What do you mean by the term Aerosols?

Particles called aerosols are ejected to the atmosphere by industries, power plants, automobiles agricultural operations and various other activities of man.

The maximum amount of aerosols are found over large urban centers and industrial cities, where they are produced by fossil fuel burning as well as the creation of solid particles from unburned hydrocarbons by photochemical reactions in the presence of ultra-violet radiation.

The burning of fossil fuels like coal or petroleum produces soot and sulphur dioxide. Sulphur dioxide in a short period of time is converted into sulphate particles. Sulphates and hydrocarbon particles are smaller than soot or smoke particles.

Most of the man-made aerosols are produced as a result of the oxidation of sulphur dioxide and nitrogen oxides. Such particles as are the oxidation products of gaseous emissions comprise sulphuric and nitric acid. Most of the microscopic particulates present in the atmosphere are of ammonium sulphate or sulphuric acid.

It is undoubtedly true that the modern century has witnessed an unprecedented growth of industries, which in turn have ejected to the atmosphere enormous quantity of particulates.

This has caused turbidity over several urban centers located in a number of industrialized countries. However, observations made of aerosols at non-urban stations in the United States of America showed that there was no increase at 18 non-urban stations.

Volcanic activity has also produced aerosols. Increases in aerosols in the modern century have been observed near the surface in the western North Atlantic, western North Pacific and the Indian Ocean in the neighbourhood of India.

However, in the most developed countries like the U.S.A., the United Kingdom and a few others vigorous efforts have been made to reduce the amount of particulates and the results have been encouraging. But in most of the European countries and the Soviet Union there is an upward trend in the aerosols because of heavy industrialization.

Since the major sources of aerosols are based near the surface, the lower layers of the atmosphere are worst affected by them. The residence times of the suspended particles in the atmosphere depend on the size of the particles, the altitude of injection and several other minor factors.

Aerosols scatter as well as absorb sunlight, and they also absorb and re-radiate back long wave infra-red radiation. A part of the scattered radiation is sent back into space and a part of it is sent towards the earth’s surface.

As a result of such scattering of solar radiation by the suspended particles there is reduction in the amount of sunlight reaching the ground. Besides, this results in an increase in the net albedo of the earth-atmosphere system.

The ultimate result is a net cooling. Certain climatic models predict that if the present aerosol concentrations are doubled, there is a possibility that there would be a cooling of between 1 and 3°C. On the other hand, the absorption of solar radiation by the suspended particles heats them as well as the air around them.

This will reduce the net albedo. In fact, the reduction or increase in the net albedo depends on several factors such as the size distribution of suspended particles, the composition of suspended particles, the composition and concentration of aerosols, long wave absorption by the aerosols, and the presence of cloud cover.

However, recently it has been established that aerosols contribute to surface cooling. According to Mitchell, the climatic effects of aerosols are a slight decrease of cloudiness and precipitation. More over, it has been proved that the man-made aerosols disturb the radiation balance of the earth.