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MODIFICATION OF THE ATMOSPHERE

Atmospheric circulation systems operate on such a large scale that one is perhaps inclined to doubt that man’s activities would have any appreciable effect on them. However, it is known that the global heat balance has changed over the last few decades, and we might ask ourselves how much of this is a result of man polluting the atmosphere. The problem is not so much to establish that man has an impact on the atmosphere but to evaluate it in comparison with the natural forces of change.

Atmospheric changes introduced by man may be grouped into three categories: the introduction of solids and gases not normally found in the atmosphere (pollutants); changes in proportions of the natural component gases of the atmosphere; and alterations of the earth’s surface in such a way as to affect the atmosphere.

Pollutants in the Atmosphere

To city-dwellers the most obvious way in which man has affected the atmosphere is through pollution. Pollutants include particulate matter, both solid and liquid particles, and gaseous substances such as sulphur dioxide (SO2), oxides of nitrogen (NO, NO2, NO3), carbon monoxide (CO) and hydrocarbon compounds. But not all man-made pollution comes from cities. Isolated industrial activities frequently create a footprint of atmospheric pollution in areas of countryside downwind from the industrial site: particularly infamous examples in Britain include smelters and brickworks. Mining and quarrying activities also send large amounts of mineral dust into the air. Even man-induced forest and grass fires as well as bonfires, can greatly add to particulate pollution at certain times of year.

Atmospheric pollutants are conducted upward from the emission sources by rising air currents as part of the normal convective processes. Larger particles settle under gravity and return to the ground as fallout. Smaller suspended particles are brought to the Earth by precipitation as washout. By a combination of the two processes the atmosphere tends to be cleaned of pollutants, and in the long run a balance is achieved between the input and output of pollutants, although there are large fluctuations in the quantities stored in the air at a given time. Pollutants are also eliminated from the air over their source areas by winds which disperse the particles into large volumes of clean air in the downwind direction. Smoke stacks are intended to take as much advantage of this as possible. The passage of a cold front accompanied by strong winds is usually very effective in sweeping away pollutants from an urban area, but during stagnant anticyclonic conditions concentrations may rise to high values, sometimes producing smog.

Once in the atmosphere, the primary pollutants undergo a number of chemical reactions, generating a secondary group of pollutants. For example, sulphur dioxide (SO2) combines with oxygen and suspended water droplets to produce sulphuric acid. This acid is harmful to organic tissues and is also very corrosive. Photochemical reactions are brought about by the action of sunlight: for example, sunlight acting on nitrogen oxides and organic compounds produces ozone (O3). Another toxic chemical produced by photochemical action is ethylene.

The harmful effects of atmospheric pollution on plant and animal life are manifold. For humans, many pollutants are irritant to the eyes and dangerous to the respiratory system. During the London smog of December 1962, more than 4,000 additional deaths occurred. Ozone in urban smog has a severe effect on plant tissues; atmospheric sulphuric acid has wiped out lichen growth in many urban areas of Britain. Lead and other toxic metal particles are a particular cause of concern for human health. In addition, pollution also causes many millions of pounds worth of damage to materials: limestone structures suffer greatly in certain British cities unless treated with preservatives.

The global effects of foreign particles in the atmosphere in altering radiation and heat balances is difficult to assess. There have always been major natural sources of particles in the atmosphere, including forest fires and large volcanic explosions. Careful monitoring by U.S. scientists of temperature trends and dust amounts at various heights in the atmosphere has led to the tentative conclusion that man’s contribution to atmospheric particles may have far-reaching effects on tropospheric processes – for example, on the rain-making mechanism – but perhaps little effect on processes in the stratosphere.

(Bryant, Richard H.. Physical Geography Made Simple, Oxford, 1993)

Date: 2014-12-22; view: 1039