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4 meters

v = 1 meter per second v = 1 meter per second

4 meters

FIGURE 4.2 (a) Effect of wind speed on pollutant concentration from constant source; (b ) Effect of variability of wind direction on pollutant concentration from constant source (continuous emission of 4 units/sec).

Turbulent motions are induced in the air flow in two ways: by thermal con-vective currents resulting from heating from below (thermal turbulence) and by disturbances or eddies resulting from the passage of air over irregular, rough ground surfaces (mechanical turbulence).

It may be generally expected that turbulent motion and, in turn, the dispersive ability of the atmosphere would be greatly enhanced during a period of good solar heating and over relatively rough terrain.

Another characteristic of the wind that should be noted is that wind speed generally increases with height in the lower levels. This is due to the decrease with height of the "frictional drag" effect of the underlying ground surface features.

Stability and Instability The stability of the atmosphere is its ability to enhance or suppress vertical air motions. Under unstable conditions the air motion is enhanced, and under stable conditions the air motion is suppressed. The conditions are determined by the vertical distribution of temperature.

In vertical motion, parcels of air are displaced. Due to the decrease of pressure with height, a parcel displaced upward will encounter decreased pressure and expand. If this expansion process is relatively rapid or over a large area so that there is little or no exchange of heat with the surrounding air or by a change of state of water vapor, the process is dry adiabatic and the parcel of air will be cooled. Likewise, if the displacement is downward so that an increase in pressure and compression is experienced, the parcel of air will be heated.

The rate of cooling of a mass of warm dry air in a dry environment with height is the dry adiabatic process lapse rate and is approximately —5.4°F/1,000 feet (—1°C/100m). The normal lapse rate (cooling) on the average is —3.5°F/1,000 feet (—0.65°C/100m). This relationship holds true in the troposphere up to about 10 km (6 miles). Temperature increases above this level in the stratosphere.

The prevailing or environmental lapse rate is the decrease of temperature with height that may exist at any particular time and place. It can be shown that if the decrease of temperature with height is greater than —5.4°F/1,000 feet, parcels displaced upward will attain temperatures higher than their surroundings. Air parcels displaced downward will attain lower temperatures than their surroundings. The displaced parcels will tend to continue in the direction of displacement. Under these conditions, the vertical motions are enhanced and the layer of air is defined as unstable.

Furthermore, if the decrease of temperature with height is less than —5.4°F/1,000 feet, it can be shown that air parcels displaced upward attain temperatures lower than their surroundings and will tend to return to their original positions. Air parcels displaced downward attain higher temperatures than their surroundings and also tend to return to their original position. Under these conditions, vertical motions are suppressed and the layer of air is defined as "stable."

Finally, if the decrease of temperature with height is equal to —5.4°F/1,000 feet, displaced air parcels attain temperatures equal to their surroundings and tend to remain at their position of displacement. This is called neutral stability.

Inversions Up to this point, the prevailing temperature distribution in the vertical has been referred to as a lapse rate, which indicates a decrease of temperature with height. However, under certain meteorological conditions, the distribution can be such that the temperature increases with height within a layer of air. This is called an inversion and constitutes an extremely stable condition.

There are three types of inversions that develop in the atmosphere: radiational (surface), subsidence (aloft), and frontal (aloft).

Radiational inversion is a phenomenon that develops at night under conditions of relatively clear skies and very light winds. The earth's surface cools by reradiating the heat absorbed during the day. In turn, the adjacent air is also cooled from below so that within the surface layer of air there is an increase of temperature with height.

Subsidence inversion develops in high-pressure systems (generally associated with fair weather) within a layer of air aloft when the air layer sinks to replace air that has spread out at the surface. Upon descent, the air heats adiabatically, attaining temperatures greater than the air below.

A condition of particular significance is the subsidence inversion that develops with a stagnating high-pressure system. Under these conditions, the pressure gradient becomes progressively weaker so that the winds become very light, resulting in a great reduction in the horizontal transport and dispersion of pollutants. At the same time, the subsidence inversion aloft continuously descends, acting as a barrier (lid) to the vertical dispersion of the pollutants. These conditions can persist for several days so that the resulting accumulation of pollutants can cause a serious health hazard.

