Inversion takes place in the atmosphere due to two reasons:
1. Radiation inversion
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2. Subsidence inversion.
Radiation Inversion:
The air near the surface cools during night by long wave radiation. Thus, the temperature is lower near the ground than at a height. This is called radiation inversion. When the sun rises and heats air near the ground, the inversion disappears.
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Subsidence Inversion:
This kind of inversion results when the upper layer of air descends (subsides) during a developing anticyclone (high pressure centres). The subsiding air warms as it contracts but the warming will be greater at the upper part than at the lower part near the ground.
Why Inversion is Important. When inversion develops, the atmosphere is said to be stable. The dense cold air lies at the bottom and has no natural tendency to rise.
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If a hot plume is omitted from a chimney, it will rise to some height, cooling as it goes up, but soon the temperature of the plume will stabilize at the temperature of the surrounding air and there will, then, be no driving force for the air to go higher. The pollutants will, thus, concentrate in the atmosphere instead of being dispersed, and this will increase the concentration and will be very harmful to mankind.
In the case of high level inversion, the lower inversion layer will form a lid for pollutants, which will cause them to accumulate and, thus, raise the concentration.
Many of the episodes of air pollution which have taken place in the world are due to inversion. Details of the famous London smog of 1952 that killed 4000 people and lasted for 5 days are given which are situated in valleys, this kind of phenomenon, where the dense air, filled with pollutants, slips down to the valleys and forms a lake of polluted air, is a very harmful situation.
The problem of air pollution takes a serious dimension when climatic conditions are adverse to its effective dispersion. Obviously, in India, climatic conditions are favorable to dispersion in most of regions and almost 90 percent of the time.
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The possibility of inversion is also much less than in cold countries but the meteorological conditions, especially the direction, could be better utilized for pollutant dispersion, which is not the case at present. However, the atmosphere cannot be used indefinitely for dumping pollutants.
One of the unfavourable factors is the absence of high grade coal. The Indian coal available for combustion/gasification consists of a fairly large amount of ash moisture as well as volatile matters.
The equipment used for combustion, both for power generation as well as in process industry, is not based on modem technology, resulting in poor combustion efficiency and, thus, a higher percentage of carbon mono-oxide in the effluent.
In this regard, a special mention could be made of fluidized bed combustion which has higher combustion efficiency than other conventional equipments.