AIR POLLUTION

1. Air pollution

Air pollution refers to the release of large amounts of harmful gases and solid particles into the atmosphere by industrial and agricultural processes, electricity generation and transport.

2. The greenhouse effect

Carbon dioxide is not normally considered to be a pollutant, but there is considerable evidence that its concentration in the atmosphere has increased appreciably during the last century, from about 0.030% to about 0.038%, largely due to the proliferation of industry, coal-fired electricity generation, and the enormous use of land and air transport, activities that are based on the combustion of fossil fuels. As a rule of thumb, every kilogram of a fossil fuel produces about 3 kilograms of carbon dioxide. The more developed a country is, the more carbon dioxide it produces. The United States, for example, produces more than half the total amount of carbon dioxide emitted anually, and there is every indication that it is soon to be overtaken by China. Therefore, the prospects for a dramatic world-wide reduction in carbon dioxide emission are not at all good.

Most of the radiation coming from the sun is in the form of visible light, about half of which can penetrate the atmosphere and reach the surface of the earth and be absorbed.

The greehouse effect. (Picture source: Wikipedia Commons. Author: ZooFari)

The radiation absorbed by the earth’s surface during the day heats up the earth’s surface (including the oceans). This means that the earth’s surface can radiate some of this heat back into space at night in the form of infrared radiation. However, the so-called GREENHOUSE GASES, such as carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), and water (H2O), and to a lesser extent, ozone (O3) (as an industrial pollutant), absorb some of this infrared radiation, which is then re-emitted back to earth. The net result is that the earth is warmer than it would be in the absence of such gases. It has been calculated that this greenhouse effect increases the earth’s temperature by about 30 ºC, making the planet liveable to humans.

So, the greenhouse effect is normal and beneficial. The problem arises due to the increase in the greenhouse gases, notably carbon dioxide, which leads to global warming and climate changes. There is much evidence that global temperatures have increased appreciably in the last 200 years or so, and already, glaciers and the ice caps are melting, and the ocean levels are rising. If this continues unchecked, serious damage will be done to the planet, with catastrophic consequences.

It should be pointed out that not everyone is convinced, and some countries, notably the United States, have so far (end of 2011) not ratified the Kyoto Protocol, established in 1997 to attempt to reduce greenhouse gas emissions. South Africa is one of the countries that support the Protocol. The main argument used against carbon dioxide being a prime cause of global warming is the fact that the energy output from the sun is not constant, and depends largely on cyclical sunspot activity.

3. Acid rain

In addition to the release of carbon dioxide into the atmosphere, the combustion of fossil fuels also produces sulphur dioxide, SO2, and oxides of nitrogen, such as nitrogen dioxide, NO2. These arise in the first place from sulphur in the fuel (coal, petrol) and from the oxidation of nitrogen taking place at the high temperatures involved in the combustion process. Motor cars exhaust fumes contain appreciable amounts of nitrogen oxides. Both oxides cause respiratory problems of varying severity in humans.

In the atmosphere, both SO2 and NO2 will form acids in the presence of oxygen and moisture, and these acids dissolve in raindrops, falling to the earth in the form of "acid rain".

Acid rain kills trees and fish, and gradually dissolves stone monuments.

4. Ozone layer depletion

Ozone, O3, is an allotrope of oxygen (), present in an upper layer of the atmosphere, called the STRATOSPHERE (). It is produced by the action of cosmic radiation on oxygen molecules:

and is present in very small amounts, less than 7x10-6 % by volume. Ozone functions as a sun screen, absorbing harmful ultraviolet rays from the sun.

CFC Trade Name
CCl3F
CCl2F2
CCl2FCClF2
CClF2CClF2
CClF2CClF2F
Freon-11 
Freon-12 
Freon-13 
Freon-14 
Freon-113 

Chlorofluorocarbons (CFCs) are man-made chemicals that contain the elements carbon, chlorine and fluorine. They were used extensively as propellants in aerosol cans, refrigerants and in the manufacture of foam plastics and printed circuit boards. Being very stable substances, once they enter the atmosphere, they survive for a long time and eventually diffuse into the stratosphere, where they can undergo decomposition to form chlorine atoms:

In turn, these chlorine atoms react with ozone to form oxygen and chlorine monoxide, ClO.

The ozone hole over Antarctica, 11 September 2003 (Source: NASA)

As a result, the ozone layer becomes depleted. A so-called OZONE HOLE develops seasonally over Antarctica, and consequently, the surface of the earth experiences increased levels of ultraviolet radiation, most noticeably in the Southern Hemisphere. The results are increased levels of skin cancers and eye diseases in humans.

The use of CFCs is banned by an international agreement called the "Montreal Protocol".


5. Additional questions