The Pasquill Stability Classes

The Monin-Obukhov length L is not a parameter that is routinely measured. Colder (1972), however, established a relation between the stability classes of Pasquill, the roughness height zo (see Section VII,B), and L. The results of his investigation are shown in Fig. 4. Alternatively, the local wind speed and cloud cover measurements are used to estimate the Pasquill stability class (Table IV). In addition, Colder developed a nonogram for relating the gradient Richardson number R-, to the more easily determined bulk Richardson number / (, ... 

Table 1 gives the equivalence between the Monin-Obukhov lengthscale and the Pasquill stability classes. 

Table 1. The equivalence between the Monin-Obukhov lengthscale, L (expressed in terms of the dimensionless ratio z/L, where z (m) is the height above the surface) and the Pasquill stability classes (Jacobson, 1999)...

Use of the equations derived in this section requires estimation of the Monin-Obukhov length L. A number of approaches are available, including the profile and gradient methods using available measurements (Arya 1999). The simplest approach based on the Pasquill stability classes will be discussed in the next section. 

The most widely used ay and ct- correlations based on the Pasquill stability classes have been those developed by Gifford (1961). The correlations, commonly referred to as the Pasquill-Gifford curves, appear in Figures 18.4 and 18.5. 

FIGURE 18.4 Correlations for crv based on the Pasquill stability classes A to F (Gifford 1961). These are the so-called Pasquill-Gifford curves. 

Colder (1972) established a relation between the Pasquill stability classes, the roughness length zo and L (Figure 16.8). To simplify calculation of /L, Colder s plot can be approximated by the correlation... 

In addition to short-term emission estimates, normally for hourly periods, the meteorological data include hourly wind direction, wind speed, and Pasquill stability class. Although of secondary importance, the hourly data also include temperature (only important if buoyant plume rise needs to be calculated from any sources) and mixing height. 

As an example of the use of the Gaussian plume equations using the Pasquill-Gifford dispersion parameters, assume that a source releases 0.37 g s of a pollutant at an effective height of 40 m into the atmosphere with the wind blowing at 2 m s . What is the approximate distance of the maximum concentration, and what is the concentration at this point if the atmosphere is appropriately represented by Pasquill stability class B ... 

Numerous analyses of data routinely collected in the United States have been performed by the U.S. National Climatic Center, results of these analyses are available at reasonable cost. The joint frequency of Pasquill stability class, wind direction class (primarily to 16 compass points), and wind speed class (in six classes) has been determined for various periods of record for over 200 observation stations in the United States from either hourly or 3-hourly data. A computer program called STAR (STability ARray) estimates the Pasquill class from the elevation of the sun (approximated from the hour and time of year), wind speed, cloud cover, and ceiling height. STAR output for seasons and the entire period of record can be obtained from the Center. Table 21-2 is similar in format to the standard output. This table gives the frequencies for D stability, based on a total of 100 for all stabilities. 

Data for one full year (1964) for Nashville, Tennessee, and Knoxville, Tennessee, 265 km (165 mi) apart, were compared to determine the extent to which the frequencies of various parameters were similar. Knoxville is located in an area with mountainous ridges oriented southwest-northeast Nashville is situated in a comparahvely flat area. The data available are the number of hours during which each of 36 wind directions (every 10° azimuth) occurred, the average wind speed for each direction, the number of hours of each Pasquill stability class for each direchon, and the mean annual wind speed. 

Figure 21-9 is a stability wind rose that indicates Pasquill stability class frequencies for each direction. For this location, the various stabilities seem to be nearly a set proportion of the frequency for that direction the larger the total frequency for that direction, the greater the frequency for each stability. Since the frequencies of A and B stabilities are quite small (0.72% for A and 4.92% for all three unstable classes (A, B, and C) are added together and indicated by the single line. 

Table 1. The Pasquill Chart for Determining the Atmospheric Stability Class...

