The pollution which is characterised by presence of large quantities of oxides of sulphur, nitrogen, smoke and fumes is known as reducing type of pollution.
Burning of fossil fuels, smelting of metallic ores and various industrial operations yield a large amount of particulate materials as; well as oxides of sulphur, carbon and nitrogen. The gaseous mixture and particulate material, under normal conditions are carried up and dispersed in the atmosphere causing little harm to the living beings.
However, in cases when there is an abrupt cooling of the lower layers of air while upper layers remain warm, the temperature profile of the atmosphere becomes inverted. Lower layers become heavier than the upper layers. Pollutants discharged into the atmosphere tend to sink down and stay near the surface. As a consequence a high concentration of pollutants gradually builds up in the lower layers of the atmospheric air.
As temperature drops water vapours condense on the surface of fine particles and form tiny liquid droplets. An accumulation of such small droplets results in formation of fog or smog, which due to still atmosphere stays near the surface of earth and forms a blanket enveloping the locality.
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Various gases, fumes and vapours dissolve or adsorb on to the fine film which is thus formed around the tiny particles. Of these gases sulphur dioxide is the most important, though other gases like oxides of nitrogen, carbon, ammonia, various hydrocarbons etc. are also there.
The tiny solid material inside the aerosol may consist of oxides of various metals and other elements which act as catalytic agents for various atmospheric reactions occurring on the aerosol surface. Sulphur dioxide is firs converted to the corresponding acid and then to sulphates upon reacting with ammonia or ammonium hydroxide bearing aerosols, ammonium sulphates and bisulphates may be formed. Likewise nitrogen dioxide is changed to corresponding acid and then to nitrates. Thus secondary aerosols are formed which bear sulphates, nitrates and acids like sulphuric and nitric acids.
The occurrence and extent of these reactions are dependent upon the quantities of the reactants and the presence of aerosols capable of providing suitable locus and the catalyst. The first documented case of this type of pollution occurred in Meuse Valley, Belgium, in 1930. The second took place in Donora, Pennsylvania in 1948, while the third and the most disastrous one took place in London in 1952.
These instances had much in common. Rapid cooling of surface layers of air prevented the normal mixing of pollutants which occurs due to the warmer layers rising up to be replaced by the upper cooler layers which sink down. Innumerable sources kept on adding various pollutants such as sulphur dioxide smoke, fumes and oxides of carbon, nitrogen etc. as the main fuel for domestic heating and power generation was coal.
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These conditions lasted for many days at a stretch during which time sufficiently high concentration of these pollutants was built up. Various atmospheric reactions occurring on the surface of aerosols caused the formation of more toxic secondary aerosols. The concentration of acids, sulphates, nitrates, oxides of carbon, hydrocarbons etc. became sufficiently high to cause adverse effects at least on the weak and infirm. Though in Meuse valley only 65 and in Donora 20 deaths were recorded, in London disaster about 95,000 people were affected and by the time fog lifted about 4000 deaths had occurred.
As hydrothermal and nuclear power has replaced coal to a varying extent in large crowded industrial cities of the developed world, the menace of reducing type of pollution has been on the decline. However, in many developing countries like India, coal, mineral oils and organic matter continues as an important source of energy and thermal power plants still use about half of the total national output of coal. The chances of occurrence of episodes of London type are far greater in rapidly expanding Indian cities as compared to developed western countries.