THE SOURCE–TRANSPORT–RECEPTOR PROBLEM

space–time, varying meteorological conditions on pollutant–transport, chemical reaction, and removal. It can be applied from around 100 ft downwind up to several hundreds of miles.

The American Meteorological Society/EPA Regulatory Model (AERMOD) is a refined model currently under development by EPA as a supplement to ISCST3 for regulatory purposes. By accounting for varying dispersion rates with height, refined turbulence based on planetary boundary layer theory, advanced treatment of turbulent mixing, plume height, and terrain effects, AERMOD improves the estimate of downwind dispersion. AERMOD, along with AERMET, a meteorological data pre-processor, and AERMAP, a terrain data prepre-processor, are state-of-the-art air quality models destined to become EPA’s regulatory model of choice.

A new version of the ISCST3 model known as ISC-PRIME has become avail-able. This model incorporates plume-rise enhancements and the next generation of building downwash effects. There are also a variety of specialized models for accidental release modeling, roadway modeling, offshore sources, and regional trans-port modeling.

4.6 THE SOURCE–TRANSPORT–RECEPTOR PROBLEM

The heart of the matter with which we are dealing is, given a source emitting a pollutant, 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 trilogy which we must quantitatively define in order to make the desired computation.

4.6.1 THE 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 sam-pling 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

1. Composition, concentration, and density 2. Velocity of emission

3. Temperature 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.

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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 The Effective Emission Height

After a hot or buoyant effluent leaves a properly designed source, such as a chimney, 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 dependant 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, credited to Briggs, will be discussed in a later section. 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 the 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 variation in topography. For the future, progress in modeling downwind

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concentrations will come through increased knowledge of wind fields and numerical solutions of the deterministic models.

4.6.2.3 Dispersion of the Pollutants

Dispersion of the pollutant depends on the mean wind speed and atmospheric turbulence. The dispersion of a plume from a continuous elevated source increases with increasing surface roughness and with increasing upward convective air cur-rents. Thus, a clear summer day produces the best meteorological conditions for dispersion, and a cold winter morning with a strong inversion results in the worst conditions for dispersion.

4.6.3 THE 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 averaging 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 concen-tration, or the contour enclosing an area of maximum concenconcen-tration, 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.

REFERENCES

1. Turner, D. B., A Workbook of Atmospheric Dispersion Estimates, 2nd ed., Lewis Publishers, CRC Press, Inc., Boca Raton, 1994.

2. Schnelle, Jr. K. B. and Dey, P. R., Atmospheric Dispersion Modeling Compliance Guide, McGraw-Hill, New York, 1999.

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