and propane into Antartic atmosphere.The alkenes would react rapidly with Ozone destroying.Chlorine atoms to produce haloalkanes and thus immobilizing then this remedial measures is not very effective as haloalkanes can once again produces chlorine radical in the stratosphere.

References

Poudel, Bhoj Raj Poudel and Dr.Megh Raj. A Text Book Of Inorganic Chemistry. Kathmandu: National Book Centre, 2012.

Sons, John Wiley and. J.D Lee Concise Inorganic Chemistry. 2007.