The heart of the matter with which we are dealing is, given a source emitting a pol- lutant, can we estimate, by calculation, the ambient concentration of that pollutant at a given receptor point? To make the calculation, it is obvious that we must have a well-defined source and that we must know the geographic relation between the source and the receptor. But we must understand the means of transport between the source and the receptor, as well. Thus, source-transport-receptor becomes the tril- ogy, which we must quantitatively define in order to make the desired computation.
4.6.1 SOURCE
Defining the source is a difficult matter in most cases. We need to consider first whether it is mobile or stationary and then whether it is emitted from a point, in a line, or more generally from an area. Then we must determine its chemical and physical properties. The properties can be determined most appropriately by sampling and analysis, when possible. It is then that we turn, for example, to estimation by a mass balance to determine the amount of material lost as pollutant. The major factors that we need to know about the source are as follows:
1. Composition, concentration, and density 2. Velocity of emission
4. Pressure of emission
5. Diameter of emitting stack or pipe 6. Effective height of emission
From these data, we can calculate the flow rate of the total stream and of the pollutant in question.
4.6.2 TRANSPORT
Understanding transport begins with three primary factors that affect the mixing action of the atmosphere: radiation from the sun and its effect at the surface of the earth, rotation of the earth, and the terrain or topography and the nature of the surface itself. These factors are the subject of basic meteorology.
The way in which atmospheric characteristics affect the concentration of air pollutants after they leave the source can be viewed in three stages:
1. Effective emission height 2. Bulk transport of the pollutants 3. Dispersion of the pollutants 4.6.2.1 Effective Emission Height
After a hot or buoyant effluent leaves a properly designed source, such as a chim- ney, it keeps on rising. The higher the plume goes, the lower will be the resultant ground-level concentration. The momentum of the gases rising up the chimney initially forces these gases into the atmosphere. This momentum is proportional to the stack gas velocity. However, stack gas velocity cannot sustain the rise of the gases after they leave the chimney and encounter the wind, which eventually will cause the plume to bend over. Thus, mean wind speed is a critical factor in determining plume rise. As the upward plume momentum is spent, further plume rise is dependent upon the plume density. Plumes that are heavier than air will tend to sink, while those with a density less than that of air will continue to rise until the buoyancy effect is spent. The buoyancy effect in hot plumes is usually the predominate mechanism. When the atmospheric temperature increases with altitude, an inversion is said to exist. Loss of plume buoyancy tends to occur more quickly in an inversion. Thus, the plume may cease to rise at a lower altitude, and be trapped by the inversion.
Many formulas have been devised to relate the chimney and the meteorological parameters to the plume rise. The most commonly used model is credited to Briggs.
See Reference 2 in Chapter 8 for a detailed discussion of the models including Briggs model. The plume rise that is calculated from the model is added to the actual height of the chimney and is termed the effective source height. It is this height that is used in the concentration-prediction model.
4.6.2.2 Bulk Transport of Pollutants
Pollutants travel downwind at the mean wind speed. Specification of the wind speed must be based on data usually taken at weather stations separated by large distances.
Since wind velocity and direction are strongly affected by the surface conditions, the nature of the surface, predominant topologic features such as hills and valleys, and the presence of lake, rivers, and buildings, the exact path of pollutant flow is difficult to determine. Furthermore, wind patterns vary in time, for example, from day to night.
The Gaussian concentration model does not take into account wind speed variation with altitude, and only in a few cases are there algorithms to account for the varia- tion in topography. For the future, progress in modeling downwind concentrations will come through increased knowledge of wind fields and numerical solutions of the deter- ministic models.
4.6.2.3 Dispersion of Pollutants
Dispersion of pollutants depends on the mean wind speed and atmospheric turbu- lence. The dispersion of a plume from a continuous elevated source increases with increasing surface roughness and with increasing upward convective air currents.
Thus, a clear summer day produces the best meteorological conditions for disper- sion, and a cold winter morning with a strong inversion results in the worst condi- tions for dispersion.
4.6.3 RECEPTOR
In most cases, legislation will determine the ambient concentrations of pollutant to which the receptor is limited. Air-quality criteria delineate the effects of air pollution and are scientifically determined dosage-response relationships. These relationships specify the reaction of the receptor or the effects when the receptor is exposed to a particular level of concentration for varying periods of time. Air-quality standards are based on air-quality criteria and set forth the concentration for a given averag- ing time. Regulations have been developed from air-quality criteria and standards, which set the ambient quality limits. Thus, the objective of our calculations will be to determine if an emission will result in ambient concentrations, which meet air- quality standards that have been set by reference to air-quality criteria.
Usually, in addition to the receptor, the locus of the point of maximum concentra- tion, or the contour enclosing an area of maximum concentration, and the value of the concentration associated with the locus or contour should be determined. The short-time averages that are considered in regulations are usually 3 min, 15 min, 1 h, 3 h, or 24 h. Long-time averages are one week, one month, a season, or a year.
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