Tuesday, March 8, 2011

Horizontal transport of air pollutants


The horizontal transport of air pollutants has been a function of the wind speed and direction, air turbulence, and the topography. Wind movements, like other weather events, could be divided into two classes, depending on their magnitude. Both the large scale (macro meteorology) and small scale (micro meteorology) events find importance in air pollution studies.
Macro meteorological events encompass the movement of large masses of air brought about by the differences in temperature and pressure and the rotation of the earth. Basic movements of air have been the heating of air masses at the equator and cooling at the poles, with a consequent north-south circulation. The rotation of the earth from west to east tends to drag the atmosphere along with the surface so that the wind in the Northern Hemisphere tends to curve toward the right and those in the Southern Hemisphere toward the left.Because of this winds in the United States are generally from west to east. The condition of the atmosphere at any one time has been i8mportant in weather forecasting and is usually shown on a weather map. These maps are based on measurements and observations at many stations throughout the country. These measurements have been including atmospheric pressure, temperature, humidity, wind speed and direction, amount types etc.
 Even a cursory examination of these maps will reveal weather atmospheric conditions in a given area have been likely to be favorable or unfavorable for dispersion of pollution.  The wind velocities shown on the map have been those of the gradient wind, which has been due to the rotation of the earth. These winds are generally along the isobars and at a height (2000ft and above) sufficiently great to be unaffected by surface friction. Flow is clockwise around high-pressure areas and counterclockwise around low-pressure areas. Near the ground, friction affects the speed and direction of the gradient wind so that surface winds generally blow between 20 and 30 across the isobars towards the low-pressure center and at half of the speed of the gradient wind. In addition to large scale effects of air movements, those of a micro meteorological nature must not be overlooked. Local circulation and temperature variations in valleys and on the slopes of hills and mountains are extremely important from an air pollution viewpoint; similarly, the presence of buildings and type of ground cover affect air movement.
  1. Frontal movements:  The migration of regions of high air pressure and low pressure has been responsible for the variations in day to day weather. The formation of low pressure systems includes the formation of a warm front and a cold a front.  In a cold front, the cold air has been moving over an area previously occupied by warm air. Warm fronts are where air has been overtaking and flowing over cold air
  2.  Effects of topography:
  • a)      Sea-land breezes: The difference in the air temperature over land and water caused by differences in the heating and cooling, causes a pressure gradient and accompanying air flow.
  • b)      Mountain valley: The differences in heating and cooling rates between the valley floor and sides can bring about a variation in the air density and pressure resulting in air flow.

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