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.

Atmosphere

The atmosphere serves as a supply of oxygen, a media for waste disposal, a filter of solar radiation, a heat transport media, and a water transport media. The atmosphere has been the layer of air that surrounds the earth. It is probably 100 miles thick, but more than one-half of its total weight is in the four miles just above the surface of the earth. The upper portion of the atmosphere is termed as the stratosphere; the lower portion, the troposphere. The stratosphere has been an isothermal region (-60 in polar regions and -90 in the tropic region) where clouds of vapor do not form and where little convection (air movement caused by temperature differences) takes place. Conversely the troposphere varies in temperature from the surface of the earth to the stratosphere’ it is having clouds of water vapor; considerable air movement, caused by temperature differences, occurs. Generally the troposphere has been 10 to 11 miles deep at the equator and only 4.5 miles deep at the north and south poles.

Applications of Meteorology to air pollution: Every air pollution problem is having three requisites: 

1. There must occur an emission of the pollutant or its precursor into the free atmosphere. 
2. After emission, it must get confined to a restricted volume of air
3. The polluted air must interface with the physical, mental or social well-being of people.

Emissions of pollutants are generally the same from day to day; the weather is the variable that triggers the air pollution episode. We cannot do much about the weather, but we can understand hoe it affects the air pollution problem if we are having knowledge of meteorology, which is that the branch of physics that treats of the atmosphere and its phenomena. Then we can apply this knowledge to solve the problem. The main areas of application of meteorology to air pollution control have been:

1.    Determination of allowable emission rates.
2.  Planning and interpreting air pollution surveys
3.  Stack design.
4.  Plant-site selection.
5.   Prediction of the air pollution potential of an area.

Movement of pollutants: Any factor that restricts the movement of air will of course prevent the movement and dispersion of pollutants entering the atmosphere. A very important factor in producing a stable atmosphere has been a negative temperature lapse rate, or atmosphere inversion. Two types of inversion are important. The radiation inversion takes place, usually at night, when the earth’s surface loses heat by radiation, thus cooling the layer of air next to the ground. The subsidence inversion has been commonplace on the west coast of continents in the temperate latitudes. It does not get restricted to the west coasts of continents. Sutton has pointed out that the typical winter “gloom” of Great Britain is a result of the presence of an anticyclone, in which there is a general slow descent of air over a wide area. The importance of wind direction and speed on the dispersal of pollutants has been obvious, but the variations of these two parameters with time of day and season of the year have been an even greater factor.