A case study is performed assuming an instantaneous release of the toxic liquid acrylonitrile from a rail tankwagon. After the release of the toxic liquid a pool of 600m is formed from which evaporation occurs, leading to a vapour cloud. This vapour cloud travels with the wind and disperses. The degree of dispersion is determined by the wind speed, the stability of the atmosphere and the surface roughness. The stability of the atmosphere is indicated by the pasquill-stabUity class. By day, the most common atmospheric stability class is class D and a wind speed of 5 m/s is assumed, this weather condition is abbreviated with D5. At night class F is the most common atmospheric stability, associated with a wind speed of 1.5 m/s this weather condition is abbreviated as FI.5. The evaporation and dispersion calculations are performed with EFFECTS 7.6. A neutral gas model is used for the dispersion calculations. 

Values of p have been found that range from 0.02-0.87. Table VI hsts values of p that can be used successfully up to aheightof200 m. This table reflects the effect of surface eonditions by preserrting values of p for urban and rural areas. Stability effeets are given according to the Pasquill-Gififord classes. 

Figure 5.2-6 shows the quantity Xj/0 along the plume centerline for effluent released at a height of 100 ft under Pasquill stability classes B, C, and D for a 6 mile/hr wind. %jJQ is also shown for a 2 mile/hr wind speed for stability class D. It will be observed that at reasonable distances from the plant % JQ decreases more or less exponentially. With the more unstable conditions (B), die maximum of occurs nearer the release point (within a few hundred meters), then drops rapidly to very low values. On the other hand, under more stable conditions (D), the peak of is located much further from the source. In the dispersion of effluents from nuclear power plants, the concentration of the effluent is therefore usually higher in the more important, populated offsite regions under stable conditions, and stable conditions are often... 

TABLE 26-28 Atmospheric Stability Classes for Use with the Pasquill-Gifford Dispersion Model... 

Critical GLC s can usually be calculated based on a unstable atmosphere, thus enabling the designer to determine a worst case scenario. For any given day, typical atmospheric stabihty data can usually be obtained from a local weather bureau, or may be estimated from the so-called Pasquill chart for the appropriate Atmospheric Stability Class (refer to Table 1). 

In the calculations that were made to predict ground level concentrations from a VCM reactor blow off, the Pasquill-Gifford-Holland dispersion model was used as a basis for these estimations. Calculations were made for six different stability classes and ground level concentrations, and at various distances from the point source of emission. 

From Figures 2 and 3, the Pasquill-Gifford dispersion coefficients are obtained for a downwind distance of 2000 meters and for atmospheric stability Class B. 

Suppose an interstate highway passes 1 km perpendicular distance from a nuclear power plant control room air intake on which 10 trucks/day pass carrying 10 tons bf chlorine each. Assume the probability of truck accident is constant at l.OE-8/mi, but if an accident occurs, the full cargo is released and the chlorine flashes to a gas. Assume that the winds are isotropically distributed with mean values of 5 mph and Pasquill "F" stability class. What is the probability of exceeding Regulatory Guide 1-78 criteria for chlorine of 45 mg/m (15 ppm). 

Atmospheric stability is frequently characterized in terms of the Pas-quill A-F stability classes (Pasquill, 1%1. The A-F categories refer to atmospheric conditions as follows A—very unstable, B—moderately un-... 

Substantial research on environmental pollution plume persistence and dispersion in air has produced a classification system called Pasquill s stability classes for plume stability in air. Unfortunately, the system does not apply as directly as we might wish since it applies at the much larger scale of stack exhausts and the like. However, some insight from the system is available. 

The Monin-Obukhov length L is not a parameter that is routinely measured. Recognizing the need for a readily usable way to define atmospheric stability based on routine observations, Pasquill (1961) introduced the concept of stability classes defined in Table 16.3. These classes have proved very useful in atmospheric diffusion calculations, as we will see shortly. 

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