INTRODUCTION — METEOROLOGICAL BACKGROUND Reduction of ground-level concentrations from a point source can be accomplished

by elevation of the point of emission above the ground level. The chimney has long been used to accomplish the task of getting the smoke from fires out of the house and above the inhabitants’ heads. Unfortunately meteorological conditions have not cooperated fully, and, thus, the smoke from chimneys does not always rise up and out of the immediate neighborhood of the emission. To overcome this difficulty for large sources where steam is produced, for example, such as power plants and space heating boiler facilities, taller and taller stacks have been built. These tall stacks do not remove the pollution from the atmosphere, but they do aid in reducing ground-level concentrations to a value low enough so that harmful or damaging effects are minimized in the vicinity of the source.

4.1.1 INVERSIONS

Inversions are the principal meteorological factor present when air pollution episodes are observed. They can be classified according to the method of formation and according to the height of the base, the thickness, and the intensity. An inversion may be based at the surface or in the upper air.

4.1.1.1 Surface or Radiation Inversions

A surface inversion usually occurs on clear nights with low wind speed. In this situation the ground cools rapidly due to the prevalence of long-wave radiation to the outer atmosphere. Other heat transfer components are negligible which means the surface of the earth is cooling. The surface air becomes cooler than the air above it, and vertical air flow is halted. In the morning the sun warms the surface of the earth, and the breakup of the inversion is rapid. Smoke plumes from stacks are quite often trapped in the radiation inversion layer at night and then brought to the ground in a fumigation during morning hours. The result is high ground-level concentration.

4.1.1.2 Evaporation Inversion

After a summer shower or over an irrigated field, heat is required as the water evaporates. The result is a transfer of heat downward, cooling the upper air by convection and forming an evaporation inversion.

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4.1.1.3 Advection Inversion

An advection inversion forms when warm air blows across a cooler surface. The cooling of the air may be sufficient to produce fog. When a sea breeze occurs from open water to land, an inversion may move inland, and a continuous fumigation may occur during the daytime.

4.1.1.4 Subsidence Inversion

In Los Angeles, the typical inversion is based in the upper air. This inversion results from an almost permanent high pressure area centered over the north Pacific Ocean near the city. The axis of this high is inclined in such a way that air reaching the California coast is slowly descending or subsiding. During the subsidence, the air compresses and becomes warmer, forming an upper-air inversion. As the cooler sea breeze blows over the surface, the temperature difference increases, and the inversion is intensified. It might be expected that the sea breeze would break up the inversion but this is not the case. The sea breeze serves only to raise and lower the altitude of the upper air inversion.

4.1.2 THE DIURNAL CYCLE

On top of the general circulation a daily, or 24-hour cycle, referred to as the diurnal cycle is superimposed. The diurnal cycle is highly influenced by radiation from the sun. When the sun appears in the morning, it heats the earth by radiation, and the surface of the earth becomes warmer than the air above it. This causes the air immediately next to the earth to be warmed by convection. The warmer air tends to rise and creates thermal convection currents in the atmosphere. These are the ther-mals which birds and glider pilots seek out, and which allow them to soar and rise to great altitudes in the sky.

On a clear night, a process occurs which is the reverse of that described above.

The ground radiates its heat to the blackness of space, so that the ground cools off faster than the air. Convection heat transfer between the lower air layer and the ground causes the air close to the ground to become cooler than the air above, and a radiation inversion forms. Energy lost by the surface air is only slowly replaced, and a calm may develop.

These convection currents set up by the effect of radiant heat from the sun tend to add or subtract from the longer-term mixing turbulence created by the weather fronts. Thus, the wind we are most familiar with, the wind close to the earth’s surface, tends to increase in the daytime and to die down at night.

There are significant diurnal differences in the temperature profiles encountered in a rural atmosphere and those in an urban atmosphere. On a clear sunny day in rural areas, a late afternoon normal but smooth temperature profile with temperatures decreasing with altitude usually develops. As the sun goes down, the ground begins to radiate heat to the outer atmosphere, and a radiation inversion begins to build up near the ground. Finally by late evening, a dog-leg shaped inversion is firmly established and remains until the sun rises in the early morning. As the sun begins to warm the ground, the inversion is broken from the ground up, and the temperature

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profile becomes “z” shaped. Smoke plumes emitted into the atmosphere under the late evening inversion tend to become trapped. Since vertical mixing is very poor, these plumes remain contained in very well-defined layers and can be readily observed as they meander downwind in what is called a fanning fashion. In the early morning as the inversion breaks up, the top of the thickening normally negative temperature gradient will encounter the bottom edge of the fanning plume. Since vertical mixing is steadily increasing under this temperature profile, the bottom of the fanning plume suddenly encounters a layer of air in which mixing is relatively good. The plume can then be drawn down to the ground in a fumigation which imposes high ground-level concentrations on the affected countryside.

A similar action is encountered in the city. However, in this case, due to the nature of the surfaces and numbers of buildings, the city will hold in the daytime heat, and thus the formation of the inversion is delayed in time. Furthermore, the urban inversion will form in the upper atmosphere which loses heat to the outer atmosphere faster than it can be supplied from the surfaces of the city. Thus, the evening urban inversion tends to form in a band above the ground, thickening both toward the outer atmosphere and toward the ground. Smoke plumes can be trapped by this upper air radiation inversion, and high ground concentrations will be found in the early morning urban fumigation.

4.1.3 PRINCIPAL SMOKE-PLUME MODELS

Even though the objective of air pollution control is to reduce all smoke emissions to nearly invisible conditions, some visible plumes are likely to be with us for quite a long while. Visible plumes are excellent indicators of stability conditions. Five special models have been observed and classified by the following names:

1. Looping 2. Coning 3. Fanning 4. Fumigation 5. Lofting

All of these types of plumes can be seen with the naked eye. A recognition of these conditions is helpful to the modeler and in gaining an additional understanding of dispersion of pollutants.

In the section immediately preceding this one, the condition for fanning followed by fumigation has been described. Lofting occurs under similar conditions to fumi-gation. However, in this case the plume is trapped above the inversion layer where upward convection is present. Therefore, the plume is lofted upwards with zero ground-level concentration resulting. When the day is very sunny with some wind blowing, radiation from the ground upward is very good. Strong convection currents moving upward are produced. Under these conditions plumes tend to loop upwards and then down to the ground in what are called looping plumes.

When the day is dark with steady relatively strong winds, the temperature profile will be neutral so that the convection currents will be small. Under these conditions the plume will proceed downwind spreading in a cone shape. Hence the name coning

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plume is applied. Under these conditions dispersion should most readily be described by Gaussian models.