Concentration of pollutants over a source area rises to its highest levels when the vertical mixing (convection) of the air is inhibited by a stable configuration of the vertical temperature profile of the air mass.
When the normal environmental lapse rate of 3.5°F per 1000 ft (0.6°C per 1(X) m) is present, there is resistance to mixing by vertical movements. Consider next that the environmental lapse rate is steepened by heating of an air layer near the ground, because of excess heat radiated and conducted from hot pavements and rooftops. When the temperature gradient (lapse rate) of the heated air becomes greater than the dry adiabatic rate of 5.5 F° per 1000 ft (1.0C° per 100 in), a condition of instability exists and a bubble of warm air can begin to rise, like a helium-filled balloon. Closed at the dry adiabatic rate, the temperature of the rising bubble then falls faster than does the' temperature of the surrounding air. When the bubble has reached an altitude at which its temperature (and therefore also its density) matches that of the surrounding air, it can rise no further and convection ceases.
Now suppose that instead of the bubble of warm air we substitute the hot air from a smokestack. The resultant rise follows essentially the same pattern, although initially faster and in the form of a vertical jet. Carrying up with it the pollutants of combustion, the rising hot air gradually cools and reaches a level of stability, where it spreads laterally. Cooling by long-wave radiation and mixing with the surrounding air will re-enforce the adiabatic cooling, since a truly adiabatic system would not be realistic in nature.
Recall that at night when the air is calm and the sky clear, rapid cooling of the ground surface typically produces a low-level temperature inversion. In cold air, the reversal of the temperature gradient may extend hundreds of feet into the air. A low-level temperature inversion represents an unusually stable air structure. When this type of inversion develops over an urban area, conditions are particularly favorable for entrapment of pollutants to the degree that heavy smog or highly toxic fog can develop. The upper limit of the inversion layer coincides with the cap, or lid, below which pollutants are held. The lid may be situated at a height of perhaps 500 to 1000 ft (150 to 300 m) above the ground.
Related to the low-level inversion, but caused in a somewhat different manner, is the upper-level inversion. Recall that anticyclones are cells of subsiding air that diverges at low levels. Within the centre of the cell, winds are calm or very gentle. As the air subsides, it is adiabatically warmed, so that the normal temperature lapse rate is displaced to the right in the temperature-altitude graph. Below the level at which subsidence is occurring, the air layer remains stagnant. The temperature curve consequently develops a kink in which a part of the curve shows an inversion. For reasons already explained, the layer of inverted temperature structure strongly resists mixing and acts as a lid to, prevent the continued upward movement and dispersal of pollutants.
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