Man introduces a large amount of gaseous wastes and fine particulate material into the atmosphere. Meteorological conditions play an important role in the dilution and dissemination of these wastes in the atmosphere.
Even in badly polluted localities there are times when the atmosphere appears quite clean. These fluctuations are not caused by gross changes in the emission of pollutants but are due to the variations in meteorological conditions of the environment such as temperature, pressure and humidity etc. (Menon 1989, Anonymous 1970)
1. Dilution of pollutants in the atmosphere:
It is the actual temperature gradient in the troposphere which determines the extent of mixing or dilution of gaseous material introduced into the atmosphere. All gases when transferred from higher pressure to a zone of lower pressure expand and undergo cooling. Similarly, contraction and warming occurs if they are transferred from a zone of lower pressure to higher pressures.
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As atmospheric pressure decreases with height, movement of a parcel of the introduced gas upwards causes its expansion and thereby lowering of its temperatures. Likewise, displacement downwards results in its contraction and warming. This happens adiabatically, i.e., no heat is either withdrawn or provided to the system.
The decline or lapse in temperature of a dry gas is 1°C for every 100 metres of height covered and is known as Dry Adiabatic Lapse Rate (DALR). It is a theoretical value calculated for the pressure differences which exist between the altitudes 100 metres apart. Any gaseous pollutant tends to expand and cool down by 1°C if it is moved 100 metres upward from its original position.