Frontal inversion forms when air masses of different temperature characteristics meet and interact so that warm air overruns cold air.

There are many and varied effects of stability conditions and inversions on the transport and dispersion of pollutants in the atmosphere. In general, enhanced vertical motions under unstable conditions increase the turbulent motions, thereby enhancing the dispersion of the pollutants. Obviously, the stable conditions have the opposite effect.

For stack emissions in inversions—depending on the elevation of emission with respect to the distribution of stability in the lower layers of air—behavior of the plumes can be affected in many different ways. On the one hand, pollutants emitted within the layer of a surface-based (radiational) inversion by low stacks can develop very high and hazardous concentrations at the surface level. On the other hand, when pollutants are emitted from stacks at a level aloft within the surface inversion, the stability of the air tends to maintain the pollutant at this level, preventing it from reaching the surface. However, after sunrise and continued radiation from the sun resulting in heating of the earth's surface and adjacent air, the inversion is "burned off." Once this condition is reached, the lower layer of air becomes unstable and all of the pollutant that has accumulated at the level aloft is rapidly dispersed downward to the surface. This behavior is called "fumigation" and can result in very high concentrations during the period. See Figure 4.3.

Precipitation Precipitation constitutes an effective cleansing process of pollutants in the atmosphere in three ways: the washing out or scavenging of large particles by falling raindrops or snowflakes (washout), accumulation of small particles in the formation of raindrops or snowflakes in clouds (rainout), and removal of gaseous pollutants by dissolution and absorption.

The most effective and prevalent process is the washout of large particles, particularly in the lower layer of the atmosphere, where most of the pollutants are released. The efficiencies of the various processes depend on complex relationships between properties of the pollutants and the characteristics of the precipitation.

Topography

The topographic features of a region include both the natural (e.g., valleys, oceans, rivers, lakes, foliages) and manmade (e.g., cities, bridges, roads, canals) elements distributed within the region. These elements, per se, have little direct effect on pollutants in the atmosphere. The prime significance of topography is its effects on the meteorological elements. As stated previously, the variation in the distribution of land and water masses gives rise to various types of circulations.

FIGURE 4.3 Diurnal and nocturnal variation of vertical mixing. (Source: M. I. Weisburd, Field Operation and Enforcement Manual for Air Pollution, Vol. 1: Organization and Basic Procedures, U.S. Environmental Protection Agency, Office of Air Programs, Research Triangle Park, NC, 1972, p. 1.24.)

Of particular significance are the local or small-scale circulations that develop. These circulations can contribute either favorably or unfavorably to the transport and dispersion of the pollutants.

Along a coastline during periods of weak pressure gradient, intense heating of the land surface, as opposed to the lesser heating of the contiguous water surface, develops a temperature and pressure differential that generates an onshore air circulation. This circulation can extend to a considerable distance inland. At times during stagnating high-pressure systems, when the transport and dispersion of pollutants have been greatly reduced, this short-period afternoon increase in airflow may well prevent the critical accumulation of pollutants.

In valley regions, particularly in the winter, intense surface inversions are developed by the drainage down the slopes of air cooled by the radiationally cooled valley wall surfaces. Bottom valley areas that are significantly populated and industrialized can be subject to critical accumulation of pollutants during these periods.

The increased roughness of the surface created by the widespread distribution of buildings throughout a city can significantly enhance the turbulence of the airflow over the city, thereby improving the dispersion of the pollutants emitted.

But at the same time the concrete, stone, and brick buildings and asphalt streets of the city act as a heat reservoir for the radiation received from the sun during the day. This, plus the added heat from nighttime space heating during the cool months of the year, creates a temperature and pressure differential between the city and the surrounding rural area so that a local circulation inward to the city is developed. The circulation tends to concentrate the pollutants in the city. This phenomenon called the urban heat island effect.

Areas on the windward side of mountain ranges can expect added precipitation due to the forced rising, expansion, and cooling of the moving air mass with resultant release of available moisture. This increased precipitation serves to increase the removal of the pollutants.

It is apparent, then, that topographical features can have many and diverse effects in the meteorological elements and the behavior of pollutants in the atmosphere.

Renewable Energy 101

Renewable Energy 101

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.

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