Explain how thermal inversions contribute to increased pollution concentration.

The dispersion of the effluent gases that exit a chyminey occurs by 2 major processes:

1) Mixing due to atmospheric turbulence

2) Mixing due to temperature gradient.

The flue gases that are emitted from stacks usually rise and expand thus releasing heat and reducing their temperature at a rate known as adiabatic lapse rate. The rate at which the temperature changes with height is known as environmental lapse rate which is generally negative in the troposphere. Any plume gas can rise only if it's density is lesser than that of the surrounding atmosphere which is achieved if it's temperature is more than than the effluent gas.

Due to various causes a temperature inversion can occur in which as we increase our altitude the temperature of the air increases. This phenomenon can be attributed to differential increase in the temperatures of land and air. As a result of this phenomenon the mixing of the gases cannot occur and the gases that emit from a chyminney form a plume in shape of blade of a ceiling fan thus the plume is known as fanning plume. The processes that cause the dispersion of the pollutants are not set thus we get a increased concentration of pollutants in the air.