Tuesday, March 8, 2011

Vertical transport of air pollutants


 The change in air temperature with change in pressure, and hence with altitude, has been an important factor in the vertical transport and dilution of air pollutants. In mixed air, the dry adiabatic lapse rate has been 5.4 per 1,000 ft, or in other words, the air temperature decreases 5.4 for each 1000 ft, increase in latitude. Generally the lapse rate of the atmosphere has been greater or less than adiabatic. When the atmospheric lapse rate has been less than adiabatic, the atmosphere is more stable and vertical transport is hindered. A negative lapse rate or inversion causes very stable air. There has been two general types of inversions-radiation and subsidence.  
  1. Radiation inversion: The radiation inversion usually takes place at night, the earth surface cooling by radiation at a faster rate then the air, causing the air near the ground being cooler than the air above. Radiation inversions take place frequently, more than two-thirds of the night of the year in some locations of the United States. The fog layers in valleys observed during the morning have been the visible evidence of radiation inversions.
  2. Subsidence inversion: Subsidence inversion has been caused by the sinking motion in high pressure areas and generally has decreasing humidities above the inversion base. The height of subsidence inversions varies from the surface to about 5000 feet.
  3. Unstable air: If the air temperature decreases more rapidly than the dry adiabatic lapse rate, the air is super adiabatic and quite unstable. In unstable air, there is substantial vertical mixing of the air and air pollutants are transported vertically and dispersed rapidly.
The change in air temperature with pressure, and hence with altitude, has been regarded as an important consideration in the incidence of air pollution. In well-mixed air the dry adiabatic lapse rate has been 5.4 per 1000 ft. In other words, the air temperature decreases 5.4for each 1000 ft above the earth’s surface. When this condition has been extant, a smoke plume will rise directly into the atmosphere until it reaches air of similar density, in turn occasioned by high temperature until it reaches air of similar density. The two most important conditions from an air pollution standpoint have been super adiabatic lapse rate and the negative lapse rate. On a summer day, rapid heating of the earth by the sun warms the air near the surface to the point where the lapse rate has been super adiabatic. The decrease in air temperature with height has been greater than the normal adiabatic lapse rate. Under this condition the atmosphere has been said to be in unstable equilibrium, and marked vertical mixing of the air results. It is a condition where pollutants get dispersed rapidly. The opposite condition has been a negative lapse rate or temperature inversion, in which case the air temperature increases with height. It may be brought about by cooling of the air near the ground because of the earth’s losing heat by long wave radiation at night.

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