What are the Causes of Stratospheric Ozone Depletion?

The three main reactants, namely nitric oxide, chlorine atoms and hydroxyl ions are normally present in the environment. Though much of these are derived from natural sources, human activity is also equally responsible for contributing a lot of such constituents in the atmosphere which give to these reactants.

Temperatures decrease with height in the troposphere, the zone of atmosphere immediately covering the surface of our planet while in stratosphere there is a rise in temperature In between the two lies a narrower region, called the tropopause, which has almost uniform temperature. It is this temperature profile which prevents to a large extent the exchange of materials between the stratosphere and the troposphere.

However, some exchange does occur and many constituents of the troposphere do reach the stratosphere. In addition to this exchange, many pollutants are injected straight into the stratosphere by supersonic transport planes, rockets and space shuttles, surface nuclear tests, volcanic emissions etc. Human activities responsible for polluting the stratosphere may be summed up as follows:

(1) Combustion of Fossil Fuels and Organic Matter:

Huge quantities of various oxides such as oxides of carbon, nitrogen and sulphur, hydrocarbons and fine particulate material is introduced daily into the troposphere some of which goes high enough to reach the stratosphere. To this may be added microbial decomposition products of organic matter which discharge plenty of hydrocarbons into the atmosphere of which methane is one of the most important constituents which reaches high enough to pollute the stratosphere.

(2) Excessive Use of Nitrogenous Fertilizers:

Microbial action on nitrogenous fertilizers produces nitrous oxide which escapes into the atmosphere. As this gas is decomposed with difficulty it tends to accumulate in the atmosphere. The estimated tropospheric load of nitrous oxide is 1.7 x 10¹⁸ gms while the stratospheric content is about 8.0 x 10″ gms. Actions of high energy radiations convert it to nitric oxide which is an important ozone-destroying constituent of the stratosphere.

(3) Excessive Use of Chlorofluorocarbons:

Chlorofluorocarbons are inert, highly stable colourless, odourless chemicals which can be easily liquified. These are mostly halogenated hydrocarbons which under the influence of high energy radiations break up to yield a chlorine atom. Chlorofluorocarbons are considered to be the most damaging pollutants of the stratosphere, as they persist in the atmosphere for periods as long as 150 years. A single chlorine atom can destroy 1, 00,000 ozone molecules.

(4) Supersonic Transports, Rockets and Space Shuttles:

Supersonic jet-liners, which fly in lower regions of the stratosphere, discharge various oxides of nitrogen, carbon, sulphur, hydrocarbons and particulate material. Space flights which are powered by powerful rockets also release large quantities of these pollutants right into the stratosphere. Ammonium perchlorate used in many of these rockets as oxidant releases plenty of chlorides.

(5) Nuclear Tests:

Surface nuclear explosions produce huge quantities of various gases, dust shoot and debris with enormous force which carries much of material straight into the stratosphere. Much of this material damages the ozone layer.

The three reactants which play a major role in destruction of ozone, namely nitric oxide, chlorine atoms and hydroxyl ions may also be derived from natural sources. The quantities of ozone destroying constituents introduced into the stratosphere by natural means are also considerable and some workers (Madura and Schauerhammer, 1992) believe that natural sources exceed the anthropogenic additions of pollutants in the stratosphere. Major natural sources of pollutants which are responsible for stratospheric ozone depletion are solar flairs and volcanic eruptions.

(1) Volcanic Eruptions:

If we look at the list of volcanic eruptions since 1500 A.D., it becomes apparent that these events are not so infrequent. One of the biggest eruptions in the recent history was that of Krakota in 1882 and a more recent one of almost similar magnitude was that of Mt. Agung in 1963.

Though events of such magnitude are rare, smaller ones are more frequent which occur almost every alternate year at one place or another on our globe. Elchichon in Mexico, Mount Pinatubo in Philippines are quite recent examples of such small eruptions.

The cumulative effect of these smaller or larger eruptions is, however, responsible for adding enormous quantities of waste gases and other pollutants which are often thrown out with force to reach high up in the atmosphere. Mt. Erebus on Antarctica alone ejects about 1000 tons of chlorine a day annually venting out 50 times chlorine than an entire year’s CFC equivalent (Rogelio Madura, 1992).

(2) Solar Flares:

Violent eruption from the sun or the solar flares force out plenty of energetic particles at velocities almost close to that of light which enter earth’s atmosphere right down to the stratospheric ozone layer. High energy content of these particles causes stratospheric nitrogen to combine with oxygen or ozone molecules and produce nitric oxide.

Solar flares are also not infrequent and some may inject catastrophically large amounts of nitric oxide into the stratosphere. The effect of high energy particles on other components and reactions occurring normally in the stratosphere should also be significant. The solar flare of August 4,1972, alone has been estimated to have caused about 13% reduction in ozone content.