Control Emmissions Technologies - Transport & Dispersion of Air Pollutants

The transport and dispersion of air pollutants in the ambient air are influenced by many complex factors. Global and regional weather patterns and local topographical conditions affect the way that pollutants are transported and dispersed. For example, the prevailing direction for weather patterns in the United States is from west to east and this is an important factor in the transport of pollutants that contribute to acid rain. Weather refers to the state of the atmosphere at a given time and place.

Figure:Temperature inversions

On a more local level, the primary factors affecting transport and dispersion of pollutants are wind and stability. Wind is the natural horizontal motion of the atmosphere. It occurs when warm air rises, and cool air comes in to take its place. Wind is caused by differences in pressure in the atmosphere. The pressure is the weight of the atmosphere at a given point. The height and temperature of a column of air determines the atmospheric weight. Because cool air weighs more than warm air, a high pressure mass of air is made up of cool and heavy air. Conversely, a low pressure mass of air is made up of warmer and lighter air. Differences in pressure cause air to move from high pressure areas to low pressure areas, resulting in wind. Wind speed can greatly affect the pollutant concentration in a local area. The higher the wind speed, the lower the pollutant concentration. Wind dilutes pollutants and rapidly disperses them throughout the immediate area.

Atmospheric stability refers to the vertical motion of the atmosphere. Unstable atmospheric conditions result in a vertical mixing. Typically, the air near the surface of the earth is warmer in the day time because of the absorption of the sun's energy. The warmer and lighter air from the surface then rises and mixes with the cooler and heavier air in the upper atmosphere causing unstable conditions in the atmosphere. This constant turnover also results in dispersal of polluted air. Stable atmospheric conditions usually occur when warm air is above cool air and the mixing depth is significantly restricted. This condition is called a temperature inversion. During a temperature inversion, air pollution released into the atmosphere's lowest layer is trapped there and can be removed only by strong horizontal winds. Because high-pressure systems often combine temperature inversion conditions and low wind speeds, their long residency over an industrial area usually results in episodes of severe smog.

The dispersion of pollutants from a source is also influenced by the amount of turbulence in the atmosphere near the source. Turbulence can be created by both the horizontal and vertical motion of the atmosphere. Other basic meteorological factors that affect concentration of air pollutants in the ambient air are: solar radiation, precipitation, and humidity. Solar radiation contributes to the formation of ozone and acts to create secondary pollutants in the air. Humidity and precipitation can also act on pollutants in the air to create more dangerous secondary pollutants, such as the substances responsible for acid rain. Precipitation can also have a beneficial effect by washing pollutant particles from the air and helping to minimize particulate matter formed by activities such as construction and some industrial processes.

Because of the factors responsible for the transport and dispersion of pollutants, air pollution produced in the United States Midwest can have adverse effects on lakes and forests in the East coast of the country. On the other hand, large cities bordered by complex topography, such as valleys or mountain ranges, often experience high concentrations of air pollutants because of the natural barrier that interrupts pollution dispersion. Los Angeles, Denver, and Mexico City are examples of cities located in basins bordered by mountain ranges. These cities experience high levels of air pollution influenced by the topography of the surrounding area. Although the overall causes of their respective pollution problems are complex, they are examples of situations where natural factors result in higher pollutant concentrations.

Although little can be done to control the natural forces that create these problems, there are some techniques that are used to help disperse pollutants. The most familiar way to disperse air pollutants is to use a smokestack. Smokestacks are tall industrial chimneys often used as a symbol for air pollution. Smokestacks are usually designed with the surrounding community in mind. Their function is to release pollutants high enough above the earth's surface so that emitted pollutants can sufficiently disperse in the atmosphere before reaching ground level. The higher the stack, the greater the likelihood that pollutants will be dispersed and diluted before affecting nearby populations.

The visible flow of pollutants from a stack is referred to as the plume. The height of the plume is influenced by the velocity and the buoyancy of the gases emitted from the stack. Often, heat energy is added to the stack gases to increase the height of the plume. This is referred to as plume rise and allows air pollutants emitted in this gas stream to be lofted higher in the atmosphere. Since the plume is higher in the atmosphere and at a further distance from the ground, the plume will disperse more before it reaches ground level.

Figure:Plume aerodynamic downwash caused by stack height and the immediate surroundings of the stack.

The shape of the plume is affected by the stack height and the immediate surroundings of the stack. As air moves over and around buildings and other structures, turbulent wakes are formed. Depending upon the release height of a plume (stack height) it may be possible for the plume to be pulled down into this wake area. This is referred to as aerodynamic or building downwash of the plume and can lead to elevated pollutant concentrations immediately downwind of the source. On the other hand, stability conditions in the atmosphere also will cause changes in the plume behavior. Stable conditions will cause the plume to be "flat" while instable conditions will cause the plume to "roll" or "loop".

Pollutants emitted from smokestacks can be transported over long distances. In general, the concentration of the pollutant decreases as it travels from the point of release and is dispersed by wind and other natural sources. Weather patterns influence the general direction and dispersion of the pollutant. As we mentioned before, pollutants released in the Midwest affect the population and the natural habitat in the East. Weather patterns are also the reason why pollution problems such as acid rain are issues of regional and international concern.

Finally, pollutant dispersion and transport can be negatively affected by climate and geographical factors. An example of how climate can affect pollutant dispersion is temperature inversion. An inversion can prevent the rise and dispersal of pollutants from the lower layers of the atmosphere and cause a localized air pollution problem like the episodes that took place in London, England and Donora, Pennsylvania.